Ohm's law Calculator

Type any two values — the other two solve instantly, with proper unit prefixes, the formulas shown, and a resistor overload check.

Enter any two values

The other two are computed instantly and marked computed. Edit any field to re-solve.

Solved circuit

Voltage—
Current—
Resistance—
Power—

Enter any two values to solve.

The full Ohm's law wheel — all 12 formulas

V = I × R · P ÷ I · √(P × R)
I = V ÷ R · P ÷ V · √(P ÷ R)
R = V ÷ I · V² ÷ P · P ÷ I²
P = V × I · V² ÷ R · I² × R

Whatever pair you know, one of these gets you each missing value — the calculator picks the right ones automatically and tells you which it used.

Ohm's law: the one relationship behind every circuit

Ohm's law says current through a conductor is proportional to the voltage across it: V = I × R. Add the power relation P = V × I and you can derive all twelve formulas on the wheel above — which means knowing any two of voltage, current, resistance and power fixes the other two. That's exactly what this calculator does, live: type any two values and the remaining pair appears instantly, tinted green and tagged computed, along with the exact formulas used. No "Calculate" button, no converting millivolts to volts by hand — pick mV, µA, kΩ or kW from the unit menus and the math adjusts.

The classic worked example: a 20 V supply across a 10 Ω resistor drives I = 20 ÷ 10 = 2 A, dissipating P = 20 × 2 = 40 W. Or from the other direction: a kettle rated 2,000 W on 230 V mains draws I = 2000 ÷ 230 ≈ 8.7 A through an effective resistance of about 26.5 Ω.

The safety check most calculators skip: resistors have power ratings — ⅛ W and ¼ W are the common hobby sizes — and exceeding the rating makes them overheat, drift, and eventually burn. Select your resistor's rating and the calculator compares it against the computed power dissipation, with the standard engineering advice to keep real dissipation under about 50-70% of the rating for a comfortable margin. A 220 Ω resistor dropping 5 V dissipates 0.11 W — fine for a ¼ W part, marginal for an ⅛ W one.

Where Ohm's law applies (and where it doesn't): it holds for resistive components — resistors, heating elements, wires — at a steady temperature. LEDs, diodes and transistors are non-ohmic: their current-voltage relationship is a curve, not a line, so you calculate the resistor in series with an LED, never the LED itself. For AC circuits with capacitors or inductors, resistance generalizes to impedance and the same formulas apply with magnitudes and phase angles.

Frequently asked questions

Which two values do I need to enter?
Any two of the four: voltage, current, resistance or power. The calculator uses the last two fields you edited as the knowns and solves the other two instantly, showing which formulas it applied.
What is Ohm's law in simple terms?
Current = voltage / resistance (I = V/R). Push harder (more volts) and more current flows; resist more (more ohms) and less current flows. Power, the rate of energy use, is voltage x current (P = V x I).
How do I calculate watts from volts and amps?
Multiply them: P = V x I. A 12 V circuit carrying 3 A delivers 36 W. This calculator does it automatically and also handles the reverse - watts and volts to amps (I = P / V).
What do the unit menus (mV, mA, kOhm, kW) do?
They convert automatically. Enter 20 mA instead of 0.02 A, or 4.7 kOhm instead of 4700 Ohm - the math uses base units internally, so mixed units always work out correctly.
Why does the resistor power rating matter?
A resistor dissipating more power than its rating overheats: its value drifts, it can discolor, smoke or fail open. Standard practice is to choose a rating at least 2x the expected dissipation - the built-in check warns you when you're close to or over the limit.
Does Ohm's law work for LEDs?
Not for the LED itself - LEDs are non-ohmic, with a sharp current-voltage curve. Use Ohm's law on the series resistor instead: R = (supply voltage - LED forward voltage) / desired current. For a 5 V supply, 2 V LED and 20 mA: R = 3 / 0.02 = 150 Ohm.
Does it work for AC mains circuits?
For purely resistive AC loads (heaters, kettles, incandescent bulbs), yes - use RMS values. For circuits with motors, capacitors or inductors, resistance generalizes to impedance (Z) and power involves a power factor; the simple P = V x I then gives apparent power, not real power.
Why did the values conflict when I entered three numbers?
Ohm's law only has two degrees of freedom - the third and fourth values are determined by the first two. This calculator always solves from the last two fields you edited and overwrites the rest, so there's never an inconsistent state.

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