Ohm's Law Calculator
Find voltage, current, resistance or power from any two of the other values. Choose what you want to find, enter the two values you know, and get the answer with the formula, step-by-step working and an optional resistor power-rating check.
Background
Ohm’s law links voltage, current and resistance: V = I·R. Power follows from the same quantities: P = V·I. Once you know any two of voltage, current, resistance and power, you can find the other two. It’s the go-to relationship for circuits, labs and “will this resistor burn?” checks.
How to use this calculator
- Choose what you want to find: voltage, current, resistance or power.
- Choose which two values you know, type them in and pick their units.
- Press Calculate (or Enter) to see the answer. If you change a value, press Calculate again. Or click a quick example to load and solve it.
- Open Check a resistor’s power rating to compare the power with a resistor rating.
- Open the step-by-step solution to see each formula with your numbers.
How this calculator works
- Normalizes units: converts your values to volts, amps, ohms and watts.
- Picks the formulas: uses Ohm’s law and the power relationships that fit the two values you know.
- Solves the other two: finds your answer and the remaining value.
- Flags impossible cases: a 0 Ω short circuit, a 0 A open circuit, or values that can’t go together.
- Checks the rating: compares the power with the resistor rating you choose.
Formulas & Equations Used
A quick memory aid: cover the quantity you want to find, and what is left shows the formula. Two letters side by side are multiplied; one letter above another is a division.
Cover V: V = I × R
Cover I: I = V ÷ R
Cover R: R = V ÷ I
Cover P: P = V × I
Cover V: V = P ÷ I
Cover I: I = P ÷ V
The triangles cover the most common formulas. The squared and square-root forms below, such as P = V² / R, are not part of a triangle.
Ohm’s law: V = I·R
Current: I = V / R
Resistance: R = V / I
Power: P = V·I = I²·R = V² / R
From power and voltage: I = P / V, R = V² / P
From power and current: V = P / I, R = P / I²
From power and resistance: I = √(P / R), V = √(P·R)
Example Problems & Step-by-Step Solutions
Example 1 — Find current
A 12 V source is connected to a 24 Ω resistor. Use I = V / R: I = 12 / 24 = 0.5 A. Power is P = V·I, which gives 6 W.
Example 2 — Find resistance
A device draws 200 mA at 5 V. Convert: 200 mA = 0.2 A. Then R = V / I gives R = 5 / 0.2 = 25 Ω.
Example 3 — Check a resistor rating
You measure 9 V across a 220 Ω resistor. Current: I = V / R gives about 0.041 A. Power: P = V² / R gives about 0.37 W. A 1/4 W resistor would overheat (147% of its rating). A 1/2 W resistor is within its rating but above the 60% rule of thumb (74%), and a 1 W resistor leaves comfortable headroom (37%).
Example 4 — Find resistance from power
A heating element runs at 12 V and 6 W. Use R = V² / P: R = 144 / 6 = 24 Ω. The current is I = P / V, which gives 0.5 A.
Frequently Asked Questions
Which values should I enter?
Any two of voltage, current, resistance and power. First choose the one you want to find, then choose which two you know.
What if I know three values?
Enter the two you trust most. To check the third, solve for it from those two and compare it with the number you have.
Why does power matter?
Power tells you how much heat a component gives off. Too much power for a resistor’s rating can make it overheat.
What’s the difference between mA and A?
A milliamp is a thousandth of an amp: 1 mA = 0.001 A, and 1 µA = 0.000001 A. Pick the unit next to each number and the calculator converts it.
What happens if a value is 0?
0 Ω is a short circuit, where current would be unlimited, so the calculator asks for a resistance above 0 Ω. A voltage with 0 A is an open circuit, with infinite resistance. With 0 V and 0 A, the resistance could be anything, so it can’t be found.
Can I enter negative values?
Voltage and current can be negative to show direction. Resistance and power can’t be negative. If voltage and current have opposite signs, the calculator asks you to check them.
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