Ohms Law Calculator
Solve for voltage, current, resistance or power using Ohm's law from any two known values, free right in your browser.
Enter Values
Minimum 0
Before you rely on this: Guidance only — not a substitute for regulated design or certification. Verify against the governing standards and a qualified, licensed professional.
How to use this calculator
- Enter any two of voltage, current or resistance.
- Press Calculate for the remaining values and the power.
How it works
Ohm's law links voltage (V, volts), current (I, amps) and resistance (R, ohms): V = I × R. Rearranged, I = V ÷ R and R = V ÷ I.
Electrical power is P = V × I (watts), so once any two of V, I and R are known, the third value and the power all follow.
Worked example
12 V and 2 A. R = 12 ÷ 2 = 6 Ω, and power = 12 × 2 = 24 W.
Common mistakes
- Mixing units — use volts, amps and ohms (not millivolts or milliamps) unless you convert first.
- Entering only one value — Ohm's law needs any two of the three to solve.
- Confusing power (watts) with energy (watt-hours) — this gives instantaneous power.
Frequently asked questions
What units does it use?
Volts (V), amps (A), ohms (Ω) and watts (W). Convert milliamps/millivolts to amps/volts first.
Which two values do I need?
Any two of voltage, current and resistance. The calculator solves the third and the power automatically.
How is power calculated?
Power = voltage × current (P = V × I). You can also use P = I²R or P = V² ÷ R.
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Cite this tool
Zerdly. (2026). Ohms Law Calculator. Retrieved from https://www.zerdly.com/tools/ohms-law-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.