LED Resistor Calculator
Find the series resistor needed to run one or more LEDs at the right current, with the nearest standard E24 value and its power rating.
- R = (Vs − n × Vf) ÷ I = (24 − 2) ÷ 0.01 = 2,200 Ω
- P = I² × R = 220 mW; rate the resistor at twice this
LED Series Resistor Formula
P = I² × R
Typical forward voltages: red about 2.0 V, green and yellow 2.1 V, blue and white 3.0 to 3.3 V.
Worked example
A red LED (2.0 V) at 10 mA on 24 V: R = (24 − 2) ÷ 0.01 = 2,200 Ω, which is a standard value. It dissipates 220 mW, so use a 0.5 W resistor.
Frequently Asked Questions
What resistor do I need for an LED on 12 V?
For a red LED at 20 mA: (12 − 2) ÷ 0.02 = 500 Ω, so use 510 Ω, rated at least 0.25 W.
Why does an LED need a resistor?
An LED has almost no resistance once it conducts. Without a current limit, the current rises until the LED fails.
Can I use one resistor for LEDs in parallel?
It is not recommended. Small differences in forward voltage make one LED take most of the current. Use one resistor per LED or string.
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