Ohm's Law Calculator — voltage and power from current and resistance

How it works

Ohm's law describes how voltage, current and resistance relate in a simple circuit. The current through most conductors is directly proportional to the voltage across them, which rearranges to:

V = I × R

where V is the voltage (volts), I is the current (amperes) and R is the resistance (ohms). This calculator solves for voltage given the current and resistance, and also reports the power the resistor dissipates:

P = I² × R

which is the same as P = VI (power equals voltage times current). Enter amperes and ohms and you get volts and watts.

The law works in any direction. If you know two of voltage, current and resistance, the third follows: V = IR, or I = V/R, or R = V/I. The calculator uses the V = IR direction; the FAQ shows how to get current or resistance instead.

Worked example

OpenStax's headlight example draws 2.50 A and has a resistance of 4.80 Ω, so the voltage across it is 2.5 × 4.8 = 12 V — a car's electrical system. The power it turns into light and heat is I²R = 2.5² × 4.8 = 30 W. The presets also cover a tiny LED (0.02 A, 100 Ω → 2 V, 0.04 W) and a heating element (12.5 A, 9.6 Ω → 120 V, 1500 W).

Frequently asked questions

What is Ohm's law?

Ohm's law states that the voltage across a resistor equals the current through it times its resistance: V = IR. Equivalently, the current is proportional to the voltage, I = V/R — double the voltage and you double the current through a fixed resistance.

How do I calculate current or resistance instead of voltage?

Rearrange the law. For current, use I = V ÷ R; for resistance, use R = V ÷ I. All three forms — V = IR, I = V/R, R = V/I — are the same relationship solved for a different unknown, so any two of the three values give the third.

What are the units — volts, amps and ohms?

Voltage is measured in volts (V), current in amperes or "amps" (A) and resistance in ohms (Ω). One volt drives one amp through one ohm. Power is in watts (W), where one watt is one joule of energy per second.

How is electrical power calculated?

Power is the rate at which a resistor converts electrical energy to heat or light: P = VI. Substituting Ohm's law gives two handy equivalents, P = I²R and P = V²/R. This calculator reports P = I²R from the current and resistance you enter.

Does Ohm's law apply to every component?

No. It holds well for *ohmic* conductors like metal wires and resistors at a steady temperature, where current stays proportional to voltage. Components such as diodes, transistors and filament bulbs are non-ohmic — their resistance changes with voltage or temperature — so V = IR is only an approximation for them.

Why is the power zero when resistance is zero?

With zero resistance there is no voltage drop (V = I × 0 = 0) and no energy is dissipated (P = I² × 0 = 0) — an ideal short circuit. Real wires have a small resistance, so a genuine short still dissipates a little power and can draw a large current.