How this watts to amps calculator works
Pick a direction, pick a supply type, and enter what you know. The arithmetic is straightforward; the part most converters get wrong is that on AC there is a third number, and leaving it out understates the current on exactly the loads where current matters most.
Every result also gives the apparent power in VA, the reactive power, and the figure a breaker would be sized to for a continuous load, because those are the numbers you actually need once you are doing anything with the answer.
The formula
AC single phase: P = V × I × PF
AC three phase: P = √3 × V × I × PF
apparent power (VA) = V × I, or √3 × V × I
continuous load breaker rating = I × 1.25
PF is the power factor, the fraction of the apparent power that does real work. It is 1 for anything purely resistive, around 0.8 for a typical induction motor, and 0.9 to 0.95 for most modern electronics with corrected supplies. The √3 in the three-phase line is there because a three-phase supply delivers on three conductors whose peaks are 120 degrees apart, so the total is not simply three times the single-phase figure. Being out by that 1.732 is the single most common three-phase error, in both directions.
Worked example
A 1,500 W space heater on 120 V. A heater is purely resistive, so the power factor is 1 and the answer is the simple one: 1,500 ÷ 120 = 12.5 A. On a 15 A circuit that is 83% of the breaker, which is why two of them on one circuit trips it and why the label warns you.
Now the same 1,500 W as a motor at 0.8 power factor. The current becomes 1,500 ÷ (120 × 0.8) = 15.6 A, which is 25% more, and it has just gone over the same 15 A breaker. The motor is doing exactly the same 1,500 watts of real work. The wiring is simply carrying more current to deliver it.
That gap is the whole reason power factor is on this page. The naive answer, watts divided by volts, is right for heaters and wrong for everything with a coil in it, and it is wrong in the direction that undersizes the circuit.
Watts, volt-amps, and the number on the label
Real power in watts is energy actually consumed. Apparent power in VA is voltage multiplied by current, which is what the conductors, the breaker and the connections physically carry. When the power factor is 1 the two are equal; the further it falls, the further they diverge.
The difference has a name, reactive power, measured in VAR, and it is not wasted energy in the way losses in a wire are. It sloshes back and forth between the supply and the load's magnetic or capacitive parts every cycle, doing no net work and going home again. A domestic meter does not bill you for it, which is why power factor is invisible on a house bill. Commercial customers are frequently billed for demand in kVA rather than kW, which is exactly why power factor correction equipment exists and why it pays for itself in a factory and never in a kitchen.
The practical version: a device labelled in VA is not telling you its watts. A 1,500 VA UPS is not a 1,500 W UPS, and the gap between them is the power factor the manufacturer assumed, often 0.6 on older units and 0.9 to 1.0 on newer ones. Our UPS runtime calculator goes into that one in detail, because it is the specification most commonly misread when someone buys a backup supply.
The 125% rule, and what it is actually about
Every result here shows the current multiplied by 1.25. That is the standard treatment for a continuous load, meaning one expected to run for three hours or more, and the overcurrent device and conductor are sized to that inflated figure rather than to the actual draw.
It is worth knowing what the rule is protecting, because it is not the appliance. Circuit breakers are calibrated at a defined ambient temperature and their tripping characteristics drift as the enclosure warms up, which it does when a load runs for hours in a panel full of other loads. The 125% margin is headroom for that thermal reality, not a claim that the appliance draws more than it says. It also means the familiar advice not to load a circuit past 80% is the same rule stated backwards: 1 divided by 1.25 is 0.8.
Two things this page deliberately does not do. It does not size conductors, because ampacity depends on insulation type, ambient temperature and how many conductors share a raceway, and getting that wrong is a fire rather than an inconvenience. And it does not account for motor inrush, which can be six to eight times the running current for a second or so at startup and is why motor circuits have their own sizing rules entirely. For the length of a run rather than its size, our voltage drop calculator handles the other half of the problem.