A 12 V car bulb drawing 2 A
Resistance is 12 / 2 = 6 Ω, and power is 12 × 2 = 24 W.
Ohm's law & power calculator
Result
Fill in the fields to see your result.
How it works
A resistive DC circuit is fully described by two independent quantities among voltage, current, resistance and power — the other two follow algebraically. Pick the pair you measured or specified, and the calculator solves the rest.
This only holds for a simple resistive load. Inductors, capacitors and AC circuits introduce phase and frequency, where plain resistance becomes impedance — Ohm’s law in this form no longer applies directly.
Formulas used
Ohm's law
V = I × R
- V
- Voltage, in volts (V)
- I
- Current, in amperes (A)
- R
- Resistance, in ohms (Ω)
Electrical power
P = V × I = I² × R = V² / R
- P
- Power, in watts (W)
Worked examples
A 60 W bulb on a 230 V mains supply
Current is 60 / 230 ≈ 0.26 A, and resistance is about 882 Ω.
Assumptions and limits
- The circuit is a simple resistive DC load.
- Voltage and current are treated as steady (not instantaneous or peak) values.
- No temperature dependence of resistance is modelled.
Frequently asked questions
What happens if I enter resistance and power?
The calculator solves I = √(P / R), then V = I × R. This pair is common when a component’s power rating and resistance are known, such as a heating element.
Why is power always shown as the main result?
Power is usually what determines a fuse rating, a running cost or a heat output, so it is shown first regardless of which two values were entered.
Does this work for AC circuits?
Only for purely resistive AC loads with V and I as RMS values. As soon as a coil or capacitor is involved, use impedance (Z) instead of R, and account for the phase angle between voltage and current.
Updated