LED Resistor Calculator
The series resistor an LED needs, and the current the nearest value really gives.
What is LED Resistor?
An LED is a current device with a nearly fixed forward voltage, so something has to set the current — that is the series resistor's entire job. The formula is a subtraction and a division, and the two things that make it useful are the direction of the rounding and the power rating: a resistor below the calculated value overdrives the LED, and a resistor that is right in Ω is still wrong in watts if it cannot shed the heat.
How it works
The voltage the resistor must drop is the supply minus the forward voltage of each LED in the string, and the resistance is that voltage divided by the desired current. It is then rounded *up* to the nearest E12, E24 or E96 value by default — the safe direction, since a larger resistor passes less current — and the current that value actually produces is reported next to the target, along with the resistor's dissipation and the smallest standard rating with a 2× margin.
- Enter your input. Type or paste your input into the field above — the tool responds instantly.
- Review the result. Output updates live as you type. Everything runs entirely in your browser.
- Copy or download. Use the Copy button to grab the result, or Download to save it as a file.
Examples
Realistic sample
Type or paste any realistic input — the tool computes the result locally and instantly.
Common mistakes
Expecting a server round-trip
Nothing is uploaded — every tool in this category runs entirely in your browser.