Calculate voltage, current, resistance and power from any two known values using Ohm’s law and Joule’s law.
I know these two values
Voltage V
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Current A
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Resistance Ω
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Power W
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Enter two values to compute.
Power wheel — all twelve Ohm’s & Joule’s formulas
The four quadrants give every way to compute one quantity when the other two are known. Ohm’s law (V = I·R) and Joule’s law (P = V·I) together yield all twelve identities shown here.
Frequently asked questions
Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature remains constant. In equation form: V = I·R, where V is voltage in volts, I is current in amperes and R is resistance in ohms. Combined with Joule’s law P = V·I, you can derive any of the four quantities from any two of the others.
Ohm’s law only holds for ohmic (linear) conductors at constant temperature. It breaks down for non-linear components such as diodes and LEDs (exponential I–V curve), incandescent bulbs (resistance rises with temperature), transistors, varistors, gas-discharge lamps and most semiconductors. For those devices you need the datasheet curve or model, not a single resistance value.
For purely resistive loads (heaters, incandescent bulbs) Ohm’s law works the same in AC as in DC, but you must use RMS values of voltage and current. When a circuit contains inductors or capacitors, resistance is replaced by impedance Z, which is frequency-dependent and complex. Power then splits into real (W), reactive (var) and apparent (VA) components — use an impedance calculator instead of this one.
Each field has a unit selector. Choose mV for millivolts (0.001 V), kV for kilovolts (1000 V); µA, mA, A, kA for current; mΩ, Ω, kΩ, MΩ for resistance; mW, W, kW, MW for power. Inputs are converted to SI base units internally, then all four results are re-displayed in the most readable unit automatically.
Subtract the LED forward voltage from the supply voltage to get the voltage drop across the resistor, then divide by the desired forward current. Example: a red LED (V_f = 2 V, I_f = 20 mA) on 5 V USB needs R = (5 − 2) / 0.020 = 150 Ω. Use the V & I mode here with V = 3 and I = 0.02, and choose the next standard value above (150 or 180 Ω). Also check that the dissipated power P = 0.06 W is well below the resistor rating.
This calculator is for educational and estimation purposes only. Mains voltage (110 / 230 V) can kill. Never work on live circuits without proper training, isolation and protective equipment. For installation work, always follow your local electrical code and consult a licensed electrician.
Results rounded for display. Always verify with the component datasheet for critical designs.
This Ohm’s law calculator solves V = I·R and P = V·I for any pair of known values among voltage (V), current (I), resistance (R) and power (P). Pick the two quantities you know, enter them with your preferred unit prefix — millivolts, volts, kilovolts; microamps, milliamps, amps; milliohms, ohms, kilohms, megohms; milliwatts, watts, kilowatts — and the calculator derives the remaining two using the twelve identities of the Ohm–Joule power wheel. Example 1: a red LED with 3 V drop and 20 mA forward current needs R = 3 / 0.02 = 150 Ω and dissipates P = 0.06 W in the resistor. Example 2: a 12 V, 21 W car bulb draws I = 21 / 12 = 1.75 A with an equivalent R ≈ 6.86 Ω. The built-in triangle and power-wheel diagrams, six common presets and a formula trace make it useful for hobbyists, students and electricians.