Ohm's Law Calculator · Practical guide
How to Find Resistance from Voltage and Current
R = V ÷ I — divide voltage by current and you have the resistance in ohms.
At a glance
The Formula
Ohm's Law rearranged for resistance reads R = V / I: divide the voltage across a component by the current flowing through it. A circuit element that has 12 volts across it while carrying 0.5 amps has a resistance of 12 ÷ 0.5 = 24 Ω.
Interactive tool
Ohm's Law Calculator
Enter your own values, then compare the result with the examples below.
Ohm's Law Results
Quick Reference: Common Voltage-Current Pairs
| Voltage | Current | Resistance | Power |
|---|---|---|---|
| 5 V | 0.1 A | 50 Ω | 0.5 W |
| 9 V | 0.3 A | 30 Ω | 2.7 W |
| 12 V | 0.5 A | 24 Ω | 6 W |
| 120 V | 10 A | 12 Ω | 1,200 W |
Measuring It in Practice
To find an unknown resistance without an ohmmeter, measure the voltage across the component with a multimeter and the current through it, then divide. This is how you identify an unmarked heating element or verify a suspect resistor under load. Keep units straight: if your current reads in milliamps, convert first — 12 V at 500 mA is 12 ÷ 0.5 = 24 Ω, not 0.024 Ω.
Power Comes Along Free
Once you know voltage and current, power is one more multiplication away: P = V × I. The 12 V, 0.5 A example dissipates 6 W. The calculator above does all of this at once — enter the voltage and current, and it returns resistance and power along with the formulas used.
Frequently Asked Questions
What units does R = V / I use?
Volts divided by amps gives ohms (Ω). If the current is in milliamps, dividing volts by milliamps gives kilohms — 10 V at 5 mA equals 2 kΩ.
Can resistance be negative or zero?
Ordinary components always have positive resistance. A reading near zero means a short circuit; an infinite reading means an open circuit. Some active electronic devices exhibit effective negative resistance, but that is a specialist topic.
Why does a light bulb's cold resistance differ from its operating resistance?
Resistance of metals rises with temperature. An incandescent filament measures far lower cold than the R = V / I value computed at full brightness, which is why bulbs usually fail at switch-on.
Does this formula work for AC circuits?
For purely resistive AC loads, yes, using RMS values. For circuits with capacitors or inductors, the equivalent quantity is impedance, which also depends on frequency.