Circuit Analysis & Fundamentals
Ohm's Law Calculator & Power Wheel
Solve for voltage, current, resistance, or power from any two known values.
About this calculator
Ohm's law (V = IR) and the power law (P = VI) are two independent equations relating four quantities: voltage, current, resistance, and power. Because they share variables, knowing any two of the four lets you solve for the other two — the relationship engineers call the "power wheel," a chart of the twelve rearrangements of these two formulas. This calculator picks the correct pair of formulas automatically based on which two quantities you know, so you don't have to look them up or rearrange algebra by hand.
It's built for quick verification work: checking a nameplate current against a known load resistance, sanity-checking a lab measurement, or teaching the relationship between the four quantities. Every result shows the exact substitution used, so you can confirm the arithmetic rather than trust a black box.
The common pitfall this calculator does not solve for you: it assumes a simple resistive DC circuit (or an AC circuit at unity power factor) where V, I, R, and P are all real, positive scalars. In an AC circuit with reactance, resistance must be replaced with impedance magnitude, and P above represents real power only — apparent power (S = VI) and reactive power (Q) require the full phasor relationship, not this calculator. See the three-phase power calculator for circuits with power factor.
Design notes & common mistakes
- P = VI is real (dissipated) power. In an AC circuit with reactance, VI is APPARENT power (VA); multiply by the power factor for real watts — this calculator assumes power factor = 1.
- Two of the four quantities fully determine the other two — entering more would over-constrain the two equations (V = IR and P = VI).
- R here is a single element's resistance. For a network, reduce series/parallel groups to one equivalent resistance first, then apply Ohm's law.
- Watch the units: mixing mA with V and Ω, or mV with A, is the most common arithmetic slip — the unit selectors convert to base units before computing.
Assumptions
- DC circuit, or AC circuit at unity power factor (P = VI with no reactive component).
- All four quantities are treated as real, positive scalars.
- Resistance is treated as constant (no temperature dependence).
When to use this calculator
Appropriate for
- Finding any two of voltage, current, resistance, and power from the other two for a DC or resistive AC element
- Quick component-level checks: sizing a resistor's power rating, finding a current from a voltage and resistance
- Teaching the Ohm's-law and power relationships
Not suitable for
- Reactive or nonlinear elements, where voltage and current are not in a simple resistive ratio
- AC circuits where phase angle and power factor matter — use the three-phase or impedance tools instead
- Circuits where temperature changes the resistance significantly during operation
What this calculator does not cover
- DC and purely resistive AC circuits only — there is no reactance, phase angle, or power factor here; for AC circuits with inductance or capacitance, work with impedance, not resistance.
- Assumes resistance is constant. Real loads — lamp filaments, motor windings, semiconductors — change resistance substantially with temperature and operating point.
- Solves a single element, not a network; reduce series/parallel combinations first (see the resistor network calculator).
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
What is the power wheel?
The power wheel is a reference chart of the twelve algebraic rearrangements of Ohm's law (V = IR) and the power law (P = VI), organized so you can look up the formula for any unknown given any two known quantities.
Does this work for AC circuits?
It works for AC circuits at unity power factor, where resistance and real power behave like the DC case. For circuits with reactance, use impedance magnitude in place of resistance and note that P becomes real power only — use a three-phase power calculator for apparent and reactive power.
Why do I only enter two values?
Ohm's law and the power law are two independent equations in four unknowns (V, I, R, P). Any two known quantities fully determine the other two, so entering more would over-constrain the system.
References
- Boylestad, R., Introductory Circuit Analysis, 13th ed., Chs. 2–4
- IEC 60027-1, Letter symbols for electrotechnology
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