Ohm's Law and Power Calculator
Enter any two of the four quantities (voltage V, current I, resistance R, power P) and the calculator solves the other two using Ohm's law and the Joule power wheel. Example: 12 V and 2 A give R = V / I = 6 ohm and P = V x I = 24 W.
Ohm law loop (V, I, R, P)
24 W
V 12 V · I 2 A · R 6 Ω · P 24 W
Enter any two of V, I, R, P and we solve the other two. DC, purely resistive assumption; AC mode adds a power factor for real power.
FAQ
- Which quantities do I need to use the calculator?
- Any two different quantities out of four: voltage V (volts), current I (amperes), resistance R (ohms) and power P (watts). From that pair, the tool solves the two missing quantities using the matching pair of power-wheel equations (12 formulas in total) and shows the exact formula used for each result.
- Which formula is used for each known pair?
- Known (V, I): R = V / I and P = V x I. Known (V, R): I = V / R and P = V x V / R. Known (V, P): I = P / V and R = V x V / P. Known (I, R): V = I x R and P = I x I x R. Known (I, P): V = P / I and R = P / (I x I). Known (R, P): V = sqrt(P x R) and I = sqrt(P / R). The calculator shows the one it applied.
- Why are some combinations rejected?
- For physical reasons. Solving R with zero current (I = 0) is a division by zero: that is an open circuit, undefined. Any division by a zero known quantity is blocked. A negative resistance, power or current is non-physical and rejected. The square-root branches (pairs that include P) require strictly positive values to stay physical.
- How are metric prefixes (mA, kohm, mW) handled?
- Each field accepts a metric prefix: voltage (mV, V, kV), current (uA, mA, A), resistance (mohm, ohm, kohm, Mohm), power (uW, mW, W, kW). Everything is converted to SI base units before computing (u = 1e-6, m = 1e-3, k = 1e3, M = 1e6), then the result is shown with a readable prefix: for example 0.0227 A becomes 22.73 mA and 1500 ohm becomes 1.5 kohm.
- Is the calculation valid for AC?
- The core V/I/R/P solve assumes a purely resistive (ohmic) load in DC. AC mode adds a power factor cos phi (0 to 1): real power P = V x I x cos phi (W) and apparent power S = V x I (VA). This is a single-phase approximation. Three-phase (factor sqrt(3) on line quantities) and non-linear loads are out of scope.
This calculator gets you a first pass at sizing, but a real power study accounts for derating, thermals and margins. Let's talk it through in a free 30-minute audit with our design team.
An electronics design house based in Montpellier, France, AESTECHNO brings 10+ years of design experience and a 100% first-pass record on CE/FCC certification. Formulas based on Ohm's law and the Joule power wheel.