How Ohm's law works
Voltage is the electrical "push," current is the flow of charge, and resistance is how strongly a component resists that flow. Ohm's law ties the three together: for a given resistance, doubling the voltage doubles the current.
Power, measured in watts, is how much energy the component uses or turns into heat each second. Because power depends on voltage and current, and Ohm's law links those to resistance, knowing any two of the four values lets you find the other two.
The formula
P = V × I P = I² × R P = V² ÷ R
I = √(P ÷ R) V = √(P × R)
Work in base units (volts, amps, ohms, watts), then add prefixes back at the end. The calculator does both steps for you.
All twelve rearrangements
| To find | From V and I | From V and R | From I and R | From P and another |
|---|---|---|---|---|
| V | — | — | I × R | P ÷ I or √(P × R) |
| I | — | V ÷ R | — | P ÷ V or √(P ÷ R) |
| R | V ÷ I | — | — | V² ÷ P or P ÷ I² |
| P | V × I | V² ÷ R | I² × R | — |
Example: a 240 Ω resistor on 12 V
You connect a 240 Ω resistor across a 12 V supply (the default values).
| Step | Result |
|---|---|
| Current: I = V ÷ R = 12 ÷ 240 | 0.05 A = 50 mA |
| Power: P = V² ÷ R = 144 ÷ 240 | 0.6 W = 600 mW |
| Check: P = V × I = 12 × 0.05 | 0.6 W |
A standard 1/4 W (250 mW) resistor would run far too hot here. Choose one rated 1 W or more.
Common mistakes
- Forgetting prefixes. 12 V ÷ 240 kΩ is 50 µA, not 50 mA. Convert kΩ and mA to base units before dividing.
- Using Ohm's law on an LED. LEDs have a nearly fixed voltage drop. For a series resistor, use R = (supply voltage − LED voltage) ÷ current.
- Ignoring power ratings. The right resistance can still burn out if its wattage rating is too low.
- Mixing up AC and DC. For mains AC, use RMS voltage and current (like 120 V); the formulas then work for resistive loads such as heaters.
Frequently asked questions
What is Ohm's law?
Ohm's law says the voltage across a resistor equals the current through it times its resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. It holds for ordinary resistors at a steady temperature; components like LEDs and diodes don't follow it.
How do I calculate electrical power?
P = V × I, voltage times current, gives power in watts. Combining it with Ohm's law gives two more forms: P = I² × R and P = V² ÷ R. Use whichever matches the values you know.
What current flows through a 240 Ω resistor on 12 V?
I = 12 ÷ 240 = 0.05 A, which is 50 mA. The resistor dissipates P = 12 × 0.05 = 0.6 W, so a common 1/4 W resistor would overheat; you'd want a 1 W part for a safe margin.
What do the prefixes mA, kΩ and MΩ mean?
They're metric multipliers. m (milli) is one thousandth, µ (micro) one millionth, k (kilo) one thousand and M (mega) one million. So 50 mA is 0.05 A, 4.7 kΩ is 4,700 Ω and 1 MΩ is 1,000,000 Ω.
How do I choose a resistor's power rating?
Calculate the power it will dissipate with P = I² × R or P = V² ÷ R, then pick a resistor rated for at least double that. Running a resistor near its limit makes it hot and shortens its life.
Rates and figures last checked October 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.