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Free online Ohm's Law calculator

Ohm's Law states that voltage equals current times resistance (V = I × R). This calculator solves for any two unknowns given any two knowns — including power (P = V × I).

Fill in any two values — the other two will be calculated.

About this calculator

How to use

  1. Enter any two of the four values — voltage (V), current (I), resistance (R), or power (P).
  2. Leave the other two fields blank.
  3. Click Calculate to see all four values and the formula used.
  4. Click Clear to reset.

Formulas

Ohm’s Law:

V = I × R

Power (Joule’s Law):

P = V × I = I² × R = V² / R

All combinations:

GivenSolve for VSolve for ISolve for RSolve for P
V, I——V / IV × I
V, R—V / R—V² / R
V, P—P / VV² / P—
I, RI × R——I² × R
I, PP / I—P / I²—
R, P√(P × R)√(P / R)——

Worked example

Inputs: V = 12 V, R = 100 Ω

I = V / R = 12 / 100 = 0.12 A
P = V² / R = 144 / 100 = 1.44 W

Inputs: I = 2 A, P = 24 W

V = P / I = 24 / 2 = 12 V
R = P / I² = 24 / 4 = 6 Ω

Frequently asked

What is Ohm's Law?
Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I × R. It was formulated by Georg Simon Ohm in 1827.
How is power included?
Power (P) relates to voltage and current by Joule's Law: P = V × I. Combined with Ohm's Law, this gives P = I² × R and P = V² / R. The calculator uses these three relationships to solve for all four quantities from any two known values.
What units are used?
Voltage is in volts (V), current in amps (A), resistance in ohms (Ω), and power in watts (W). All standard SI units.
Why does the calculator return no result sometimes?
If the two known values imply a division by zero (for example, voltage = 0 with power given, or resistance = 0 with voltage given), the result is mathematically undefined and the calculator returns an error.
What is a typical example?
A 12 V car battery connected to a 100 Ω resistor: I = 12 / 100 = 0.12 A, P = 12² / 100 = 1.44 W.
How do I share my calculation?
Click Share with my numbers to copy a URL with your two input values pre-filled.