β‘ Ohm's Law Calculator
Solve V=IR for voltage, current, or resistance. Calculate power too.
How to Use This Calculator
This calculator solves Ohm's law for any one of the three quantities. Choose what to solve for, enter the other two values, and the answer appears instantly.
Use the Solve for dropdown to pick voltage, current, or resistance.
Enter the two known values in their fields. Current is in amps, so 500 mA is 0.5 A.
Click Calculate. The missing value appears with the correct unit.
Double-check with the power formula P = V x I if you are sizing a real resistor, so it does not overheat.
Ohm's Law
Ohm's law says voltage pushes current through resistance in direct proportion: double the voltage across a fixed resistor and you double the current. It is the first quantitative law of circuits, and nearly every electronics calculation starts here. The three rearrangements, V = IR, I = V/R, R = V/I, are the same relationship viewed from three sides.
The law holds exactly for ohmic materials, where resistance stays constant as voltage changes. Real components drift: a filament bulb's resistance climbs as it heats up, which is why bulbs draw a surge of current when first switched on. For most design work with resistors at modest power, the ideal law is accurate enough.
Worked Example
Where This Comes Up
Ohm's law is the daily arithmetic of electronics: choosing LED resistors, checking whether a fuse will blow, sizing wires so voltage drop stays small, and debugging dead circuits with a multimeter. Electricians use it on mains wiring, car mechanics on 12 V systems, and students on nearly every circuits homework problem.
Mind the milli prefixes: 20 mA is 0.02 A, and 4.7 kΞ© is 4,700 Ξ©. Mixing milliamps with amps is the most common source of answers that are off by a factor of 1,000. Also, resistance is not always what the color bands say once a resistor heats up, so for precision work measure it in circuit or derate generously.
Frequently Asked Questions
What are the units in Ohm's law?
Voltage in volts (V), current in amperes or amps (A), resistance in ohms (Ξ©). Power, when you need it, is in watts: P = V x I.
What is an ohmic material?
A material where current is proportional to voltage, giving a straight-line V-I graph. Most metals at constant temperature qualify. Diodes, filament bulbs, and semiconductors are non-ohmic: their resistance changes with voltage or temperature.
How do I also find the power?
Use P = V x I, or combine with Ohm's law: P = I^2R or P = V^2/R. A 12 Ξ© resistor carrying 2 A dissipates 2^2 x 12 = 48 W.
Can I use Ohm's law for AC circuits?
Only for the resistive part. AC circuits with capacitors or inductors need impedance, which includes phase shifts that simple resistance does not capture.
How do I pick a resistor to protect an LED?
Subtract the LED's forward voltage from the supply voltage, then divide by the desired current: R = (V_supply - V_LED) / I. For a 9 V supply, 2 V LED, and 20 mA: R = 7 / 0.02 = 350 Ξ©.