VFD Sizing Calculator - Select the Right Motor Drive

Q: What is a VFD sizing calculator?

A VFD sizing calculator computes the minimum required Variable Frequency Drive rating for an AC motor, taking motor power, service factor, and duty cycle as inputs to output a recommended standard drive size in kW and estimated input kVA.

Q: How do I calculate VFD size for a motor?

Multiply motor kW by service factor then by (1 + application margin). Example: 15 kW motor, SF 1.15, pump (10% margin) = 15 × 1.15 × 1.10 = 18.98 kW. Select the next standard size — 22 kW.

Q: What margin should I add when sizing a VFD?

Use 10% for fans/pumps (variable torque), 15% for conveyors/compressors (constant torque), and 20–25% for cranes/mills/hoists (high inertia).

Q: What is the difference between VFD kW and kVA ratings?

kW is real output power driving the motor. kVA is apparent power drawn from the supply — higher due to power factor and efficiency losses. A 30 kW VFD draws ~34.4 kVA at PF 0.90 and efficiency 97%.

Q: How does ambient temperature affect VFD sizing?

VFDs derate at 1% per °C above 40°C. A 45 kW drive at 55°C is only effective at 38.25 kW. Select a larger size or install panel cooling.

Q: What is VFD service factor and why does it matter?

Service factor (SF) on the motor nameplate indicates continuous overload capacity. A 10 kW motor with SF 1.15 can deliver 11.5 kW. The VFD must handle this full output — always include SF in the sizing formula.

Q: How do I size a VFD for a crane or hoist?

Use 20–25% margin on motor kW × SF. Verify 150–200% overload rating for the duty cycle. Consider brake choppers for regenerative braking energy.

Motor & Supply Parameters

Motor Power

Rated shaft power from nameplate

Supply Voltage

Service Factor (SF)

1.15

Standard motors: 1.0 – 1.15

Region / Standards

Application / Duty Class

Custom margin: %

Results & Recommendation

Enter motor details & press Calculate to get VFD recommendation

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What Is a VFD Sizing Calculator and How Does It Work?

Quick Answer: A VFD sizing calculator finds the minimum Variable Frequency Drive rating for your AC motor by applying the formula: Motor kW × Service Factor × (1 + Application Margin), then rounding up to the nearest standard drive size. Enter your motor power, select your duty type (fan, conveyor, crane), and get an instant kW, kVA, and standard size recommendation — no manual calculation needed.

Choosing the wrong VFD (Variable Frequency Drive) can cost thousands in equipment damage, energy waste, and unexpected downtime. Most engineers and technicians struggle with VFD sizing because they don’t account for service factors, duty cycles, and application-specific margins — and a single missed step means nuisance trips, premature drive failure, or money wasted on an oversized unit. This guide covers the complete VFD sizing methodology: the formula, application margins, derating factors, standard sizes, real worked examples, and regional standards (IEC, NEMA, AS/NZS) — plus a free calculator that does the math instantly.

What Is a VFD and Why Does Sizing Matter?

Quick Answer: A Variable Frequency Drive (VFD) controls AC motor speed by varying supply frequency and voltage. Proper sizing — matching the drive to motor rating, service factor, and application duty — prevents overload trips, motor damage, energy waste, and premature equipment failure.

A Variable Frequency Drive controls AC motor speed by varying the frequency and voltage supplied to the motor. Think of it as a sophisticated dimmer switch for industrial motors — but instead of reducing brightness, it precisely controls speed, torque, and acceleration.

Why proper VFD sizing is critical

Motor protection: An undersized VFD can’t handle peak currents during startup or load changes, causing nuisance trips, thermal stress, and premature failure.
Energy efficiency: Oversizing by more than 25–30% reduces efficiency at partial loads — you pay for capacity you don’t need.
Cost optimization: A 30 kW drive costs considerably more than a 22 kW unit. Proper sizing avoids unnecessary capital expenditure.
Application performance: Different applications demand different torque characteristics. Fans need variable torque profiles; conveyors require constant torque. The VFD must match these requirements.

According to the European Commission’s Motor Challenge Programme, undersized drives account for over 34% of unplanned motor system failures in manufacturing plants. Getting the sizing right the first time is not optional — it’s the foundation of reliable motor control.

How to Use the VFD Sizing Calculator

Quick Answer: Enter motor power (kW or HP), supply voltage, service factor, and application type. Press Calculate to get your required VFD kW, estimated input kVA, and the nearest standard drive size — plus application-specific guidance on cooling and overload.

What you’ll need before you start

Motor nameplate rating (kW or HP)
Supply voltage (230 V, 400 V, 480 V, 575 V, 690 V, or custom)
Motor service factor — found on the nameplate, typically 1.0 to 1.15
Application type: fans/pumps, conveyors, cranes, or custom
Installation region (for standards-specific notes)

Step-by-step instructions

Enter motor power — type the nameplate rating and select kW or HP. The calculator converts automatically (1 HP = 0.7457 kW).
Select supply voltage — choose a standard voltage or enter a custom value.
Set service factor — drag the slider to your motor’s SF (default 1.15 for standard motors).
Choose application type — click the preset matching your load. For custom applications, enter your own margin percentage.
Select region — affects the standards notes in results (IEC, NEMA, or AS/NZS).
Press Calculate — results appear instantly: required VFD kW, input kVA, and recommended standard drive size.
Copy or print — use the action buttons for a text summary or printable spec sheet.

Understanding your results

Calculator Output Fields Explained
Result field	What it means
Motor (converted)	Your motor rating in both kW and HP
Required VFD (calculated)	Minimum VFD rating after applying SF and margin
Input kVA (estimate)	Apparent power drawn from supply — for transformer/cable sizing
Recommended standard VFD	Nearest standard size equal to or greater than required kW
Drive utilisation bar	How loaded the recommended drive will be at full motor output

Pro Tip: If the utilisation bar shows above 90%, seriously consider the next standard size up — especially in hot climates or high-altitude installations where derating applies.

The VFD Sizing Formula Explained

Quick Answer: VFD Required (kW) = Motor Power (kW) × Service Factor × (1 + Application Margin). This is the base formula — but ambient temperature, altitude, and cable length all affect the final drive selection beyond this calculation.

The core formula

VFD_required_kW = motor_kW × SF × (1 + marginPct)

Variable A — Motor power (kW): The nameplate rated output. If rated in HP, convert first:

motor_kW = motor_HP × 0.7457

Variable B — Service factor (SF): A multiplier showing how much overload the motor can sustain continuously. A 15 kW motor with SF 1.15 can actually deliver 17.25 kW. The VFD must handle this, not just the nameplate rating. Ignoring SF is the most common VFD sizing error.

Variable C — Application margin: Extra headroom based on load characteristics. Values: 10% (variable torque), 15% (constant torque), 25% (high inertia). The margin covers load transients, SF utilization, and minor ambient variation.

Result: The minimum kW rating your VFD must meet. Round up to the nearest standard size from the IEC progression.

Estimating input kVA

VFDs draw apparent power (kVA) from the supply — not just real power (kW). This matters for sizing transformers, generators, and protective devices.

Input_kVA = Required_kW ÷ (Power Factor × Efficiency)
— Conservative values used in calculator: PF = 0.90, η = 0.97

Example: 30 kW required VFD → Input kVA = 30 ÷ (0.90 × 0.97) = 34.4 kVA

Always size upstream transformers and protective devices for kVA, not just kW.

VFD Sizing Calculator: Flowchart for Selecting the Correct VFD Size from Motor Rating — VFD sizing flowchart: from motor nameplate data to recommended standard drive size.

Application-Specific Margin Guidelines

Quick Answer: Use 10% margin for fans and pumps (variable torque), 15% for conveyors and compressors (constant torque), and 25% for cranes, mills, and hoists (high inertia). Getting this wrong is the single most common VFD sizing error — an undersized margin leads to continuous thermal stress and reduced drive lifespan.

VFD application margin comparison showing variable torque, constant torque, high inertia and servo applications — Torque-speed profiles for the four main VFD application types, with recommended sizing margins.

Variable torque — 10% margin (fans, pumps, blowers)

Centrifugal loads follow the affinity laws: torque increases with speed squared (T ∝ N²), power with speed cubed (P ∝ N³). They’re the most VFD-friendly loads — low starting current and they rarely demand peak motor output. A 15 kW pump motor running at 80% speed might only need 7.7 kW.

Worked example — cooling tower fan

Motor: 30 kW | SF: 1.15 | Margin: 10% (variable torque) Required = 30 × 1.15 × 1.10 = 37.95 kW Input kVA = 37.95 ÷ (0.90 × 0.97) = 43.5 kVA

→ Select standard 45 kW VFD

Constant torque — 15% margin (conveyors, compressors, extruders)

Constant torque loads demand full torque at any speed. A conveyor belt loaded with material needs the same force whether running at 20 Hz or 50 Hz. Higher starting currents and load transients require more VFD headroom. According to ABB’s Technical Guide Book (3AFE64514482), constant torque applications should be sized at “at least 1.1–1.2 times the motor full-load current rating.”

Worked example — screw compressor

Motor: 22 kW | SF: 1.10 | Margin: 15% (constant torque) Required = 22 × 1.10 × 1.15 = 27.83 kW Input kVA = 27.83 ÷ (0.90 × 0.97) = 31.9 kVA

→ Select standard 30 kW VFD

High inertia / frequent starts — 25% margin (cranes, mills, hoists)

These are the most demanding applications. High rotational inertia requires extended acceleration — and sustained high-current draw. Cranes start and stop dozens of times per hour. Peak currents can reach 150–200% of rated motor current, and thermal stress accumulates fast. Most standard VFDs provide 150% overload for 60 seconds. For crane and hoist applications, verify whether this is sufficient for your actual duty cycle — some require 200% for extended periods, which means a heavy-duty drive, not just a larger standard unit.

Worked example — overhead crane hoist

Motor: 45 kW | SF: 1.15 | Margin: 25% (high inertia) Required = 45 × 1.15 × 1.25 = 64.69 kW Input kVA = 64.69 ÷ (0.90 × 0.97) = 74.1 kVA

→ Select standard 75 kW VFD

Application margin reference table

TABLE 1: VFD Application Margin Guidelines
Application type	Typical loads	Recommended margin	Key characteristic
Variable torque	Fans, pumps, blowers	10%	Torque ∝ Speed², low start current
Constant torque	Conveyors, compressors, extruders	15%	Full torque at all speeds
High inertia	Cranes, mills, centrifuges	20–25%	Repeated high-current starts
Servo / precision	Machine tools, packaging	12–15% + mfr data	Rapid acceleration, position control
Custom	Variable / special	User-defined	Consult load profile data

Critical Derating Factors

Quick Answer: VFDs lose effective capacity at high ambient temperatures (1% per °C above 40°C) and high altitudes (1% per 100 m above 1,000 m). Always calculate combined derating before finalizing selection — particularly critical for outdoor, rooftop, or high-altitude installations in Pakistan, India, and similar regions.

Ambient temperature derating

Standard VFDs are rated for 40°C (104°F) ambient. Above this, the drive must be derated.

Effective kW = Rated kW × [1 − 0.01 × (T_ambient − 40)]

A 45 kW drive in a 50°C electrical room: 45 × [1 − 0.01 × 10] = 40.5 kW effective. You’d need a 55 kW drive to deliver 45 kW output.

Altitude derating

Air density drops with elevation, reducing cooling effectiveness. Above 1,000 m, apply derating.

Effective kW = Rated kW × [1 − 0.01 × (Altitude − 1000) ÷ 100]

At 2,000 m: the drive loses another 10% — same as a 10°C excess temperature.

Pro Tip — Combined derating: Multiply all derating factors together. A 45 kW drive at 50°C and 2,000 m: 45 × 0.90 × 0.90 = 36.45 kW effective. You may need a 45 kW nominal drive just to deliver what a 37 kW would in ideal conditions.

VFD derating factors including temperature, altitude, cooling and cable length considerations — Key VFD derating factors — temperature, altitude, enclosure IP rating, and cable length all reduce effective output capacity.

Derating factors reference table

TABLE 2: VFD Derating Factors and Formulas
Derating factor	Threshold	Impact	Rate / Formula
Ambient temperature	Above 40°C	High	1.0% per °C — kW_eff = kW × [1 − 0.01 × (T − 40)]
Altitude	Above 1,000 m	Medium	1.0% per 100 m — kW_eff = kW × [1 − 0.01 × (Alt − 1000)/100]
Long motor cables	Over 100 m	Medium	Add output reactor; may require 5–10% larger drive
Enclosure rating IP65/66	Sealed drives	Medium	10–15% vs IP20 — multiply by 0.85–0.90
Switching frequency	Above 4 kHz PWM	Low	3–5% per step — check manufacturer curve
Combined	All factors	High	kW_eff = kW × f_temp × f_alt × f_IP × f_other

Standard VFD Sizes and Ratings

Quick Answer: IEC standard VFD sizes follow motor frame progressions: 0.75, 1.1, 1.5, 2.2, 3.0, 4.0, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90, 110, 132, 160, 200 kW. Always select the first standard size equal to or greater than your calculated requirement — never round down.

VFD manufacturers — ABB, Siemens SINAMICS, Schneider Altivar, Rockwell PowerFlex — produce drives in standardized power ratings aligned with IEC motor frame sizes and common electrical distribution equipment ratings.

Standard VFD power ratings chart from 0.75 kW to 200 kW with typical applications — IEC standard VFD power progression from 0.75 kW to 200 kW — select the first size equal to or above your calculated requirement.

TABLE 3: Standard VFD Sizes with Approximate Current Ratings at 400 V
VFD size (kW)	Approx HP	Output current @ 400 V	Typical application
0.75	1.0	2.3 A	Small pumps, mixers
1.5	2.0	4.1 A	Packaging, small fans
2.2	3.0	5.8 A	Machine tools, small conveyors
4.0	5.4	9.6 A	Ventilation fans, feeders
7.5	10.1	16.7 A	HVAC, moderate pumps
11	14.7	24.2 A	Chiller pumps, cooling towers
15	20.1	32.0 A	Process pumps, air handlers
18.5	24.8	38.0 A	Medium conveyors, blowers
22	29.5	45.0 A	Compressors, large conveyors
30	40.2	60.0 A	Large fans, industrial pumps
37	49.6	73.0 A	Screw compressors, extruders
45	60.3	87.0 A	Large compressors, crushers
55	73.7	105.0 A	Industrial blowers, extruders
75	100.6	144.0 A	Large process equipment
90	120.7	170.0 A	Chillers, large mills
110	147.5	205.0 A	Industrial air compressors
132	177.0	245.0 A	Heavy industrial drives
160	214.6	302.0 A	Mining equipment, large mills
200	268.2	377.0 A	Large industrial processes

* Current ratings approximate at 400 V, 3-phase. Always verify against manufacturer nameplate data.

Real-World VFD Sizing Examples

Quick Answer: Three worked examples — HVAC fan (variable torque, 30 kW → 45 kW VFD), screw compressor (constant torque, 37 kW → 55 kW VFD), and overhead crane hoist (high inertia, 15 kW → 22–30 kW VFD) — show how the formula produces practical drive selections.

Example 1: HVAC centrifugal fan (variable torque)

Application: Building ventilation, continuous 24/7 operation

Motor: 18.5 kW (25 HP) | Voltage: 400 V | SF: 1.15 | Duty: Variable torque Required = 18.5 × 1.15 × 1.10 = 23.41 kW Input kVA = 23.41 ÷ (0.90 × 0.97) = 26.82 kVA

→ Standard selection: 30 kW VFD (28% overhead — ideal for ambient variation and future expansion)

Input cable should be rated for at least 27 kVA. The 30 kW drive provides comfortable overhead for ambient variation and future expansion.

Example 2: Industrial screw compressor (constant torque)

Application: Compressed air system, frequent load cycling

Motor: 37 kW (50 HP) | Voltage: 480 V | SF: 1.10 | Duty: Constant torque Required = 37 × 1.10 × 1.15 = 46.81 kW Input kVA = 46.81 ÷ (0.90 × 0.97) = 53.62 kVA

→ Standard selection: 55 kW VFD (17% overhead — handles load swings without nuisance trips)

Compressors experience significant load swings during unload-to-load transitions. The 55 kW drive handles these transients reliably.

Example 3: Overhead crane hoist (high inertia)

Application: Material handling, 30+ starts per hour

Motor: 15 kW (20 HP) | Voltage: 400 V | SF: 1.15 | Duty: High inertia Required = 15 × 1.15 × 1.25 = 21.56 kW Input kVA = 21.56 ÷ (0.90 × 0.97) = 24.71 kVA

→ Standard selection: 22 kW (minimum) or 30 kW (recommended for enhanced overload margin)

For crane applications, many engineers specify 30 kW here for enhanced overload capacity during emergency stops and load swing events. Always verify the drive’s overload specification — 150–200% for extended periods may be required.

Regional Standards: IEC vs NEMA vs AS/NZS

Quick Answer: Europe follows IEC 61800-3 (EMC) and IEC 60204-1 (machine safety). USA follows NEMA MG1 Part 31 and NEC Articles 430/440. Australia applies AS/NZS 61800-3. The calculator’s region setting adjusts the guidance notes in your results accordingly.

Europe — IEC standards

IEC 61800-3 governs electromagnetic compatibility. Key requirements: screened motor cables, AC line reactors or EMC filters for Category C2/C3 environments, and Safe Torque Off (STO) for machine safety per IEC 62061.

USA — NEMA / NEC

NEMA MG1 Part 31 specifies motors intended for inverter supply — look for “Inverter Duty” on the motor nameplate. NEC Article 430.122 requires supply conductors sized at 125% of VFD rated input current. UL 508C certification applies to the VFD assembly.

Australia — AS/NZS

AS/NZS 61800-3 mirrors IEC with additional state-specific workplace safety requirements. Supply voltage is typically 400 V or 415 V. Enclosure IP ratings must comply with AS/NZS 60529.

TABLE 4: Regional Standards Comparison
Region	Primary standard	Motor standard	Installation code	Common voltage
Europe	IEC 61800-3	IEC 60034	IEC 60204-1	400 V / 690 V
USA	NEMA MG1 Pt.31	NEMA MG1	NEC 430/440	230 V / 460 V / 575 V
Australia	AS/NZS 61800-3	AS/NZS 60034	AS/NZS 60204-1	400 V / 415 V
International	IEC 61800-3	IEC 60034	IEC 60204-1	Varies

Advanced Selection & Installation Guidance

Proper VFD installation diagram showing circuit protection, cooling, grounding and cable routing — Proper VFD installation: circuit protection, cooling clearances, screened motor cables, and grounding best practices.

Harmonics and EMC filtering

VFDs generate harmonic currents that distort power quality — causing transformer heating, voltage distortion, and interference with nearby equipment. Solutions include AC line reactors (3–5% impedance), DC bus chokes, and active harmonic filters for large installations above 50 kW. Budget 10–20% additional cost for proper filtering and EMC compliance per IEEE Std 519.

Energy savings and ROI

For centrifugal loads, the affinity laws show dramatic energy savings at reduced speeds: power drops with the cube of speed (P ∝ N³). A 30 kW fan at 80% speed consumes only 30 × (0.80)³ = 15.36 kW — a 49% saving. At 8,000 operating hours per year and $0.12/kWh, that’s over $14,000 annual savings per motor. VFD payback periods on pump and fan applications commonly fall below six months.

Industry 4.0 connectivity

Modern VFDs from ABB, Siemens SINAMICS, and Rockwell PowerFlex support Ethernet/IP, Profinet, Modbus TCP, and EtherCAT for real-time monitoring, predictive maintenance, and SCADA integration. Specify the communication protocol during initial selection — retrofitting communication cards adds cost and complexity.

Common VFD Sizing Mistakes to Avoid

✗ Mistake 1: Ignoring service factor. Sizing the VFD to nameplate kW only means the motor can exceed VFD rating when operating at SF capacity. ✓ Fix: Always multiply motor kW × SF before applying margin.

✗ Mistake 2: Wrong application margin. Using 10% (fan margin) for a constant-torque conveyor — the drive runs near thermal limits continuously. ✓ Fix: Match margin to actual load profile. When in doubt, choose the higher category.

✗ Mistake 3: Forgetting ambient derating. Specifying a drive for a poorly ventilated MCC room without derating — thermal trips in summer. ✓ Fix: Measure installation site temperature. Apply 1%/°C above 40°C. Install cooling if needed.

✗ Mistake 4: Long cable runs without output reactors. Running 200 m motor cables without considering capacitance — overvoltage spikes at motor terminals, insulation failure. ✓ Fix: Install output reactors or dV/dt filters for cable runs over 100 m.

✗ Mistake 5: Combined derating not calculated. Applying temperature derating OR altitude derating separately, not multiplied together. ✓ Fix: Effective kW = rated kW × f_temp × f_alt × f_IP (multiply all factors).

Frequently Asked Questions

What is a VFD sizing calculator?

A VFD sizing calculator is a tool that computes the minimum required Variable Frequency Drive rating for a given AC motor. It takes motor power, service factor, and application duty cycle as inputs, then outputs a recommended standard drive size in kW and an estimated input kVA for upstream electrical sizing. Our free calculator also provides application-specific notes on cooling, overload capacity, and regional standards compliance.

How do I calculate VFD size for a motor?

Multiply motor power (kW) by the service factor, then multiply by (1 + application margin). For a 15 kW motor with SF 1.15 driving a pump (10% margin): 15 × 1.15 × 1.10 = 18.98 kW required. Select the next standard size — 22 kW. Estimate input kVA: 18.98 ÷ (0.9 × 0.97) = 21.74 kVA. Always round up to the nearest standard size — never down.

What margin should I add when sizing a VFD?

The margin depends on the application: 10% for variable torque loads (fans, centrifugal pumps), 15% for constant torque loads (conveyors, screw compressors), and 20–25% for high inertia applications (cranes, mills, hoists). These margins cover load transients, service factor utilization, and minor derating from ambient conditions. When uncertain about the application category, always choose the higher margin.

What is the difference between VFD kW and kVA ratings?

kW (kilowatts) is real output power — what actually drives the motor shaft. kVA (kilovolt-amperes) is apparent power drawn from the electrical supply, which is higher than kW due to power factor and efficiency losses. A VFD with 30 kW output draws approximately 34.4 kVA from the supply (at PF 0.90 and efficiency 97%). kVA matters for sizing upstream transformers, generators, cables, and protective devices.

How does ambient temperature affect VFD sizing?

Most VFDs are rated for 40°C (104°F) ambient. Above this, derate by approximately 1% per °C. A 45 kW drive at 55°C ambient is only effective at 38.25 kW (45 × 0.85). Either select the next standard size up, install panel air conditioning or heat exchangers, or specify a drive with an extended temperature rating. This is particularly important for installations in hot climates or poorly ventilated electrical rooms.

What is VFD service factor and why does it matter?

Service factor (SF) is a multiplier printed on the motor nameplate indicating continuous overload capacity. A 10 kW motor with SF 1.15 can deliver 11.5 kW without damage. The VFD must handle this full output, so SF must be included in the sizing formula: VFD required = motor kW × SF × (1 + margin). Ignoring SF is one of the most common — and costly — VFD selection errors.

Can I use a smaller VFD than the motor rating?

Generally no. The VFD must be rated equal to or greater than the calculated required kW (motor kW × SF × margin). Using a smaller drive risks nuisance overcurrent trips, thermal shutdown, and premature drive failure. The only exception is when the motor will never operate above a known partial load — consult a qualified engineer before intentionally undersizing.

How do I size a VFD for a crane or hoist?

Use a 20–25% application margin on top of motor kW × service factor. Verify the drive’s overload rating (typically 150% for 60 seconds) is sufficient for your starting duty cycle. Applications with very frequent starts may require 200% overload capability or a specialized heavy-duty drive. Consider brake choppers and resistors for regenerative deceleration energy. Always consult both the crane OEM and VFD manufacturer for high-cycle duty applications.

Conclusion

Proper VFD sizing comes down to one formula: motor kW × service factor × (1 + duty margin), rounded up to the nearest standard size. Apply the right margin — 10% for fans, 15% for conveyors, 25% for cranes — and always check for temperature and altitude derating at your installation site.

You now have the complete methodology to confidently select any Variable Frequency Drive. Use the free VFD sizing calculator at the top of this page to get an instant recommendation. Verify against the motor nameplate, cross-check your installation conditions with the derating tables, and confirm the final selection with your VFD vendor before purchase.

Have a complex application — multi-motor systems, regenerative loads, or extreme environments? Contact us — we’re happy to help with project-specific sizing guidance.

→ Try the free VFD Sizing Calculator now

Sources & Further Reading

Last Updated: June 2026 | Next Review: December 2026

Recommended reading: Energy Calculators — Solvebility | VFD Sizing Calculator