How to use the Ohm's Law Calculator
- Choose which two values you know: voltage and current, voltage and resistance, or current and resistance.
- Enter those two values in the matching fields. The third field is ignored.
- Read the resistance, voltage, current and power.
- Use the power figure to check that parts and wires can handle the load.
Formula used
V = I × R, I = V ÷ R, R = V ÷ I. Power P = V × I = I² × R = V² ÷ R. Only the two values matching your choice are used.
Worked example
Here is the Ohm's Law Calculator worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| I know | Voltage and current |
| Voltage (volts) | 12 |
| Current (amps) | 2 |
| Resistance (ohms) | 6 |
| Result | |
| Resistance (Ω) | 6 |
| Voltage (V) | 12 |
| Current (A) | 2 |
| Power (W) | 24 |
The formulas
Ohm’s law is V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms. Rearranged, I = V ÷ R and R = V ÷ I. With 12 V and 2 A, the resistance is 12 ÷ 2 = 6 Ω. Power is P = V × I, so that circuit uses 12 × 2 = 24 W.
Using voltage and resistance instead: 12 V across 6 Ω gives a current of 2 A. Using current and resistance: 2 A through 6 Ω gives 12 V.
Power relationships
Combining the formulas gives P = I² × R and P = V² ÷ R. A 100 Ω resistor with 5 V across it carries 0.05 A and dissipates 0.25 W. Doubling the voltage across the same resistor doubles the current and quadruples the power: at 10 V the power is 1 W.
Resistors have a power rating, such as 0.25 W or 0.5 W, and they overheat if the power exceeds it. Choose one with a comfortable margin.
Practical examples
An LED that needs 20 mA from a 5 V supply, with an LED drop of 2 V, needs a resistor of (5 − 2) ÷ 0.02 = 150 Ω, which you can check here with 3 V and 0.02 A. A 60 W bulb on a 230 V supply draws about 0.26 A, and a 1,500 W heater on 120 V draws 12.5 A.
Ohm’s law helps choose resistors, fuses and wire sizes, and understand why devices get warm. Always follow electrical safety rules and local regulations, and have mains work done by a qualified person.
Limits of the law
Ohm’s law holds for resistors and similar parts whose resistance stays constant. Diodes, LEDs, transistors and lamps do not follow it in a simple way, because their resistance changes with voltage and temperature. In AC circuits with coils or capacitors, you use impedance and the power factor instead of just resistance.
For circuits with several resistors, first find the combined resistance (add them in series, or add the reciprocals in parallel) and then apply the law.
Frequently asked questions
Which two values are used?
Only the pair you choose. The third field is ignored, and the calculator shows the value that follows from the pair.
What is the formula for power?
P = V × I, which also equals I² × R and V² ÷ R.
Does it work for AC circuits?
For purely resistive loads, yes, using RMS values. Circuits with coils or capacitors need impedance and power factor.
What are the units?
Volts for voltage, amps for current, ohms for resistance and watts for power.
How do I convert milliamps?
Divide by 1,000. 20 mA is 0.02 A.
Why is zero not allowed for the current or resistance?
Because the formulas divide by them, and zero would give an undefined result.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.