Watts / Amps / Volts Calculator

Enter any two values — the rest is solved instantly, with the formula shown. Commas are fine (1,500).

Power

Voltage

Current

Resistance (DC only)

–Power

–Voltage

–Current

–Resistance

Enter two values. The steps appear here.

Quick answerWatts = volts × amps on DC. On AC multiply by the power factor, and on three-phase by √3 as well: P = √3 × V × I × PF. A 1,500 W heater on 120 V draws 1,500 / 120 = 12.5 A.

Will it fit the circuit? Breaker load check

Uses the current above. A load that runs for three hours or more counts as continuous, and a standard breaker should carry no more than 80% of its rating for it.

Enter values above to see the load on 15, 20, 30, 40 and 50 A breakers.

80% rule: NEC 210.20(A) requires a branch-circuit overcurrent device rated at least 100% of the non-continuous load plus 125% of the continuous load — the same thing as loading it to 80% (1 / 1.25). Wire sizes: NEC 240.4(D) small-conductor limits for copper. See sources. Local codes and equipment instructions govern; have a licensed electrician size real circuits.

Ohm's Law Formulas

V = I × RVoltage

I = V / RCurrent

P = V × IPower

R = V / IResistance

P = I² × RPower from I,R

P = V² / RPower from V,R

AC: P = V × I × PF×√3 for three-phase

AC: |Z| = V / Iimpedance, not resistance

Related tools

Can you convert watts to volts?

Not on their own. Watts measure power and volts measure potential, so the conversion needs one more quantity. With the current you divide: V = P / I. With the resistance you take a square root: V = √(P × R). Anything that claims to turn watts straight into volts has assumed a current for you.

Two worked examples at the same wattage

A 10 W load drawing 0.5 A sits at 10 / 0.5 = 20 V. The same 10 W drawing 2 A sits at 10 / 2 = 5 V. The wattage did not change; the current decided the voltage. That is why a “10 watts to volts” search has no single answer.

Volts, amps and watts: the three formulas

Power is the product of the other two, which gives one formula for each unknown: P = V × I · V = P / I · I = P / V. On AC add the power factor, and for three-phase add √3 — the calculator above applies both when you pick those modes.

Watts to volts at common currents

Volts for a given power at a known current (V = P / I). Figures are the formula applied, not measurements.
Power	0.5 A	1 A	2 A	5 A	10 A	15 A
5 W	10.0 V	5.0 V	2.5 V	1.0 V	0.5 V	0.3 V
10 W	20.0 V	10.0 V	5.0 V	2.0 V	1.0 V	0.7 V
25 W	50.0 V	25.0 V	12.5 V	5.0 V	2.5 V	1.7 V
60 W	120 V	60.0 V	30.0 V	12.0 V	6.0 V	4.0 V
100 W	200 V	100 V	50.0 V	20.0 V	10.0 V	6.7 V
500 W	1,000 V	500 V	250 V	100 V	50.0 V	33.3 V
1,000 W	2,000 V	1,000 V	500 V	200 V	100 V	66.7 V
1,500 W	3,000 V	1,500 V	750 V	300 V	150 V	100 V

Read across a row to see how the same power lands at different voltages as the current changes. For resistance instead of current, use V = √(P × R): 10 W through 40 Ω is √400 = 20 V, the same answer as 0.5 A above.

Current Drawn at Common Voltages

Produced by the calculator above on its DC tab, where power factor is 1 and the arithmetic is simply watts divided by volts. Find the appliance rating on the left and read the current it draws at each supply voltage. The same appliance pulls roughly half the current on a 230 V supply that it does on 120 V, which is why circuit ratings differ so much between countries.

Power	110 V	120 V	220 V	230 V	240 V
60 W	0.55 A	0.50 A	0.27 A	0.26 A	0.25 A
100 W	0.91 A	0.83 A	0.46 A	0.43 A	0.42 A
300 W	2.73 A	2.50 A	1.36 A	1.30 A	1.25 A
500 W	4.55 A	4.17 A	2.27 A	2.17 A	2.08 A
750 W	6.82 A	6.25 A	3.41 A	3.26 A	3.13 A
1,000 W	9.09 A	8.33 A	4.55 A	4.35 A	4.17 A
1,200 W	10.9 A	10.0 A	5.45 A	5.22 A	5.00 A
1,500 W	13.6 A	12.5 A	6.82 A	6.52 A	6.25 A
1,800 W	16.4 A	15.0 A	8.18 A	7.83 A	7.50 A
2,000 W	18.2 A	16.7 A	9.09 A	8.70 A	8.33 A
2,400 W	21.8 A	20.0 A	10.9 A	10.4 A	10.0 A
3,000 W	27.3 A	25.0 A	13.6 A	13.0 A	12.5 A
5,000 W	45.5 A	41.7 A	22.7 A	21.7 A	20.8 A

Every value in this table is the calculator’s own output · DC / power factor 1 · single-phase and three-phase AC add a power factor and a √3 term — use the tabs above for those · not a substitute for an electrician

Sources for the numbers on this page

1 horsepower (electric) = 746 W, exact — NIST Special Publication 811, Appendix B.9. The hp option in the calculator uses this factor.
80% continuous-load rule — NFPA 70, National Electrical Code, section 210.20(A) (overcurrent device not less than the non-continuous load plus 125% of the continuous load) and the Article 100 definition of a continuous load (maximum current expected to continue for three hours or more). NFPA 70 free access.
14 / 12 / 10 AWG copper at 15 / 20 / 30 A — NFPA 70, section 240.4(D), small-conductor overcurrent limits. Other sizes depend on Table 310.16, insulation temperature and terminal ratings, so the breaker check does not guess them.
√3 for three-phase — balanced three-phase power with line-to-line voltage: P = √3 × V_L-L × I_L × PF. It is geometry, not a rule of thumb.

Checked 2026-10-09. Local amendments to the NEC can be stricter; the authority having jurisdiction decides.

What this page assumes, and where it stops

Jurisdiction

United States codes and agency guidance. Local codes may be stricter.

Assumptions

Only what you enter on this page is used. Personal circumstances, exemptions, and special cases are not applied.
Results are estimates for orientation, not determinations.
Everything runs in your browser. Nothing you type is sent to a server.

Where you should not rely on this

This is general guidance and does not replace training, local codes, or professional judgment. In an emergency, get to safety and call your local emergency number first. Work that touches wiring, gas, or structure should be done or inspected by a licensed professional.

Official sources

Page reviewed 2026-09-04 · rules current as of that review were not independently verified against every jurisdiction · next review scheduled annually

Frequently asked questions

Which two values do I need to enter?

Any two of watts, volts and amps — the calculator solves for the rest. With DC it uses the plain power formula; with AC it also applies the power factor you set.

Why does three-phase give a different answer?

Three-phase power includes a factor of the square root of three (about 1.732). The calculator applies it automatically when you select three-phase, so the same volts and amps yield roughly 1.73 times the power.

What power factor should I use?

The page starts at 1.0, which is exact for purely resistive loads such as heaters and incandescent lamps. Motors and many electronics run below 1. Use the figure on the equipment nameplate, or divide watts by volt-amperes (W / VA) when both are printed.