Ohm's Law Calculator
Enter any two of voltage (V), current (I), resistance (R) or power (P). The calculator returns the other two using Ohm's law and the DC power equations.
Voltage (V)
12
- Current (A)
- 2
- Resistance (Ω)
- 6
- Power (W)
- 24
Ohm's law: V = I·R. Power: P = V·I = I²·R = V²/R. Enter any two of V, I, R, P; the calculator solves the other two. All values are positive — for AC circuits with reactance, use an impedance calculator instead.
How to use this calculator
Fill in any two of the four boxes — voltage in volts, current in amps, resistance in ohms, or power in watts — and leave the others at 0. The calculator detects which pair you've supplied and solves for the remaining two. The defaults show a worked example: 12 V at 2 A through a resistor.
How the calculation works
Ohm's law states that the current through a conductor is directly proportional to the voltage across it: V = I·R, where R is the resistance. DC power follows from energy per unit charge: P = V·I, which combined with Ohm's law gives the equivalents P = I²·R = V²/R. From any two of {V, I, R, P}, the other two are uniquely determined.
Worked example
A 12 V battery drives 2 A through a resistor. Resistance R = V/I = 12/2 = 6 Ω. Power dissipated P = V·I = 12 × 2 = 24 W. If you instead knew R = 60 Ω across a 120 V supply, current I = V/R = 2 A and power P = V²/R = 14,400/60 = 240 W.
Frequently asked questions
What is Ohm's law in plain English?
Push (voltage) divided by squeeze (resistance) equals flow (current). Double the voltage across a fixed resistor and you double the current; double the resistance at fixed voltage and you halve the current. The formula V = I·R links the three.
Does Ohm's law work for AC circuits?
Only for purely resistive AC loads (heaters, incandescent bulbs). Once a circuit contains capacitors or inductors, you replace resistance R with complex impedance Z, and current and voltage stop being in phase. For AC reactive circuits, use an impedance calculator and work with RMS values.
Why do I get power three different ways (P = V·I, I²R, V²/R)?
They're algebraic rearrangements of the same equation. P = V·I is the definition (energy per charge × charges per second). Substituting V = I·R gives P = I²·R; substituting I = V/R gives P = V²/R. Use whichever pair of values you happen to know.
What units do I enter — millivolts, milliamps, kilo-ohms?
Use base SI units: volts, amps, ohms, watts. Convert before entering: 5 mA = 0.005 A, 4.7 kΩ = 4,700 Ω, 250 mV = 0.25 V. Mixing prefixes is the most common cause of wrong answers — pick base units once and stay consistent.
What if I enter all four values?
Any value you supply (greater than zero) is taken as given and not recalculated. The calculator only fills in fields you've left at 0. If you over-specify with inconsistent numbers, the result will reflect what you typed — re-zero a field to have it solved.
Can current or voltage be negative?
Sign conventions matter in circuit analysis (passive sign convention, KVL loops), but Ohm's-law magnitudes are positive. This calculator clamps inputs to ≥ 0; if you're tracking signs for a mesh-current solution, drop the sign here and reinstate it at the end.