Watts Amps Volts Calculator
Convert between watts, amps and volts on a DC or resistive circuit, and get the resistance in ohms, from any voltage and current.
How it works
P = V × I · R = V ÷ I · (P in watts, V in volts, I in amps, R in ohms)
For a DC circuit or a purely resistive AC load, power is the product of voltage and current, and Ohm’s law ties the same two quantities to resistance. Fix any two and the others follow. On reactive AC loads — motors, transformers — real power also depends on the power factor, so use the three-phase or power-factor tools there; this covers the resistive case that heaters, lamps and battery circuits obey.
Worked example
A 120 V circuit drawing 5 A: power = 120 × 5 = 600 W (0.6 kW). Its effective resistance is 120 ÷ 5 = 24 Ω. Double the current to 10 A and the power doubles to 1,200 W while the resistance halves to 12 Ω.
Frequently asked questions
Does this work for AC appliances?
It is exact for resistive loads — incandescent lamps, heating elements, kettles — where voltage and current stay in phase. For motors, fluorescent ballasts and electronics you must include power factor, which the 3-phase and kVA tools handle.
How do I get amps from watts?
Rearrange P = V × I to I = P ÷ V. A 600 W resistive load at 120 V draws 600 ÷ 120 = 5 A. This calculator does the same starting from volts and amps.
What is the resistance actually telling me?
It is the load’s effective opposition to current at that operating point (V ÷ I). For a heater it is fairly constant; for a lamp it rises sharply once the filament is hot, so cold-start resistance is much lower.
Related calculators
Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.