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Ohm's Law Calculator
Last updated: June 19, 2026
An Ohm's law calculator is an academic physics utility that computes electrical relationships using the formula voltage equals current multiplied by resistance. The calculator supports calculations across four key electrical properties: voltage (volts), current (amperes), resistance (ohms), and power (watts). By entering any two known variables, the calculator applies algebraic Ohm's law and Joule's law equations to solve for the two unknown parameters. Physics students, electronics hobbyists, and technicians use this tool to verify circuit parameters and prevent component damage.
Pick the two quantities you know and clear the others. The calculator returns voltage, current, resistance, and power, and shows which formulas were used to derive the unknowns.
Quick Answer
Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two known values to compute the remaining parameters.
Enter exactly two values
The other two are computed using Ohm's law and the power relationships.
e.g. 12
e.g. 2
e.g. 6
e.g. 24
Formulas
- V = I × R
- I = V / R
- R = V / I
- P = V × I
- P = I² × R
- P = V² / R
Computed quantities
12 V, 2 A, 6 Ω, 24 W
R = V / I · P = V × I
Ohm's law is V = I × R for ideal resistors. The power relationships follow by substitution: P = V × I, P = I² × R, and P = V² / R. Real circuits have temperature dependence, reactance, and non-ideal behavior; this calculator assumes a simple resistive DC or RMS AC circuit.
Examples
12 V, 2 A
R = 6 Ω · P = 24 W
120 V, 60 W
I = 0.5 A · R = 240 Ω
5 A, 4 Ω
V = 20 V · P = 100 W
100 W, 50 Ω
V ≈ 70.71 V · I ≈ 1.41 A
How it works
Ohm's law is the single equation relating voltage, current, and resistance. Combined with P = V × I, it produces the six standard formulas used to find any unknown from any two knowns.
Ohm's law · V = I × R
Power · P = V × I = I² × R = V² / R
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Frequently asked questions
Ohm's law is V = I × R: the voltage across a resistor equals the current through it times its resistance. The law applies to ideal resistors in DC circuits and to RMS values in AC circuits with purely resistive loads.
Start with P = V × I (the basic definition of electrical power) and substitute Ohm's law. Replace V with I × R to get P = I² × R. Replace I with V / R to get P = V² / R. All three forms describe the same power dissipation.
Because two independent values determine the other two through Ohm's law and the power relationships. Entering three creates an overdetermined system; if the third does not match what Ohm's law predicts, there is no consistent answer. Clear extra fields before computing.
An ohm is the SI unit of electrical resistance: 1 ohm is the resistance that produces 1 ampere of current when 1 volt is applied. Symbol is the Greek letter omega (Ω). Larger units include kΩ (1,000 Ω) and MΩ (1 million Ω).
For purely resistive AC loads, yes; substitute RMS voltage and RMS current. For circuits with capacitance or inductance, the relationships use complex impedance instead of pure resistance, and power has real (W), reactive (VAR), and apparent (VA) components. This calculator handles the basic resistive case only.
No. The calculator is a math tool for the basic electrical relationships. Real electrical work involves code compliance, conductor sizing, overcurrent protection, grounding, and safety procedures that this tool does not address. Consult a licensed electrician for permitted electrical work.
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