Ohm's Law Calculator
Enter any two known values — voltage, current, resistance or power — and the remaining two are calculated instantly. Handles all formula paths: V=IR, P=VI, P=I²R and V²/R.
Last updated: May 2026
Enter any two values above to calculate the remaining two.
Handles all formula combinations: V=IR · P=VI · P=I²R · P=V²/R
Ohm's Law — all four formula paths
Most calculators only solve for one variable at a time and require you to pick the formula manually. This tool takes any two known values and calculates both remaining unknowns in one step. Particularly useful when the only known pair is resistance and power (P=I²R path) or voltage and power (V²/R path) — formula paths that are easy to miscalculate under time pressure.
Formula reference
| Known | Solve for I | Solve for V | Solve for R | Solve for P |
|---|---|---|---|---|
| V + I | — | — | V ÷ I | V × I |
| V + R | V ÷ R | — | — | V² ÷ R |
| V + P | P ÷ V | — | V² ÷ P | — |
| I + R | — | I × R | — | I² × R |
| I + P | — | P ÷ I | P ÷ I² | — |
| R + P | √(P ÷ R) | √(P × R) | — | — |
Typical use cases
- Checking a current-limiting resistor before wiring an LED circuit
- Verifying a resistor's power rating before soldering (P = I²R)
- Calculating load current from a known supply voltage and impedance
- Reverse-engineering component values from measured bench readings
Frequently Asked Questions
What happens if I enter all four values?
The calculator uses the first valid pair it finds in priority order: V+I, then V+R, then V+P, then I+R, then I+P, then R+P. To get a specific formula path, clear the fields you don't want used as inputs. For most bench checks, entering exactly two values gives the clearest result.
How do I convert mA to A before entering the value?
Divide milliamps by 1000. A 20 mA LED becomes 0.02 A. The quick-value buttons handle common mA values for you — click "20 mA" and 0.02 is entered automatically. For microamps (µA), divide by 1,000,000 (e.g., 500 µA = 0.0005 A).
Why does power dissipation matter when choosing a resistor?
Standard 1/4 W resistors fail if they dissipate more than 0.25 W continuously. A 220 Ω resistor on a 12 V supply dissipates 0.655 W — that needs a 1 W or larger rated component. Use P = V²/R or P = I²R to check before ordering. Running a resistor above its rated wattage causes drift, overheating or permanent failure.
Does Ohm's Law apply to AC circuits?
For resistive loads (heaters, incandescent bulbs, resistors) yes — apply the same formulas with RMS voltage and current. For reactive components (capacitors, inductors, motors) the relationship involves impedance (Z) rather than pure resistance (R), and the formulas become V = I × Z. For AC power calculations including power factor, use the Watts / Volts / Amps Calculator.
How do I check if a resistor value is in the E12 or E24 series?
Common E12 values (10% tolerance): 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 Ω (and ×10 multiples). E24 adds: 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91. If your calculation gives 185 Ω, the nearest E12 is 180 Ω or 220 Ω. Pick the one that keeps current within safe limits — typically the higher value for LED circuits.