Ohm's Law Calculator

Fill in any two of voltage, current, resistance and power. The other two are calculated instantly.

Ohm's law formulas

Ohm's law links voltage (V), current (I) and resistance (R): V = I × R. Electrical power (P) is the rate energy is used: P = V × I. Putting the two together gives every formula you need from any two known values.

• Voltage: V = I × R, V = P ÷ I, or V = √(P × R)
• Current: I = V ÷ R, I = P ÷ V, or I = √(P ÷ R)
• Resistance: R = V ÷ I, R = V² ÷ P, or R = P ÷ I²
• Power: P = V × I, P = I² × R, or P = V² ÷ R

Worked examples

Current through a resistor

A 220 Ω resistor is connected across a 12 V supply. The current is I = V ÷ R = 12 ÷ 220 = 0.0545 A, or about 54.5 mA. The power is P = V² ÷ R = 144 ÷ 220 = 0.655 W, so a 1 W resistor is needed; a common 0.25 W resistor would overheat.

Resistance of a heating element

A 1,000 W electric kettle runs on 230 V mains. Its current is I = P ÷ V = 1000 ÷ 230 = 4.35 A, and its element resistance is R = V² ÷ P = 52,900 ÷ 1000 = 52.9 Ω.

Units and prefixes

Voltage is measured in volts, current in amperes (amps), resistance in ohms and power in watts. In electronics small currents are given in milliamps (1 mA = 0.001 A) and large resistances in kilohms (1 kΩ = 1,000 Ω) or megohms (1 MΩ = 1,000,000 Ω). You can type these prefixes directly into the calculator.

Frequently Asked Questions

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