Ohm's Law Calculator
Solve for voltage, current, resistance, or power in a DC circuit given any two known electrical values, using Ohm's law and the power law.
How It's Calculated
Formula
V = I \times R \\ P = V \times I \\ I = \frac{V}{R} \\ R = \frac{V}{I} \\ P = I^2 R \\ P = \frac{V^2}{R}Ohm's law describes a fixed relationship between voltage (V, in volts), current (I, in amperes), and resistance (R, in ohms) in a DC circuit: V = I x R. Combined with the power law, P = V x I, these two equations link all four quantities together, so knowing any two of {voltage, current, resistance, power} is enough to solve for the other two algebraically. This calculator covers all six possible known-value pairings and derives the matching formula for each — for example, knowing voltage and power gives current as I = P/V and resistance as R = V²/P, while knowing resistance and power gives current as I = √(P/R) and voltage as V = √(P x R). This is a V1 DC/magnitude calculator: it does not model AC phase angles, impedance, reactive power, power factor, or complex numbers, and all known input values are entered as positive numbers to avoid divide-by-zero and undefined results.
Worked Examples
Known: Voltage 12V, Current 2A
- Formula: R = V/I, P = V*I
- Resistance = 12 / 2 = 6 Ω
- Power = 12 x 2 = 24 W
Known: Resistance 6Ω, Power 24W
- Formula: I = √(P/R), V = √(P*R)
- Current = √(24/6) = √4 = 2 A
- Voltage = √(24 x 6) = √144 = 12 V
Frequently Asked Questions
Why do I need to pick which two values I know?
Ohm's law and the power law together relate four quantities (voltage, current, resistance, power), but which formula solves for the two you're missing depends on which two you already have. Selecting a mode tells the calculator exactly which two values you're providing and which algebraic rearrangement to apply.
Does this support AC circuits with phase or reactance?
No. This is a V1 DC/magnitude calculator using plain Ohm's law and power law arithmetic. It does not model AC phase angles, impedance, reactive power, power factor, or complex numbers.
Why must the known values be greater than zero?
A zero known value would make at least one of the derived formulas undefined — dividing by zero current or zero resistance, for example. Requiring known values to be positive keeps every one of the six modes well-defined without special-casing zero pair by pair.
Why do the Known Value fields say their meaning depends on the mode?
This calculator uses two generic fields — "Known Value 1" and "Known Value 2" — for all six modes rather than a separate labeled field per mode, so switching modes doesn't require re-learning a new form layout. Each field's helper text spells out exactly what it means in the currently selected mode, and the "Formula Used" result restates which formula produced your answer.