How the Volts, Amps & Watts Calculator works
Formulas
The calculator finds one of watts, amps or volts from the other two. P is real power in watts, V is voltage, I is current in amps and PF is the power factor.
DC P = V × I I = P ÷ V V = P ÷ I
single-phase AC P = V × I × PF I = P ÷ (V × PF) V = P ÷ (I × PF)
balanced three-phase AC P = √3 × V × I × PF I = P ÷ (√3 × V × PF) V = P ÷ (√3 × I × PF)
apparent power (AC) VA = V × I (× √3 for three-phase)
In code this is one factor: factor = PF × √3 for three-phase, PF for single-phase and 1 for DC. Watts are first × second × factor; amps and volts are first ÷ (second × factor). The apparent power shown next to an AC watts result is the same calculation with a power factor of 1.
For three-phase AC, V is the RMS line-to-line voltage and I the RMS line current, the definitions in Equation 1 of DOE's fact sheet Determining Electric Motor Load and Efficiency. P is the total three-phase real power.
Inputs
- Both values must be numbers greater than zero. An empty field, zero, a negative number or text gives the message "Enter a number greater than zero in both fields."
- For AC, the power factor must be greater than 0 and no more than 1; anything else gives "Power factor must be greater than zero and no more than 1." For DC the power factor field is hidden and ignored.
- Extreme inputs whose result overflows, or underflows to zero, are refused with a message rather than shown.
There is no default power factor for real equipment. The field starts at 1, which is correct for a purely resistive load such as a heating element and wrong for most motors and electronic power supplies.
Rounding
The arithmetic runs at full JavaScript double precision. Results are rounded for display only, to at most two decimal places with thousands separators: 480 V three-phase at 20 A and a power factor of 0.85 shows 14,133.53 W. A positive result below 0.005 shows as "less than 0.01" rather than 0. Inputs are echoed in the calculation line as typed, up to six decimal places.
The two watts-to-amps tables on the calculator page are computed at build time by the same function the page runs, from the same file, and rounded the same way.
Assumptions and limits
- AC values are RMS and the waveform is sinusoidal. Three-phase mode assumes a balanced supply and load.
- Results are steady-state. They do not include motor starting current, compressor surge, inverter losses or wiring losses.
- The tool explains ratings. It is not a method for sizing wire, breakers or an installation.
Sources
- U.S. Department of Energy, Energy Saver, Estimating Appliance and Home Electronic Energy Use: "If the wattage is not listed on the appliance, you can still estimate it by finding the electrical current draw (in amperes) and multiplying that by the voltage used by the appliance. Most appliances in the United States use 120 volts. Larger appliances, such as clothes dryers and electric cooktops, use 240 volts." The live page returned 404 on 2026-10-25, so it is cited as the Internet Archive capture of 2026-07-01.
- U.S. Department of Energy, Motor Challenge fact sheet, Determining Electric Motor Load and Efficiency, Equation 1: "Pi = V x I x PF x √3 / 1000", where V is the "RMS voltage, mean line-to-line of 3 phases", I the "RMS current, mean of 3 phases" and PF the "power factor as a decimal".
- U.S. Department of Energy, Motor Challenge fact sheet, Reducing Power Factor Cost: power factor "is defined as the ratio of real (working) power to apparent (total) power".
Change log
- 2026-10-25 — first release.
Cite this page
GridComfort, "Volts, Amps & Watts Calculator — methodology," https://gridcomfort.com/tools/volts-amps-watts-calculator/methodology/, updated 2026-10-25.