All calculatorsOhm's Law Calculator
Solve voltage (V), current (I), resistance (R), and power (P) instantly. Enter any two values and we’ll compute the rest — with unit support, steps, and a mini power/overload gauge.
Background
Ohm’s Law links voltage, current, and resistance: V = I·R. Once you know any two, you can find the others — and also compute electrical power: P = V·I. This is the go-to tool for circuits, labs, and “will this resistor burn?” checks.
How to use this calculator
- Enter any two known values (V, I, R, or P).
- Leave the value you want to find blank (or overwrite it).
- Pick units (V/mV/kV, A/mA, Ω/kΩ/MΩ, W/mW/kW).
- Click Calculate (or enable Auto-calculate).
- Optionally select a resistor power rating to check overheating risk.
How this calculator works
- Normalize units: convert everything into base units (V, A, Ω, W).
- Pick a consistent formula set: use Ohm’s Law and power relationships.
- Solve unknowns: compute missing values from any two knowns.
- Check power rating: compare computed power to the selected resistor rating.
Formula & Equation Used
Ohm’s Law: V = I·R
Current form: I = V / R
Resistance form: R = V / I
Power: P = V·I = I²·R = V² / 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 → 6 W.
Example 2 — Find resistance
A device draws 200 mA at 5 V. Convert: 200 mA = 0.2 A. Then R = V / I → R = 5 / 0.2 = 25 Ω.
Example 3 — Check resistor rating
You measure 9 V across a 220 Ω resistor. Current: I = V / R → 0.041 A. Power: P = V² / R → 0.37 W. A 1/4 W resistor would likely overheat; 1/2 W is safer.
Frequently Asked Questions
Q: Which two values should I enter?
Any two of V, I, R, or P work. The calculator solves the remaining values consistently.
Q: Why does power matter?
Power tells you how much heat a component dissipates. Too much power for a resistor’s rating can cause overheating.
Q: What’s the difference between mA and A?
Milliamp is thousandths of an amp: 1 mA = 0.001 A. The calculator converts automatically.
Q: What if I enter three or four values?
The calculator checks consistency. If values conflict (due to rounding or measurement error), you’ll get a callout.
