ElectroHub

Ohm's Law Calculator

Ohm's law ties the three quantities that describe any resistive circuit. Give the calculator any two and it returns the missing one — and, because it is nearly always the next thing you need, the power being turned into heat.

Ohm's Law — V = I × R

Enter any two of voltage, current or resistance. Leave the third blank to solve for it. Power (P = V × I) is computed automatically.

Enter at least two values to compute the third.

Common trap: Current is not used up as it flows around a circuit — the same current that leaves the battery returns to it. What a resistor drops is voltage, and what the circuit converts is energy.

The formula

V = I × R

V
voltage across the element (volts)
I
current through it (amperes)
R
resistance (ohms)

Worked example

12 V across a 250 Ω resistor. Find the current and the power.

  1. Rearrange for current: I = V / R
  2. I = 12 / 250 = 0.048 A = 48 mA
  3. Power: P = V × I = 12 × 0.048

I = 48 mA and the resistor dissipates 0.576 W — so a ¼ W part is not enough.

Where you'll use it

The single most used equation in electronics. Note it only holds for ohmic materials: diodes, LEDs and filament bulbs do not obey it, which is why an LED needs its own resistor calculation.

The laws behind it

Parts this applies to

Resistor (fixed)Potentiometer / Rheostat / TrimmerResistor Array / NetworkThermistor (NTC / PTC)Varistor / MOVSupercapacitorLED (light-emitting diode)MOSFET

Related calculators

Electrical Power CalculatorP = V × I = I² × R = V² / RLED Resistor CalculatorR = (V_supply − V_forward) / I_forwardSeries and Parallel Resistance CalculatorSeries: R = R₁ + R₂ + …Joule Heating CalculatorH = I² × R × t