Watts to Amps Calculator

How to use the watts to amps calculator

Enter the power in watts and pick the circuit voltage — 120 V for standard US outlets, 240 V for large appliances, or a custom value — and you get the current in amps, plus the minimum breaker for a continuous load. This is the breaker-and-wire-sizing question: amps are what circuits and breakers are rated in, so converting a device’s watts to amps tells you whether a given circuit can carry it.

The math is amps = watts ÷ volts, and it holds exactly for DC and for resistive AC loads — heaters, incandescent bulbs, kettles, anything that’s essentially a heating element. Motors and some electronics have a power factor below 1, so they pull somewhat more current than this for the same watts. For those, treat the result as a floor and check the nameplate amps; the math here is the resistive case.

The 80% rule is the safety margin that governs real wiring. Breakers and circuits are sized so that a continuous load stays at 80% of the rating: a 15 A circuit should carry no more than 12 A continuously, a 20 A circuit no more than 16 A. So a device drawing 12 A is already at the limit of a 15 A circuit. The tool shows the minimum breaker that keeps your draw inside that 80% band.

US homes run two voltages, and picking the right one is half the answer. Standard outlets and most plug-in devices are 120 V; big appliances — electric dryers, ranges, water heaters, EV chargers — use 240 V, which halves the amps for the same watts (the reason they use it: thinner wire and less loss). Choose the voltage that matches the circuit you’re putting the load on, not where the device was made.

Why this matters: overloading a circuit trips the breaker at best and overheats the wiring at worst. The safe way to plan is to add up the amps of everything sharing a circuit and compare the total against its rating with the 80% margin in mind. It’s the same question an electrician asks before adding a load — “what else is on that circuit?” — and the reason high-draw appliances often get a dedicated line.

The formula

Current is power divided by voltage. This is exact for DC and resistive loads; motors draw a little more for the same watts, which the note covers:

amps = watts ÷ volts

Worked example with the defaults — 1,200 W on a 120 V circuit: 1,200 ÷ 120 = 10.0 amps. On a 240 V circuit the same 1,200 W is only 5 amps, which is exactly why large appliances are wired for 240 V.

Power factor: motors, compressors, and some electronics draw more current than watts ÷ volts suggests, because their power factor is below 1. For resistive loads — heaters, kettles, incandescent bulbs — the formula is exact. When in doubt, use the device’s nameplate amps and treat this figure as a minimum.

Frequently asked questions

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