Ohm's law calculator
V = R·I and P = V·I: any two values and the tool works out the other two, with proper prefixes (mA, kΩ, mW).
Fill any two boxes (e.g. voltage and resistance) and leave the others empty: the tool works out the rest.
The 12 Ohm's law formulas
| Find | Formulas |
|---|---|
| V (volt) | V = R·I · V = P/I · V = √(P·R) |
| I (ampere) | I = V/R · I = P/V · I = √(P/R) |
| R (ohm) | R = V/I · R = V²/P · R = P/I² |
| P (watt) | P = V·I · P = V²/R · P = R·I² |
Ohm's law
In a conductor, voltage is proportional to current: V = R·I, where R is resistance in ohms. Adding power (P = V·I) yields the 12 combinations of the classic formula wheel: any two quantities always give you the other two.
Practical examples
An LED wanting 20 mA from a 9 V battery with a 2 V LED drop needs a (9−2)/0.02 = 350 Ω resistor. A 2000 W heater at 230 V draws 2000/230 = 8.7 A. An 8 Ω speaker fed 20 W sees √(20·8) = 12.6 V.
Careful
These formulas hold for DC and resistive loads (resistors, filament bulbs, heaters). Motors, power supplies and reactive AC loads involve power factor and the maths changes. This is a hobby/teaching tool: mains wiring is a job for an electrician.