How the number is made
How to use this calculator
Type the watts and the amps. Choose DC, single-phase AC, or three-phase AC. For AC, set the power factor. The volts appear in the result card, with the steps beside it. Submit the form if JavaScript is off. The same formulas run on the server, so the first number is already on the page.
This page checks the voltage implied by compatible power and current values for one operating condition. It does not choose a suitable supply, determine a wall-outlet voltage or authorize changing equipment connections. Use the documented circuit and equipment conditions. If the calculated voltage is outside 1 to 1000 volts, the page shows the default example with a warning rather than presenting it as your result.
For AC, enter real input power, RMS current and power factor for the same operating condition. Three-phase results are line-to-line RMS values for a balanced load. Power factor is real power divided by apparent power, not efficiency. This is an algebraic check, not a recommended supply voltage.
The formulas
DC: V = P / I
Single-phase AC: V = P / (I × PF)
Three-phase AC: V = P / (I × √3 × PF)
P is watts, V is volts, I is amps, and PF is power factor. The DOE Fundamentals Handbook gives P = V × I for DC and P = VRMS × IRMS × PF for single-phase AC. Solving for voltage gives the first two formulas above. For balanced three-phase wye or delta loads, DOE Volume 3 gives P = √3 × VLL × Iline × PF; solving for voltage gives line-to-line RMS voltage. Unbalanced loads are outside this model.
DC
Direct current has no power factor. The volts are watts divided by amps. A 1500 watt load at 12.5 amps is 120 volts on DC. The same watts at a higher current would be a lower voltage. That is why a low-voltage run that still carries a lot of watts also carries a lot of amps. The DC choice ignores whatever power factor is typed in the form, and the steps say so.
If the division would land outside 1 to 1000 volts, the engine refuses the case. A huge watt figure at a tiny current is not a household circuit. The warning on this page is that refusal, not a rounded guess.
Single-phase AC
At a power factor of 1, single-phase volts match the DC formula. At 0.8, the computed voltage is higher for fixed watts and amps because the denominator is smaller. That does not make the result an appropriate supply voltage. If it differs from the documented supply, check that the inputs describe the same operating condition.
A useful check is the other way around. If a load lists 1800 watts and you measure 15 amps on a 120 volt circuit at a power factor of 1, this page should return 120 volts. If it does not, the watts on the label are not the watts in the circuit, or the meter is not reading the same current you typed.
Three-phase
For the same watts, amps and power factor, the balanced three-phase formula gives a lower numerical voltage than the single-phase formula because its denominator includes the square root of 3. The result is line to line. Do not reinterpret it as phase-to-neutral voltage; a line-to-neutral conversion needs the actual system connection and conditions.
Type the current of one conductor, not the sum of three. The three-phase formula already accounts for the three legs through the square-root factor. Adding the three clamp readings and then pasting the sum will drive the voltage far too low.
Worked example
Type 1800 watts, 15 amps, single-phase, power factor 1. The engine returns 120 volts. The step is 1800 divided by 15. Nothing else is added. Change the power factor to 0.8 and the volts rise, because the formula divides by 0.8 as well. Change the circuit to three-phase if that is the system you have. The reference table below is the same engine: DC volts at 10 amps, and single-phase volts at 12.5 amps and power factor 1. It is not a list of supply voltages you should force a circuit to use.
What this page does not do
This calculation does not identify a panel supply, convert one electrical system to another, or select installation equipment. Use watts to amps for the inverse current calculation with consistent inputs. Shareable calculations include your values in the page address, which may appear in browser history and server logs. Do not enter confidential information. Search engines should use the clean calculator address.
Volts at 10 A DC and 12.5 A AC
The 10 A column is DC. The 12.5 A column is single-phase at power factor 1. Every cell is computed by the same function as the result card.
| Watts | Volts at 10 A DC | Volts at 12.5 A AC |
|---|---|---|
| 60 | 6 | 4.8 |
| 100 | 10 | 8 |
| 240 | 24 | 19.2 |
| 500 | 50 | 40 |
| 1000 | 100 | 80 |
| 1500 | 150 | 120 |
| 1800 | 180 | 144 |
| 2400 | 240 | 192 |
Common questions
How do I convert watts to volts?
Divide watts by amps for DC, or for single-phase AC at a power factor of 1. If the power factor is lower, divide by amps times that power factor. Three-phase also divides by the square root of 3, and the answer is line to line.
Why did the page refuse my numbers?
The engine only returns voltages from 1 to 1000. Watts that are huge next to a tiny current, or tiny next to a huge current, fall outside that range. Check the units. Kilowatts typed as watts, or milliamps typed as amps, are the usual cause.
Does power factor change the volts?
For fixed real power and current, a lower power factor gives a higher calculated RMS voltage within the selected AC model. This algebraic relationship does not select an appropriate supply. DC does not use power factor.
Is a three-phase result hot to hot?
Yes. The three-phase formula returns line-to-line RMS voltage for a balanced load. It is not a hot-to-neutral result on a wye system; this calculator does not convert between those values.
Where does the current check come from?
From watts to amps . The next-step button carries the watts, the new volts, the circuit, and the power factor. That is the power-chain step after a voltage has been found.
How to read a shared result
A shared address preserves watts, amps, circuit and power factor. The server calculates those inputs even without JavaScript; enhanced calculations update the same result in place. The clean canonical address identifies the calculator and opens its default example, not your particular scenario.
Results are rounded to four decimal places. An illustrative 1800 watts at 15 amps gives 120 volts for DC or single-phase AC at power factor 1. The table uses the same rounding. This is an arithmetic reference, not a precision instrument or a circuit-planning method.
Keep the assumptions with the number. A voltage without the watts, the amps, the circuit, and the power factor cannot be checked later. The card lists those four items above the steps so a printed page still shows them. The print button hides the menu and the form. The steps stay.
What a voltage result is not
It is not permission to reconnect a load to a different system. A heater built for 240 V is not a 120 V heater even if the watts and a new current could be made to divide to 120. The element, the plug, and the listing stay with the original voltage. Use this page to check arithmetic, not to redesign listed equipment.
It is also not a voltage-drop result. This page has no run length, no wire metal, and no circular mils. If the question is how many volts you lose along a feeder, gather the full installation details for a qualified review; the voltage-drop review guide does not calculate that result.
Sources
- DOE Electrical Science Volume 4: DC power identity (formula-only use). Accessed 2026-10-26.
- DOE Electrical Science Volume 3: sinusoidal RMS AC power (formula-only use). Accessed 2026-10-26.
- DOE Electrical Science Volume 3: balanced three-phase line-value power (formula-only use). Accessed 2026-10-26.