How to use the Ohm's Law
- Enter any two of voltage, current, resistance or power.
- Leave the remaining fields blank — the calculator solves for them.
- Check the power figure against the component's rating, not just its resistance.
- Derate power ratings by roughly half for continuous operation in still air.
How the calculation works
V = I × R holds for ohmic components, meaning those whose resistance is constant across the operating range. Rearranged it gives current for a known supply and load, or the resistance needed to limit current to a target. Power follows as P = V × I, which substitutes into I²R and V²/R — the two forms used most in practice because they express dissipation in terms of what you can measure.
Power is where designs fail rather than where they are calculated wrong. A resistor that computes correctly for resistance may still be dissipating three times its rating, so the wattage figure is the one to check against the datasheet. Semiconductors, lamp filaments and motors are not ohmic — their resistance changes with temperature and load — so treat an Ohm's law result on those as an approximation at one operating point.
V = I × R ; P = V × I = I²R = V²/RSource: Ohm's law and Joule's first law; standard electrical circuit theory.
Worked example
Driving a 20 mA LED with a 2.1 V forward drop from a 12 V supply through a series resistor.
- Voltage across the resistor = 12 − 2.1 = 9.9 V.
- R = V / I = 9.9 / 0.020 = 495 Ω, so use the nearest standard 510 Ω.
- Power in the resistor = V × I = 9.9 × 0.020 = 0.198 W.
A 510 Ω resistor dissipating about 0.2 W — a 0.25 W part is marginal, so a 0.5 W resistor is the safer choice.
Frequently asked questions
Does Ohm's law work for AC circuits?+
Only for purely resistive loads. With capacitance or inductance present you need impedance and phase angle, and real power also requires the power factor, so a DC-style result overstates the current a reactive load draws in phase.
Why does my measured current differ from the calculation?+
Supply voltage sags under load, wiring and connections add resistance, and component tolerances are typically 5%. A discrepancy of a few percent is normal; a large one usually means an unexpected series resistance or a non-ohmic component.
How much power headroom should a resistor have?+
Aim for at least double the calculated dissipation in free air, and more in an enclosure or at elevated ambient temperature. Manufacturers' ratings assume specific mounting and cooling conditions.
Can I use this to size a fuse?+
Use it to find normal operating current, then choose a fuse above that but below the circuit's damage threshold, allowing for inrush current on motors and power supplies. Fuse selection also depends on breaking capacity and time-current characteristics.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.