Ohm's Law Calculator
Solve Ohm's Law and the power formulas for Voltage, Current, Resistance and Power. Pick which two you know (V+I, V+R, V+P, I+R, I+P, or R+P) and it computes the other two, with the formulas and calculation steps shown.
Input
Pick the two values you already know — the other two are calculated.
In volts (V).
In amps (A).
Output
| Quantity | Value |
|---|---|
| No data yet | |
Guides
Ohm's Law ties voltage, current, and resistance together in one equation: V = I × R. Add in the power formulas — P = V × I, P = I² × R, and P = V² ÷ R — and any two of the four quantities (Voltage, Current, Resistance, Power) are enough to solve for the other two. This calculator does exactly that: pick which pair you know, enter the values, and it works out the rest instantly, showing the formula and the calculation step used for each result.
How to use it
- Choose which two values you know from the Known Values dropdown: Voltage & Current, Voltage & Resistance, Voltage & Power, Current & Resistance, Current & Power, or Resistance & Power.
- Enter the two known values — the calculator only shows the fields relevant to your selection, so you always know exactly what to fill in.
- The Results table fills in automatically with all four values (Voltage, Current, Resistance, Power), and the Formulas & Steps panel shows exactly how each unknown was derived.
All values use standard SI units: volts (V), amps (A), ohms (Ω), and watts (W).
Why any two values are enough
Ohm's Law itself (V = I × R) only relates three quantities, so knowing any two of {V, I, R} always fixes the third. Bringing power into the picture — P = V × I — adds a fourth quantity and three more derived relationships (P = I²R, P = V²/R, and their inverses), which is why any pair out of {V, I, R, P} pins down the other two, not just the three in the classic triangle. This calculator covers all six possible pairs, not just the textbook V-I-R case.
What if my two known values include zero?
A few combinations divide by one of the known values, so a zero there would make the answer mathematically undefined (division by zero) rather than a real circuit result:
- Voltage & Current known: Current can't be zero (used to solve for Resistance).
- Voltage & Resistance known: Resistance can't be zero (used to solve for Current and Power).
- Voltage & Power known: neither can be zero (each is used as a divisor for the other unknown).
- Current & Power known: Current can't be zero (used to solve for Voltage and Resistance).
- Resistance & Power known: Resistance can't be zero (used to solve for Current).
- Current & Resistance known: no restriction — this pair never divides by either value, so zero is a valid (if trivial) input.
The calculator flags these cases with a clear message instead of showing "Infinity" or "NaN".
Is this the same as a resistor calculator?
No — this tool solves the electrical relationships between voltage, current, resistance, and power directly from any two known values. If you're instead trying to decode a resistor's value from its color bands, or size a current-limiting resistor for an LED, those are separate, more specialized calculators.
Does it handle very large or very small numbers?
Yes. Enter values in plain volts, amps, ohms, and watts — including decimals (e.g. 0.5 A, 0.002 Ω) — and the results are computed with full floating-point precision, rounded to six significant figures for display.
Privacy
All calculations run entirely in your browser (or via the API, entirely on request) — no circuit values are stored or sent anywhere beyond the calculation itself.