About this tool
Work out the series resistor needed to drive an LED safely from the supply voltage, LED forward voltage and forward current. It also suggests the nearest standard E12 value and the power the resistor dissipates — all in your browser.
Frequently asked questions
How is the resistor calculated?
It uses R = (Vsupply − Vforward) / I, where I is the forward current in amperes (the milliamp input divided by 1000).
What is the E12 value?
E12 is a standard series of preferred resistor values. The calculator rounds the exact result to the closest real-world resistor you can actually buy.