Ohm’s Law Calculator
Pick which two quantities you already know — voltage, current, resistance, or power — and this tool fills in the rest using Ohm’s law and the electric-power identity.
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Result
The formula
Ohm’s law links voltage (V), current (I), and resistance (R): V = I × R. Instantaneous power in a DC (or resistive AC) circuit is P = V × I, which rearranges to P = I²R and P = V²/R. Any two independent values determine the other two.
Worked example
- Known: V = 12 V, I = 2 A
- R = V ÷ I = 12 ÷ 2 = 6 Ω
- P = V × I = 12 × 2 = 24 W
Result: R = 6 Ω · P = 24 W
How Ohm’s law and power are calculated
Choose a known pair, enter positive values in SI units (volts, amperes, ohms, watts), and we solve the matching rearrangement. The same four numbers always satisfy both V = IR and P = VI when the circuit is purely resistive.
The six known pairs
From V and I: R = V/I and P = VI. From V and R: I = V/R and P = V²/R. From V and P: I = P/V and R = V²/P. From I and R: V = IR and P = I²R. From I and P: V = P/I and R = P/I². From R and P: V = √(PR) and I = √(P/R).
Units
Use volts (V), amperes (A), ohms (Ω), and watts (W). Milliamps or kilohms must be converted first (e.g. 500 mA = 0.5 A, 4.7 kΩ = 4700 Ω). Prefix mistakes are the most common source of wrong answers.
DC vs AC
For DC and for AC loads that behave resistively, these formulas use RMS voltage and current. Reactive circuits (inductors, capacitors) need impedance and phase — real power is then V I cos φ, which this calculator does not apply.
What we do not model
No temperature coefficients, wire gauge tables, three-phase factors, or component derating. Treat results as ideal lumped-element estimates for education, breadboarding, and quick checks.
Interesting facts
Named for Georg Ohm
Georg Simon Ohm published the proportional link between voltage and current in metallic conductors in 1827. The unit of resistance — the ohm (Ω) — commemorates that work.
Power is not a fourth independent law
P = VI follows from the definition of electrical power. Combined with Ohm’s law it yields the familiar I²R and V²/R heating formulas used for resistors and cables.
Why √(P R) appears
When only resistance and power are known, V = √(P R) because P = V²/R. The positive square root is the physical magnitude for ordinary DC polarity conventions.
SI coherence
One volt across one ohm drives one ampere and dissipates one watt — the SI electrical units are built so these identities stay numerically clean.
Frequently asked questions
The core statement is V = I × R (voltage equals current times resistance). Rearrangements I = V/R and R = V/I cover the same relationship.
Multiply: P = V × I. Example: 12 V and 2 A give 24 W. That is the instantaneous power for DC, or average power for resistive AC using RMS values.
Yes. R = V²/P. Choose “Voltage & power” in this calculator, enter V and P, and read resistance (and current) from the result grid.
Then V = P/I and R = P/I². Select “Current & power”, enter I and P, and the tool returns voltage and resistance.
Not as a single fixed R. Diodes are nonlinear; use the datasheet forward voltage and a series resistor sized with Ohm’s law around the LED drop, not a bulk “LED resistance.”
Enter amperes. Convert first: 250 mA = 0.25 A. Mixing mA with ohms without converting understates current by 1000×.
It is fine for rough resistive estimates using RMS volts and amps. Motors, switch-mode supplies, and anything with a power factor need a different treatment.
This tool solves for magnitudes in the usual teaching form. Zero or negative inputs would imply open circuits, shorts, or sign conventions we do not model here.
References
- NIST Guide to the SI SI definitions and relationships for the volt, ampere, ohm, and watt.
- BIPM SI Brochure International System of Units — ampere and derived electrical units.
- OpenStax College Physics — Ohm’s Law Clear textbook treatment of Ohm’s law, resistance, and power.