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Ohm's Law Calculator

Enter any 2 of V, I, R to calculate all 4 values including power (P).

Input (any 2)
ℹ Enter any 2 values to calculate the third.
Results
Voltage (V)—
Current (I)—
Resistance (R)—
Power (P = V×I)—
💡 Usage & Formula

Ohm's Law defines the relationship between Voltage, Current, Resistance, and Power in electrical circuits.

Formulas:

  • Voltage: V = I * R
  • Current: I = V / R
  • Resistance: R = V / I
  • Power: P = V * I = I^2 * R = V^2 / R

Usage: Enter any two parameters to automatically calculate the remaining two parameters.

When you need it: The everyday sanity check — finding the third of voltage, current or resistance, or the power a part dissipates, before you pick a resistor wattage or a wire gauge.

Worked example: 5 V across 220 Ω → I = 5 / 220 = 22.7 mA and P = V² / R = 25 / 220 = 114 mW, so a 1/4 W resistor has comfortable margin.

Tips & gotchas:

  • Power has three equivalent forms — P = VI = I²R = V²/R — use whichever two quantities you already know.
  • Derate resistor power to about 50% of its rating for temperature and reliability margin.
  • Ohm's law applies to linear resistors, not diodes or LEDs — those follow their own V–I curve.
  • Copper and most resistors drift with temperature, so hot resistance differs from the datasheet's 25 °C value.
📖 References: Ohm's law (Wikipedia)