Amp Watt Calculator
Amps tell you how much current is flowing, watts tell you how much power that flow delivers, and volts are the missing middle term that connects them. This amp watt calculator solves for any of the three: pick what you want to find, pick your circuit type (DC, single-phase AC, or three-phase AC with line-to-line or line-to-neutral voltage), add a power factor for AC loads, and the tool returns the answer with the exact formula and substitution printed underneath, so you can check it by hand or show your working.
The calculator is pre-filled with the simplest case on purpose: 10 amps at 12 volts DC returns 120.00 W (0.120 kW). From that same card, without a second tool, you can run the questions that actually come up on a job or in a workshop: a 5 amp motor load at 120 V with a power factor of 0.8 (480.00 W), a three-phase machine at 15 amps and 208 V line-to-line with a power factor of 0.9 (4,863.60 W), or the reverse problem, how many amps an 1,800 W heater draws at 120 V (15.00 A). Every one of those examples is worked in full below.
For load planning and learning. Final breaker and wire sizing also depends on continuous-load rules, wire gauge, temperature and local code: confirm safety-critical work with a licensed electrician.
Quick Answer
- Quick Answer
- Worked examples, checked against this exact tool
- The four formulas in one place
- Why AC needs a power factor, and what to enter
- Line-to-line vs line-to-neutral, with the numbers side by side
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
Worked examples, checked against this exact tool
Each row below is a setting of the calculator above, and the result column is what it prints. Run any of them to confirm the card and the article agree.
| Case | Settings | Result |
|---|---|---|
| DC power (pre-filled) | Watts mode, DC, 10 A, 12 V | 120.00 W (0.120 kW) |
| Single-phase AC power | Watts mode, AC single-phase, 5 A, 120 V, PF 0.8 | 480.00 W |
| Three-phase power (line-to-line) | Watts mode, 3-phase L-L, 15 A, 208 V, PF 0.9 | 4,863.60 W (4.864 kW) |
| Reverse: heater current | Amps mode, AC single-phase, 1,800 W, 120 V, PF 1.0 | 15.00 A |
| Reverse: supply voltage | Volts mode, DC, 120 W, 10 A | 12.00 V |
The three-phase row deserves a slow look, because it is where most hand calculations go wrong. The working the tool prints is 1.732 × 15 × 208 × 0.9 = 4,863.60. Drop the power factor and you get 5,404 W, an 11% overstatement. Drop the 1.732 and use the single-phase formula instead and you get 2,808 W, a 42% understatement that would undersize everything downstream. The formula choice is the calculation; the multiplication is the easy part. For the current side of three-phase systems specifically, the 3-phase current calculator goes deeper on that one direction.
The four formulas in one place
Every conversion on this page is one of four equations, or one of them rearranged. They come from the electrical power law, the companion to Ohm’s law: power equals voltage times current, adjusted for how AC loads actually draw current.
Why two three-phase versions? Because nameplates quote voltage two ways. A 480 V service is a line-to-line figure; the same system’s line-to-neutral voltage is 277 V. Feed 277 into the √3 formula and you undercount by a factor of about 1.73; feed 480 into the 3-times formula and you overcount by the same factor. The calculator forces you to declare which voltage you hold, which is the step paper calculations skip. Once watts are known, running cost is the natural next question for a steady load, and the bitcoin per watt calculator shows the same watt figure carried into cost per day for a 24/7 load.
Why AC needs a power factor, and what to enter
On DC, current and voltage rise and fall together, so every amp does work. On AC, inductive loads (motors, transformers, magnetic ballasts) pull current that sloshes back and forth magnetising coils without delivering net power. Power factor is the share that does real work: watts divided by volt-amps. Ignore it and a motor’s wattage comes out low while its actual current draw, the thing that heats wires and trips breakers, stays high.
| Load type | Typical power factor | Note |
|---|---|---|
| Resistive heating, incandescent lamps | 1.0 | All current becomes heat or light |
| Large induction motors, loaded | 0.85 to 0.95 | Check the nameplate first |
| Small induction motors | 0.7 to 0.8 | Lightly loaded motors run lower |
| Electronic fluorescent ballasts / LED drivers | 0.9 to 0.99 | Magnetic ballasts: 0.7 to 0.9 |
| Computers and consumer electronics | 0.6 to 0.8 | Better power supplies sit higher |
Two rules of thumb cover most visits. If the load makes heat on purpose, enter 1.0 and stop worrying. If it spins, pumps or cools, use the nameplate PF; absent a nameplate, 0.85 (the pre-filled value) is a fair planning figure for a working motor, and 0.8 is the conservative choice. Never enter a PF above 1: the calculator rejects it, because the number cannot exist. The loads readers size most often with this tool are HVAC units, and the AC tonnage calculator is the companion step for working out what size of unit a space needs before you price its circuit.
Line-to-line vs line-to-neutral, with the numbers side by side
The same physical machine can be described with either voltage, and both calculator paths must land on the same watts. Take a 10 A three-phase load at PF 0.9. Quoted line-to-line at 480 V: 1.732 × 10 × 480 × 0.9 = 7,482.46 W. Quoted line-to-neutral at 277 V: 3 × 10 × 277 × 0.9 = 7,479.00 W. The 3.46-watt gap (about 0.05%) is not a formula error: 480 and 277 are rounded nominal partners (480 ÷ √3 = 277.1), so the two descriptions were never perfectly consistent to begin with. On a real nameplate, use the voltage printed there with its matching formula and the answer is the machine’s answer.
What competing calculators miss
The top-ranking amps-to-watts tools are accurate within their lane. The lane is the problem.
Winning move 1: all three directions. Every major competitor converts amps to watts only. The reverse questions are at least as common in practice: how many amps does this heater draw, what voltage does this power and current imply? Here they are one menu change away, with the labels re-drawing themselves so you always know which value you are entering.
Winning move 2: both three-phase conventions, declared. Most tools print a single three-phase formula and never say whether their volts are line-to-line or line-to-neutral. Guess wrong and the answer is off by a factor of 1.73 in one direction or the other. This calculator makes the convention an explicit input and prints it inside the formula string of your result.
Winning move 3: the working, shown and validated. Each result carries its formula and substitution line, so a student, an apprentice or a reviewer can verify the arithmetic without re-deriving it. Inputs are validated the way the physics demands: zero and empty values get plain errors, and a power factor above 1 is rejected instead of silently producing power from nowhere.
Frequently asked questions
Multiply amps by volts for DC: W = A x V. For single-phase AC, multiply by the power factor as well: W = A x V x PF. For three-phase, use W = 1.732 x A x V x PF with line-to-line voltage, or W = 3 x A x V x PF with line-to-neutral voltage. This calculator runs all four versions and prints the formula and substitution with the result.
It depends entirely on the voltage, because watts equal amps times volts. At 12 V, 1 amp is 12 W. At 120 V it is 120 W, and at 240 V it is 240 W, before any power factor adjustment on AC. There is no fixed wattage for an amp, which is why the calculator always asks for volts as well.
1,200 W for DC or for a resistive AC load with a power factor of 1.0 (10 x 120). For a motor load at a power factor of 0.8, the same 10 amps at 120 V delivers 960 W of real power (10 x 120 x 0.8). The current is identical; the useful power is not, which is the whole point of the power factor input.
Rearrange the same formulas: A = W / V for DC, A = W / (V x PF) for single-phase AC, and A = W / (1.732 x V x PF) for three-phase line-to-line. In this calculator, set Solve for to Amps. Example: an 1,800 W heater on 120 V single-phase at PF 1.0 draws 1,800 / 120 = 15.00 A.
Power factor is the ratio of real power (watts, doing work) to apparent power (volt-amps). It runs from 0 to 1. Resistive loads such as heaters and incandescent lamps sit at 1.0. Loaded induction motors run about 0.85 to 0.95, small motors 0.7 to 0.8, and many electronic loads 0.6 to 0.8. If a nameplate lists PF, use it. For a mixed motor load with no data, 0.8 is a reasonable planning figure; for anything that makes heat, use 1.0.
Line-to-line voltage is measured between two hot conductors (208 V or 480 V in North American commercial systems). Line-to-neutral is measured from one hot conductor to neutral (120 V or 277 V in the same systems). Use the 1.732 formula with line-to-line values and the 3-times formula with line-to-neutral values. Both describe the same total power: 10 A at 480 V line-to-line with PF 0.9 is 7,482.46 W, and at 277 V line-to-neutral it is 7,479.00 W, the tiny gap coming from the rounded nominal voltage pair.
On a 120 V circuit with a power factor of 1.0, 1,500 / 120 = 12.5 A. That is most of a standard 15 A branch circuit (1,800 W ceiling, and 80% of that, 1,440 W, is the usual continuous-load planning limit). Run the heater and a second large load on the same circuit and tripping the breaker is a matter of time.
No. A power factor of 1.0 means every volt-amp becomes a watt of real power, which only a purely resistive load achieves. Anything above 1 is a data-entry or measurement error, and this calculator rejects it on AC modes with a plain error message rather than returning a fantasy number.
No. Watts measure power right now; watt-hours measure energy over time. A 100 W bulb running for 10 hours consumes 1,000 Wh, which is 1 kWh, the unit on your electricity bill. This calculator works in watts, amps and volts only. Multiply the watts result by hours and divide by 1,000 to get kilowatt-hours.
Use it for load planning, not for final sizing. Breaker and wire decisions also depend on continuous-load rules (the NEC’s 80% convention for loads running 3 hours or more), wire gauge and material, ambient temperature, conduit fill and voltage drop, plus your local code. Do the conversion here, then have a licensed electrician confirm the installation.
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References
- AmpsToWatt.com, “Amps to Watts Calculator” – states the DC, single-phase and both three-phase formulas used here, with the NEC 80% continuous-load caution for breaker planning. AmpsToWatt: Amps to Watts Calculator. Accessed October 2026.
- Portlandia Electric Supply, “How to Convert Amps to Watts: Formula & Calculator” – journeyman-level treatment of the three formula families, power factor discipline and the cost of using the wrong one. Portlandia Electric: Convert Amps to Watts. Accessed October 2026.
- EcoFlow, “Amps to Watts Conversion Made Simple” – clear line-to-line versus line-to-neutral three-phase teaching with 400 V worked examples. EcoFlow: Amps to Watts Conversion. Accessed October 2026.
- EasyUnitConverter, “Amps to Watts Calculator” – DC conversion reference table (amps x volts) used to spot-check this calculator’s DC outputs. EasyUnitConverter: Amps to Watts. Accessed October 2026.
- GraphCalc, “Amps to Watts Calculator” – single and three-phase formulas with power factor ranges and the same standard worked examples. GraphCalc: Amps to Watts Calculator. Accessed October 2026.
- Sage Calculator, “Amp To Watt Calculator” – explains the power law relationship between amps, volts and watts for DC and AC systems. Sage Calculator: Amp To Watt Calculator. Accessed October 2026.
- CalculatorsConversion, “Amps to Watts Calculator: Free Converter, Formula & Table” – inverse amps formula and household reference values such as 15 A at 120 V equalling 1,800 W. CalculatorsConversion: Amps to Watts. Accessed October 2026.