Ohm's Law Calculator
Ohm's Law is the foundational relationship in electrical engineering, linking voltage (V), current (I), resistance (R), and power (P) in a resistive circuit. The core formula, V = I * R, was established by Georg Ohm in 1827 and remains the starting point for analysing every DC circuit. From just two known quantities you can derive the other two: current equals voltage divided by resistance (I = V / R), resistance equals voltage divided by current (R = V / I), and power equals voltage multiplied by current (P = V * I). Power can also be expressed as P = I^2 * R or P = V^2 / R, allowing calculation even when only current and resistance are known. This calculator accepts any two of the four quantities and instantly solves for the remaining pair. It is useful for sizing resistors, checking wire ratings, specifying power supplies, and troubleshooting circuits. Select the two values you know, enter them, and the results update automatically. All formulas and a worked example are shown below. For AC circuits with reactive loads, consult impedance-based tools in addition to this calculator.
Ohm's Law formulas
The four quantities are related by two independent equations. Given any two, the others follow.
V = I × R
I = V / R
R = V / I
P = V × I = I² × R = V² / R
Worked example
A 12 V battery drives a circuit through a 6 Ω resistor. Find current and power.
- Current: I = V / R = 12 / 6 = 2.00 A
- Power: P = V × I = 12 × 2 = 24.00 W
- Verify: P = I² × R = 4 × 6 = 24 W (consistent)
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, provided temperature remains constant. The relationship is V = I * R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. It was formulated by German physicist Georg Ohm in 1827.
What are the four Ohm's Law formulas?
The four core formulas are: Voltage V = I * R; Current I = V / R; Resistance R = V / I; Power P = V * I. Power can also be expressed as P = I^2 * R and P = V^2 / R. These let you solve for any unknown if you know any two values.
Does Ohm's Law apply to AC circuits?
Ohm's Law in its basic form applies to resistive DC circuits. In AC circuits, the equivalent concept uses impedance (Z) instead of resistance, incorporating reactive components from capacitors and inductors. For purely resistive AC loads, the DC form still holds.
What units does Ohm's Law use?
Voltage is measured in volts (V), current in amperes (A), resistance in ohms (Omega), and power in watts (W). Using these SI units in the formulas gives consistent results without any conversion factors.
Why can't I enter all four values?
Only two independent values are needed because the other two are mathematically determined by those two. Entering all four would be over-constrained and could produce contradictions. Select the two values you know and the calculator solves for the rest.
Sources
- NIST: SI Units: Electric Current.
- IEEE Std 100-2000: The Authoritative Dictionary of IEEE Standards Terms (definition of Ohm's Law).
Reviewed by the CalculatorHub team, edited by James Graham. 14 June 2026. General information only.