Watts to Amps to Volts Calculator (DC and AC)
Solve for current, power or voltage in a DC, single-phase AC or three-phase AC circuit from the other two, with the power factor applied and power entered in watts, kilowatts or horsepower. Shows kW, hp and apparent power alongside the answer.
Last updated: October 2026
Current
12.5 A
Formula below · 2 sources (NIST, engineeringtoolbox.com) · Updated Oct 2026
Compare with similar
About this calculator
Electrical power is voltage times current. For DC, P = V × I exactly, so I = P ÷ V and V = P ÷ I. For AC the voltage and current are sine waves that may not peak together, and only the in-phase part delivers real power: P = V × I × PF, where the power factor PF (0 to 1) is the cosine of the angle between them. Resistive loads such as heaters, kettles and incandescent lamps have PF = 1; motors, transformers and older fluorescent ballasts draw extra reactive current and sit around 0.8-0.9; the product V × I without the power factor is the apparent power in volt-amperes (VA), which is what wiring and transformers must carry.
In a balanced three-phase system with the line-to-line voltage V (for example 400 V or 480 V), P = √3 × V × I × PF, where I is the line current. The √3 (1.732) comes from the 120° spacing of the phases. If you have the line-to-neutral voltage instead (230 V or 277 V), the formula is 3 × Vₗₙ × I × PF, which gives the same result.
The calculator rearranges whichever form matches your circuit for the quantity you choose: I = P ÷ (k × V), V = P ÷ (k × I) or P = k × V × I, with k = 1 for DC, PF for single-phase and √3 × PF for three-phase. Power may be entered in watts, kilowatts (× 1,000) or horsepower (× 746 W, the electric horsepower used for motor ratings). Note that a motor nameplate horsepower is mechanical output; the electrical input is higher by the motor's efficiency, so divide by the efficiency (typically 0.85-0.95) for the true input power before converting to amps, or use the motor full-load current calculator.
The tiles always show all three quantities plus kW, hp and apparent power, so you can read the answer in whichever unit the breaker, cable or generator is rated in.
How to use
Example (the defaults): a 1,500 W space heater on a 120 V single-phase circuit, power factor 1. I = 1,500 ÷ (120 × 1) = 12.5 A. That is most of a 15 A circuit, and over the 12 A limit for a continuous load on one, which is why 1,500 W is the usual ceiling for plug-in heaters in North America. Three-phase: a 10 kW load at 480 V line-to-line with power factor 0.85. Choose three-phase, enter 10 with kW, 480 V and 0.85. I = 10,000 ÷ (1.732 × 480 × 0.85) = 14.15 A. The apparent power tile shows 11,765 VA. Horsepower: 5 hp at 230 V single-phase, PF 0.85 (nameplate output, ignoring motor losses). 5 × 746 = 3,730 W; I = 3,730 ÷ (230 × 0.85) = 19.08 A. DC: a 100 W load on a 12 V battery draws 100 ÷ 12 = 8.33 A, so a 100 Ah battery runs it for roughly 12 hours.
Frequently asked questions
How do I convert watts to amps?
Divide the watts by the volts, and for AC also by the power factor: I = P ÷ (V × PF). A 1,200 W microwave on 120 V with PF 1 draws 10 A; a 2,000 W kettle on 230 V draws 8.7 A. For three-phase, divide by √3 as well: I = P ÷ (1.732 × Vₗₗ × PF). Choose Current in the calculator and enter the power, voltage and power factor.
How many watts is one amp?
It depends on the voltage: watts = volts × amps. One amp is 12 W at 12 V, 120 W at 120 V and 230 W at 230 V (for a resistive load). The same current carries more power at a higher voltage, which is why transmission lines run at thousands of volts and why 240 V appliances can be more powerful on the same wire size.
What power factor should I use?
Use 1 for purely resistive loads: heaters, incandescent bulbs, kettles, toasters. Use the nameplate value for anything else; typical figures are 0.8-0.9 for induction motors at full load (lower at light load), 0.9-0.99 for modern electronics with power factor correction, and 0.5-0.7 for small unloaded motors and older magnetic ballasts. DC circuits have no power factor, and the calculator ignores the field when DC is selected.
Why does the three-phase formula have √3 in it?
In a balanced three-phase system the three line-to-line voltages are 120° apart, and the line-to-line voltage is √3 times the line-to-neutral voltage. Total power is 3 × Vₗₙ × I × PF; substituting Vₗₙ = Vₗₗ ÷ √3 gives √3 × Vₗₗ × I × PF. The calculator expects the line-to-line voltage (208, 400, 480 V); if you only know the line-to-neutral value, multiply it by 1.732 first.
Can I use this to size a breaker or wire?
It gives you the load current, which is the starting point, but not the answer. Codes such as the NEC require continuous loads to be sized at 125% of the load current, apply derating for temperature and bundled conductors, and set voltage-drop limits for long runs; motors have their own tables based on full-load current rather than nameplate watts. Take the amps from here to the wire size, voltage drop and circuit breaker sizing calculators, and have a licensed electrician confirm the installation.
Is horsepower 746 W or 735.5 W?
Both exist. The electric horsepower used for motor ratings in North America is exactly 746 W, and the mechanical horsepower is 745.7 W; the metric horsepower (PS, CV) used in Europe is 735.5 W. This calculator uses 746 W. The difference is 1.4%, smaller than the uncertainty in a typical power factor, but choose the figure that matches your nameplate if it matters.
Related calculators
Sources & references
Think a result is wrong? Report a problem with this calculator.