Ohm's Law Calculator
Solve V, I, R, P from any two known values.
Overview
The Ohm's Law Calculator computes any two of voltage, current, resistance and power when you supply the other two. Enter, for example, 12 volts and 4 amps and out come the resistance (3 Ω) and power dissipation (48 W). Every combination of two knowns is supported.
It is built for electronics hobbyists building circuits, electricians sizing components, students checking lab measurements and engineers ballparking heat dissipation. The four-quantity relation is so foundational it deserves a calculator rather than reciting it from memory each time.
How it works
Ohm's Law states V = I * R, where V is voltage in volts, I is current in amps and R is resistance in ohms. Combined with the power identity P = V * I, you can derive any pair from any other pair: P = I² * R = V² / R.
The calculator detects which two values you supplied and applies the relevant formula. Internally everything reduces to V, I, R, P via the same four equations: V = I * R, I = V / R, R = V / I, P = V * I.
Examples
V = 12, I = 2 → R = 6 Ω, P = 24 W
V = 230, R = 100 → I = 2.3 A, P = 529 W
I = 0.5, R = 1000 → V = 500 V, P = 250 W
P = 60, V = 120 → I = 0.5 A, R = 240 Ω
FAQ
Does Ohm's Law apply to AC circuits?
Use rms values for V and I and treat impedance instead of pure resistance. For purely resistive AC loads the formula is unchanged.
What if the resistance is negative?
In passive components it isn't. Some active devices (like tunnel diodes in a narrow region) exhibit negative differential resistance, but Ohm's Law as a static relation assumes positive R.
Is power always positive?
In a resistor, yes — it dissipates. In an active source, V and I can be opposite signs, giving negative "power dissipated" which means the component is supplying power.
Why are calculators useful here? It's just one equation.
Because there are four equivalent forms and people swap which two are knowns. A single calculator removes the chance of dividing the wrong way.
Can I use it for non-linear loads?
Strictly, Ohm's Law applies only to linear (ohmic) elements. For non-linear loads use the operating-point values and treat them as locally valid.