How is the LED resistor calculated?

R = (Vsupply − Vforward) / Iforward, with current in amps. The resistor drops the difference between the supply and the LED's forward voltage at the chosen current.

Why pick the next-higher standard resistor?

Choosing the next-higher E12 value keeps the actual current at or below your target, protecting the LED. A lower value would push more current through it.

Is anything uploaded?

No — the calculation runs entirely in your browser, nothing is sent anywhere.

LED Resistor Calculator

Series resistor for an LED at the right current.

LED resistor calculator

To drive an LED safely you add a series resistor that drops the difference between the supply voltage and the LED’s forward voltage at your chosen current: R = (Vsupply − Vf) / I.

Enter the supply voltage, the LED forward voltage and the target current to get the exact resistor, the nearest standard E12 part, the real current that part produces, and the power the resistor dissipates — all in your browser.