Ohm’s law is V = I × R: the voltage across a resistor equals the current through it times its resistance. Rearrange it for whichever you need, I = V ÷ R or R = V ÷ I, and the power it burns is P = V × I. That is most of practical electronics in one line. Solve for any of the three below.
Inputs
Result
10.0
mA · current
V = I × R, rearranged. Power is V × I.
Power dissipated
33 mW
The three forms
They are one equation, written for whatever you are missing. Know the current and the resistance and you want the voltage: V = I × R. Know the voltage and the resistance and you want the current: I = V ÷ R. Know the voltage and the current and you want the resistance: R = V ÷ I. Keep the units honest, volts and amps and ohms, and the arithmetic is exact. This calculator takes current in milliamps and converts for you.
Power comes with it
Any part carrying current at a voltage is dissipating power, P = V × I, and for a resistor that also equals I²R or V² ÷ R. It leaves as heat. A resistor rated for less than it dissipates runs hot, drifts, and eventually fails, so once you have the value, check the power and pick a part rated above it with margin. The resistor power calculator does that step.
From a real board
The status LED on the OTD L1.01 ESP32-S3 USB-C breakout is a plain Ohm’s-law problem (One Thousand Drones, L1.01 build guide). The 3.3 V rail drives a red LED that drops about 1.8 V, so the resistor sees the difference, 1.5 V. Aim for a gentle 3 mA and the resistor is R = 1.5 V ÷ 3 mA = 500 Ω, so you fit the nearest standard value and move on. Every current-limiting resistor on the board is the same three-line calculation. See the L1.01 build.
References
- Georg Ohm’s relation between voltage, current, and resistance, in any introductory circuits text.
- One Thousand Drones. ESP32-S3 USB-C breakout (L1.01), indicator LED. Build the board.