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Watts to Amps Calculator (120 V)

Convert an appliance wattage into the current it draws on a 120 V circuit, for breaker and cable checks.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Current draw

12.5 A

High confidence

I = P ÷ V at 120 V single phase. For a continuous load, compare the answer against 80% of the breaker rating rather than the full rating.

Conversion factor applied
0.01 A per W

The practical question behind this conversion is almost always whether something can be plugged in where you want to plug it. Dividing wattage by 120 V gives the current, and current is what the breaker and the conductor care about. The trap is the continuous-load rule: a circuit expected to run for three hours or more is generally limited to 80% of its rating, which turns a 15 A circuit into a 12 A allowance, or roughly 1,440 W. That is precisely why a 1,500 W space heater is a nuisance on a general-purpose circuit despite appearing to fit. Note also that appliances rated in VA rather than watts are already stating volts times amps, so the VA figure is the one to divide — using the watt figure on an inductive load will understate the current the supply actually has to deliver.

Add the equipment this sizes

This result is a specification — 12.5 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Rearranged Ohm's law: I = P / V
  • Continuous loads are commonly limited to 80% of the branch-circuit rating under NEC 210.19(A)

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Frequently asked questions

How much of a 15 A circuit can I actually use?
For a continuous load — one running three hours or more — the usual limit is 80% of the breaker rating, so 12 A on a 15 A circuit, or about 1,440 W at 120 V. That is why a 1,500 W heater is awkward on a 15 A circuit even though the arithmetic says it fits.
Does this cover starting current?
No. A motor can pull several times its running current while it comes up to speed. That surge rarely changes the cable size, because it is too brief to heat the conductor, but it does influence breaker type and can cause nuisance tripping on a device with a fast magnetic response.
What if the appliance is rated in VA rather than watts?
Use the VA figure directly for current calculations — it is already volts times amps. Watts is the smaller number on an inductive load, and using it would understate the current the supply has to deliver.