Electronics / Circuits
Ohm's Law Calculator
Enter any two of voltage, current, resistance, and power to find the other two, with SI prefixes such as 4.7k, 220m, and 2M2 understood and shown in the results.
Ohm's Law Calculator: Ohm's law says the current through a resistor equals the voltage across it divided by its resistance (I = V ÷ R), and the power it dissipates is the voltage times the current (P = V × I). Combining the two gives every value from any pair. For example, 12 V across 4.7 kΩ drives 2.553 mA and dissipates 30.64 mW. Runs 100% locally in your browser with zero server file uploads.
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Fill in any two values. Prefixes work: 4.7k, 220m, 2M2, 100µ (or 100u).
V = I × R · P = V × I = I² × R = V² ÷ R
A worked example: an LED resistor
A red LED drops about 2 V and is happy at 10 mA. From a 5 V supply, the resistor must drop the other 3 V, so R = 3 V ÷ 0.01 A = 300 Ω; the nearest standard value above is 330 Ω. It dissipates 3 V × 0.01 A = 30 mW, well within a 0.25 W rating.
Find the colour bands for the resistor you choose with the resistor colour code tool.
When Ohm's law does not apply
It holds for ordinary resistors at a steady temperature. Diodes, LEDs, lamps whose filament heats up, and motors do not have a fixed resistance, so their current must be read from their datasheet curves; for alternating current with coils or capacitors, impedance replaces resistance.
How to use it
- Type any two values, using prefixes like k, M, m, or µ if you like.
- Read the other two values, worked out with Ohm's law and the power formula.
- Clear the fields to start a new calculation.
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Frequently asked questions
What do the prefixes mean?
p is pico (10⁻¹²), n nano (10⁻⁹), µ or u micro (10⁻⁶), m milli (10⁻³), k kilo (10³), M mega (10⁶), and G giga (10⁹). Lower-case m and upper-case M differ by a factor of a billion.
What does 4k7 mean?
Resistor notation that puts the prefix where the decimal point goes: 4k7 is 4.7 kΩ, 2M2 is 2.2 MΩ, and 4R7 is 4.7 Ω. It avoids losing a decimal point on a printed schematic.
How do I choose a resistor's power rating?
Pick one rated for at least twice the power it will dissipate; common through-hole resistors are rated at 0.25 W.
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