Ohm's Law Calculator
Solve Ohm's law for voltage, current or resistance — V = I R — with the power dissipated.
Any two of volts, amps and ohms give the third — and the watts.
How the ohm's law calculator works
Ohm's law says the current through a resistor is the voltage across it divided by its resistance: I = V ÷ R. Rearranged, V = I R and R = V ÷ I. Choose what to solve for, enter the other two, and the calculator also gives the power, P = V I = I² R = V² ÷ R — the figure that decides whether a resistor gets warm or burns.
It holds for ohmic conductors at constant temperature; filament lamps and diodes are not ohmic, and their "resistance" changes with current.
Formula: V = I R · P = V I
Worked examples
| Inputs | Result | Note |
|---|---|---|
| 12 V across 6 Ω → current | 2 | 2 A, 24 W |
| 0.02 A through 330 Ω → voltage | 6.6 | 6.6 V — an LED resistor |
| 230 V, 8.7 A → resistance | 26.4368 | a 2 kW heater: 26.4 Ω |
FAQFrequently asked questions
What is Ohm's law?
Current is proportional to voltage for a conductor at fixed temperature: V = I R. It is a property of materials, not a law of nature — some devices do not obey it.
How do I size a resistor for an LED?
R = (supply − LED forward voltage) ÷ desired current. For 9 V, a 2 V LED and 20 mA: (9 − 2) ÷ 0.02 = 350 Ω — use 330 or 390.
Why does power matter?
Because it becomes heat. A ¼ W resistor carrying 24 W will burn; check P = V I against the component rating.
Does Ohm's law apply to AC?
For pure resistance, yes, with rms values. Capacitors and inductors add reactance, and the generalised form uses impedance.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Measurement Institute — Australia's measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.