Ohm's Law Calculator
Enter any two of voltage, current, and resistance to calculate the other two, plus power.
Last updated: September 5, 2026
Frequently asked questions
What is Ohm's law?
V = I × R - voltage equals current multiplied by resistance. It describes the relationship between these three quantities in an electrical circuit: for a fixed resistance, more voltage drives more current.
How do I calculate power from Ohm's law values?
P = V × I (power equals voltage times current) - once you know voltage and current from Ohm's law, power follows directly. You can also derive P = I²R or P = V²/R by substituting Ohm's law into the power formula.
What is the current if a 10-ohm resistor has 20 volts across it?
2 amps - using I = V/R = 20/10 = 2 A.
Why do I need to enter exactly two known values?
Ohm's law (V=IR) relates three quantities - knowing any two lets you solve for the third algebraically, but with only one known value, there isn't enough information to determine the other two uniquely.
How do I calculate resistance if I know voltage and current?
R = V / I. For a circuit with 12 volts and 3 amps of current: R = 12 / 3 = 4 ohms.
What is the voltage across a 5-ohm resistor carrying 4 amps?
20 volts - using V = I × R = 4 × 5 = 20 V.
Can I enter voltage and power instead of voltage and current?
No - this calculator solves specifically from two of voltage, current, and resistance. If you have voltage and power instead, first find current with I = P/V, then enter voltage and that current to get resistance and confirm power.