The Ohm's Law calculator solves for any two of the four electrical quantities — Voltage (V), Current (I), Resistance (R), and Power (P) — from any two known values. Covers V=IR and all power relationships: P=VI=I²R=V²/R.
Enter Any Two Values
Fill in two fields and leave the others blank. The calculator will compute all remaining values.
Formulas Used
Ohm's Law Formula Triangle
How to Use the Ohm's Law Calculator
Ohm's Law (V = IR) connects voltage, current, and resistance in electrical circuits. Combined with the power equation P = VI, any two of the four quantities uniquely determine the other two. This calculator accepts any combination of two known values.
Step 1: Enter Two Values
Fill in exactly two of the four input fields: Voltage (V), Current (I), Resistance (R), or Power (P). Leave the other two blank. All six combinations of two knowns work.
Step 2: Click Calculate
The calculator determines which pair you entered and applies the appropriate formulas. For example, if you enter V=12V and R=24Ω: I = V/R = 0.5A, P = V²/R = 6W.
Practical Examples
LED circuit: 3.3V LED with 470Ω current-limiting resistor. Enter V=3.3 and R=470 → I = 7mA, P = 0.023W. Perfect for battery-powered devices.
Heater sizing: A 1500W heater on 120V: I = P/V = 12.5A. This exceeds a 10A breaker, requiring a 15A or 20A circuit.
Wire sizing: 100W bulb on 120V: I = 0.833A, R = 144Ω. Very low current — thin wire is fine.
FAQ
What is Ohm's Law?
Ohm's Law states that voltage equals current times resistance: V = I × R. This fundamental relationship connects three key electrical quantities. Combined with the power formula P = V × I, you can calculate any of four quantities if you know any two.
What are all the power formulas?
Power can be calculated as P = V × I (voltage × current), P = I² × R (current squared × resistance), or P = V²/R (voltage squared divided by resistance). Choose the formula based on which two quantities you know.
What units are used in Ohm's Law?
Voltage is in Volts (V), current is in Amperes or Amps (A), resistance is in Ohms (Ω), and power is in Watts (W). For small signals, you might use millivolts (mV), milliamps (mA), kilohms (kΩ), or milliwatts (mW).
Is this calculator free?
Yes, completely free with no signup required. All calculations run in your browser.
How do I use this with a real circuit?
For a simple circuit: if you have a 9V battery and a 470Ω resistor, enter V=9 and R=470 to find I=0.0191A (about 19.1mA) and P=0.172W. This tells you the current draw and heat dissipated in the resistor.