LED Resistor Calculator — series resistor for any LED
Size the current-limiting resistor for any LED, with forward-voltage presets per colour and support for series and parallel arrays.
Example: Driving a red LED at 20 mA from 5 V needs a 150 Ω resistor — use 150 Ω.
Formula
R = (V_supply − n × V_forward) / I_forwardWorked example
Driving a red LED at 20 mA from 5 V needs a 150 Ω resistor — use 150 Ω.
V_R = V_supply − n × V_f
V_R = 5 V − 1 × 2 V
V_R = 3 V
R = V_R / I_f
R = 3 V / 20 mA
R = 150 Ω
P_R = V_R × I_f
P_R = 3 V × 20 mA
P_R = 60 mW
That is 60% of the power burned as heat in the resistor.
Frequently asked questions
What resistor do I need for an LED on 5 V?
For a typical red LED (2.0 V forward) at 20 mA, you need (5 − 2.0) / 0.02 = 150 Ω. For a blue or white LED (3.2 V forward) it is (5 − 3.2) / 0.02 = 90 Ω, so you would fit 100 Ω. Always round up to the next stock value rather than down.
What happens if I use no resistor at all?
An LED has no meaningful internal resistance, so current is limited only by the supply. It rises until the die overheats, forward voltage drops, current rises further, and the part fails — often within seconds. The one exception is LEDs sold with a driver already built in.
Should I round the resistor up or down?
Up. A larger resistor means less current, which costs a little brightness. A smaller one means more current than you designed for, which costs LED lifetime. This calculator recommends the next E24 value at or above the ideal.
Can I put several LEDs on one resistor?
In series, yes — that is what the LED count field does, and it is the efficient arrangement because every LED sees identical current. In parallel it is a bad idea: small forward-voltage differences make one LED hog the current, run hottest, and fail first. Parallel strings each need their own resistor.
Why do my LEDs look different brightnesses?
Usually too little voltage across the resistor. When the supply barely exceeds the LEDs' combined forward voltage, normal part-to-part forward-voltage spread swings the current a lot. Either raise the supply or switch to a constant-current driver.
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