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SZ-06Sizing · Electrical

Ohm’s law and watts–amps calculator

Type any two of volts, amps, ohms and watts and get the other two — or convert kW and kVA to amps for single- and three-phase loads with a power factor. Drawn to scale. It converts quantities; it does not size wire or breakers.

Volts, amps, ohms, watts

What are you converting?

Example: a 12 Ω heating element on 120 V. Type any two values.

Worked out Example

current 10 A, power 1,200 W.

from 12 Ω and 120 V

See it drawn to scale Change the numbers

Volts, amps, ohms and wattsDrawn to scale on both axes
Volts against amps for the example: power as an area, resistance as a slopeVolts against amps. The point 10 A, 120 V is the corner of a rectangle whose area is the power, 1,200 W; the line through it from zero has the slope of the resistance, 12 Ω.AVolts against ampsP = area · R = slope051015050100150200AmpsVolts10 A120 VPower = V × I1,200 WR = V ÷ I12 Ω
Volts given
120 Vvoltage
Amps worked out
10 Acurrent
Ohms given
12 Ωresistance
Watts worked out
1,200 Wpower

I = 120 V ÷ 12 Ω = 10 AP = 120² ÷ 12 = 1,200 W

Already showing the example.

Read with the result

  • DC, or AC in a purely resistive load with rms values. For motors and other inductive loads use AC power.HyperPhysics, “Electric Power”.
  • Calculated current is not a wire or breaker size. Conductors and overcurrent devices follow the electrical code and the equipment’s listing and instructions.

Change the numbers

Type any two. The other two are worked out in the dashed boxes; type in one and it becomes a given.

Voltage across the load.

Current through it.

Resistance of the load.

Power it uses.

Result: 10 A · 1,200 W. Back to the result

How the math works

Ohm’s law and the power relations, as HyperPhysics and NIST state them, plus the Department of Energy’s three-phase power equation. Every result keeps full precision; only the display is rounded.

  1. CurrentI = V ÷ RRearranged: V = I × R and R = V ÷ I.
  2. PowerP = V × I = I² × R = V² ÷ RHolds for DC, and for AC in a resistor with rms values.
  3. Single-phase ACkW = V × I × PF ÷ 1,000 · I = kW × 1,000 ÷ (V × PF)
  4. Three-phase ACkW = √3 × V × I × PF ÷ 1,000DOE Equation 1: V is the line-to-line voltage, I the line current, for a balanced load. √3 is computed (1.7320508…), so results can differ from a 1.732 hand calculation in the fifth figure.
  5. kVA to ampsI = kVA × 1,000 ÷ V (single) · ÷ (√3 × V) (three)Apparent power ignores the power factor.
  6. kW and kVAkW = kVA × PFPower factor = real power ÷ apparent power (DOE). The triangle’s third side, reactive power, is √(kVA² − kW²).

Worked examples

  • 1,500 W heater on 120 VI = 1,500 ÷ 120 = 12.50 A · R = 120² ÷ 1,500 = 9.6 Ω
  • 5 kW at 240 V, PF 0.855 × 1,000 ÷ (240 × 0.85) = 24.51 A (20.83 A at PF 1.0)
  • 10 kW three-phase, 208 V, PF 0.910 × 1,000 ÷ (√3 × 208 × 0.9) = 30.84 A
  • 75 kVA three-phase at 480 V75 × 1,000 ÷ (√3 × 480) = 90.21 A
  • DOE’s motor: 469.7 V, 37 A, PF 0.763√3 × 469.7 × 37 × 0.763 ÷ 1,000 = 22.97 kW (DOE prints 22.9)

Before you use the number

  • Power factor changes the answer. Heaters and incandescent lamps are resistive, PF 1.0. Motors and other inductive loads run lower, and their PF changes with load (DOE). Use the nameplate or a measurement; the calculator assumes no “typical” motor PF.
  • Three-phase means line-to-line voltage and a balanced load. For a line-to-neutral voltage or an unbalanced load the three-phase formulas do not apply as written.
  • kVA is not kW. kW = kVA × PF.
  • Current is not a wire or breaker size. Conductor and overcurrent sizing follow the electrical code and the equipment’s listing and instructions; this calculator does not apply them.
  • Sinusoidal loads. The relations hold for rms values of sine-wave voltage and current; results are not exact for nonlinear loads such as many electronic power supplies.

Long run? Check the voltage at the far end with the voltage drop calculator.

Sources

Opened and checked October 4, 2026. The worked examples are our own arithmetic.

  1. NIST Guide to the SI, Chapter 4: The Two Classes of SI Units and the SI PrefixesNational Institute of Standards and Technology
  2. Ohm’s LawHyperPhysics, Georgia State University
  3. Electric PowerHyperPhysics, Georgia State University
  4. Power Factor for AC PowerHyperPhysics, Georgia State University
  5. Determining Electric Motor Load and Efficiency (fact sheet)U.S. Department of Energy, Motor Challenge
  6. Reducing Power Factor Cost (fact sheet)U.S. Department of Energy, Motor Challenge

  • 4.05Guide

    Voltage Drop Explained: The K Formula, 3% and 5%

    How to work voltage drop with the K-factor formula, copper vs aluminum, single vs three phase, and what the NEC's 3% and 5% notes really say.

    √3 × 12.9 × 150 × 200 ÷ 211,600 = 3.17 V

    Length
    6 min
    Updated

  • 4.01Guide

    BTU Per Square Foot: The Room Chart vs Manual J

    What ENERGY STAR's room AC chart says about BTU per square foot, how its four adjustments work, and why whole houses need a Manual J load.

    9,000 × 0.90 + 2 × 600 = 9,300 BTU/h

    Length
    6 min
    Updated

Tools for the job

Tool listSeveral trades4 itemsSpecs read

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  1. Meters and testersFluke 87V

    87V Industrial Multimeter

    • True-RMS; 1,000 V AC/DC, 10 A (20 A for 30 seconds), resistance to 50 MΩ, capacitance to 9,999 µF, frequency to 200 kHz.

    Specs: Fluke product page, read

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  2. Circuit tracers and analyzersFluke 2052

    2052 Advanced Wire Tracer Kit

    • Traces energized and de-energized wires in walls, ceilings and floors; finds opens, shorts, breakers and fuses.

    Specs: Fluke product page, read

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  3. Combustion and electrical testingFluke 902 FC

    902 FC True-RMS HVAC Clamp Meter

    • Measures 600 A AC, 600 V AC/DC, resistance to 60 kΩ, capacitance to 1,000 µF and 200 µA DC for flame sensors.

    Specs: Fluke product page, read

    View on Amazon for Fluke 902 FC(paid link)

  4. Thermal imagersFluke TiS20+ MAX

    TiS20+ MAX Thermal Imaging Camera

    • 120 x 90 (10,800-pixel) infrared resolution, 60 mK thermal sensitivity, 9 Hz.

    Specs: Fluke product page, read

    View on Amazon for Fluke TiS20+ MAX(paid link)

How tools are chosen for gear listsAll 11 electrical tools