The formulas
Ohm's law ties voltage, current and resistance together. Combined with the power equation, any two quantities give you the other two.
V = I × R P = V × I
I = V ÷ R P = I² × R
R = V ÷ I P = V² ÷ R
I = √(P ÷ R) V = √(P × R)
Examples
- A 1,500 W heater on 120 V draws 1,500 ÷ 120 = 12.5 A and has a resistance of 120² ÷ 1,500 = 9.6 Ω.
- A 4.7 kΩ resistor across 24 V passes 24 ÷ 4,700 = 5.1 mA and dissipates 24² ÷ 4,700 = 0.12 W, so a 1/4 W part is fine.
- A 100 W, 8 Ω speaker load sees √(100 × 8) = 28.3 V RMS at full power.
AC circuits
These relationships apply directly to DC and to purely resistive AC loads such as heaters and incandescent lamps. For motors, transformers and electronics, use impedance instead of resistance and account for power factor. The electrical power calculator handles kW, kVA and power factor for single- and three-phase systems.
Frequently asked questions
- How many amps is 1000 watts?
- At 120 V, 1000 ÷ 120 = 8.3 A. At 240 V, 4.2 A. This assumes a resistive load; motors draw more because of power factor and efficiency.
- Why do I have to enter exactly two values?
- Two independent quantities fully define the circuit. A third would either be redundant or contradict the first two.
Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.