Ohm's Law & Power Calculator
Lock any two known values (V, I, R, P) — the other two are solved instantly.
Enter any two known values and click their label to lock them. The other two are solved automatically.
Select 2 more value(s) to lock by clicking the label.
Formula
V = I × R P = V × I
Any two of voltage, current, resistance and power fix the other two.
The most useful equation in electronics
Ohm's law links voltage (V), current (I) and resistance (R); combined with the power law P = V·I it forms the wheel shown above. Lock any two quantities and the calculator derives the rest.
Locked values glow blue, solved values glow green — a quick visual of which relationship is doing the work.
Worked example
- Driving a 20 mA LED that drops 2 V from 5 V: the resistor sees 3 V at 0.02 A → R = 150 Ω.
- That resistor dissipates P = V·I = 3 × 0.02 = 60 mW, comfortably inside a ¼ W part.
- Double the current and power quadruples (P = I²·R) — why heat sneaks up on you.
Where it's used
Sizing resistors, checking whether a trace or component can handle the current, estimating battery drain, and almost every other back-of-envelope calculation in a build.