Ohm's law calculator
Fill in exactly two of the four boxes (voltage, current, resistance, power) and leave the other two empty. The calculator solves the rest with Ohm's law and the power formula. You can type prefixes: 4.7k means 4,700 and 20m means 0.02.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter two known values in the matching boxes, for example 12 in Voltage and 2 in Current.
- Leave the other two boxes empty.
- The two unknown values are shown in the result. Values use the prefix that keeps the number readable, such as mA or kΩ.
| Known |
Resistance R |
Current I |
Voltage V |
Power P |
| V and I |
V ÷ I |
|
|
V × I |
| V and R |
|
V ÷ R |
|
V² ÷ R |
| V and P |
V² ÷ P |
P ÷ V |
|
|
| I and R |
|
|
I × R |
I² × R |
| I and P |
P ÷ I² |
|
P ÷ I |
|
| R and P |
|
√(P ÷ R) |
√(P × R) |
|
Worked example
A 5 V supply across a 220 Ω resistor gives a current of 5 ÷ 220 = 0.02273 A, shown as 22.73 mA. The power is 5² ÷ 220 = 0.1136 W, shown as 113.6 mW, which a quarter-watt resistor handles comfortably.
Tips
- Keep resistor power below its rating, with a margin. A rule of thumb is to stay under half of the rating.
- For batteries and USB supplies, use the voltage under load, not the label.
Frequently asked questions
What is Ohm's law?
Ohm's law says voltage equals current times resistance, V = I × R. Combined with the power formula P = V × I it gives all the other forms, such as P = I² × R and P = V² ÷ R, which the calculator uses depending on the two values you enter.
What do m, k and M mean in the boxes?
They are SI prefixes. m is milli (0.001), k is kilo (1,000) and M is mega (1,000,000). Prefixes are case sensitive, so 5m is 0.005 and 5M is 5,000,000. You can also add µ or u for micro, n for nano, p for pico and G for giga.
How do I find the resistor for an LED?
Use the voltage across the resistor, supply minus the LED's forward voltage, and the current you want. For a 5 V supply, an LED dropping 2 V and 20 mA, the resistor sees 3 V, so enter 3 and 20m to get 150 Ω. Choose the next higher standard value.
Why does the calculator say two values are required?
Four quantities are linked by two equations, so any two determine the other two. With one value there are many answers, and with three or more the entries could contradict each other.
Does it work for AC circuits?
For resistive loads with RMS values for voltage and current, yes. Reactive loads such as motors and coils also have a power factor and reactance, which this tool does not include.