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Ohm's law calculator

Pick what you want to find, then type the other two. Ohm's law, V = I × R, gives the third, and the power each part turns into heat comes along with it.

Across the part.

4.7k = 4,700 Ω.

Current: I = V ÷ R

20 mA
Voltage
9 V
Current
20 mA
Resistance
450 Ω
Power, P = V × I
180 mW

The three forms

V = I × R
  • I = V over R: the current, from the voltage across a resistor and its resistance
  • R = V over I: the resistance that gives a current at a voltage

Volts, milliamps and kilohms fit together without any zeros to count: V = mA × kΩ. So 9 V across 1 kΩ is 9 mA.

A worked example

A 9 V battery sits across a 450 Ω resistor. The current is:

I = 9 V over 450 Ω = 0.02 A = 20 mA

and the resistor turns P = V × I = 9 × 0.02 = 0.18 W into heat. That's most of a ¼ W resistor's rating, so a ½ W part is the safer choice.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Ohm's law” is lesson 5 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 4 before it first.

Or start from the very first lesson, no account needed

The ideas behind it

Questions people ask

What is Ohm's law?

Voltage equals current times resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. It holds for resistors and wires, not for LEDs or diodes.

How do I type milliamps or kilohms?

Use the letter for the prefix: 20m is 20 mA (0.02 A), 4.7k or 4k7 is 4,700 Ω, 1M is a million ohms. You can leave the unit off.

How do I work out the power too?

The calculator shows it under the answer: P = V × I. Keep a resistor at half its power rating or less, so a common ¼ W part should burn no more than about 0.125 W.

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