Ohms Law Calculator
Solve voltage, current, resistance and power from any two known values.
About Ohms Law Calculator
Ohm's Law (V = I × R) and the Power Law (P = V × I) together define the four electrical quantities. Any two quantities uniquely determine the other two. The calculator solves all six possible input pairs: V+I, V+R, V+P, I+R, I+P, R+P. Edge cases like zero voltage, zero current or zero resistance are flagged explicitly because they indicate open or short circuits where some quantities are undefined or infinite.
Formulas
Ohm's Law
Voltage = Current × Resistance
- V
- Voltage (volts)
- I
- Current (amperes)
- R
- Resistance (ohms)
Power Law
Power = Voltage × Current = Current² × Resistance = Voltage² ÷ Resistance
- P
- Power (watts)
Worked example
With the default values below, Ohms Law Calculator returns Voltage (V) of 12.0000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- I know…one of Voltage (V) and Current (I), Voltage (V) and Resistance (R), Voltage (V) and Power (P), Current (I) and Resistance (R), Current (I) and Power (P), Resistance (R) and Power (P)
- Voltage (V) and Current (I)
- Voltage (V)minimum 0 V
- 12 V
- Current (I)minimum 0 A
- 3 A
- Resistance (R)minimum 0 Ω
- 4 Ω
- Power (P)minimum 0 W
- 36 W
Results
- Voltage (V)
- 12.0000 V
- Current (I)
- 3.0000 A
- Resistance (R)
- 4.0000 Ω
- Power (P)
- 36.0000 W
Step by step
- 1Known values= V = 12.0000 V
- 2Ohm's Law (V = I × R)
- 3Power Law (P = V × I)
- 4Check: P = I² × R= 36.0000 W
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the voltage across a resistor is directly proportional to the current flowing through it, with the resistance as the constant of proportionality: V = I × R.
Why are there six input combinations?
There are four variables (V, I, R, P). Choosing any two gives a unique solution for the other two: C(4,2) = 6 combinations. This calculator handles all six.
What does it mean when resistance is zero?
Zero resistance is an ideal short circuit. Ohm's Law says current would be infinite (V ÷ 0), which is a real circuit fault. The calculator flags this rather than returning Infinity.
Is this valid for AC circuits?
For pure resistive loads, yes. AC circuits with capacitors or inductors have impedance (Z), not just resistance, and require phasor math. This calculator covers the resistive case.