Ohm's Law Calculator
Solve for voltage, current, resistance, or power instantly. Enter any two — get the other two.
Enter any two values to derive the other two.
Ohm's Law: V = I × R. Power: P = V × I = I²·R = V²/R.
Given any two of {V, I, R, P}, the other two are determined. This calculator handles all six input combinations. Values in SI units: volts, amperes, ohms, watts.
How to calculate watts from volts
Power (W) = Voltage (V) × Current (A). If you know voltage and resistance instead: P = V²/R. If you know current and resistance: P = I²·R.
How to calculate current
Current (A) = Voltage (V) / Resistance (Ω). At 12 V through 100 Ω, current is 0.12 A (120 mA). To calculate current from power: I = P/V or I = √(P/R).
Frequently asked
- How many ohms in a volt?
- Volts and ohms are different units and don't convert directly. Voltage divided by resistance gives current (I = V/R).
- How to calculate watts from volts?
- Power (W) = V × I. If you only have voltage and resistance, use P = V²/R.
- How to calculate current?
- Current (A) = Voltage / Resistance, or Power / Voltage, or √(Power / Resistance).
- What is Ohm's Law used for?
- Sizing resistors, choosing wire gauge, calculating LED current-limiting, checking safe operating limits — any DC circuit analysis.
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