LED Resistor Calculator
Calculate the current-limiting resistor for LEDs: exact ohms, nearest standard E24 value, and power rating.
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20 mA suits most indicator LEDs.
Results
- Exact resistance
- 150 Ω
- Nearest standard value (E24, rounded up)
- 150 Ω
- Actual current with that resistor
- 20 mA
- Resistor dissipation
- 0.06 W
Use a resistor rated 0.125 W or more (≥ 2× dissipation).
R = (Vsupply − n × Vf) ÷ If. Rounding up to the standard value keeps current slightly below target, which is the safe direction. Forward voltage varies between parts — check the datasheet for precision work.
About LED Resistor Calculator
Enter the supply voltage, LED forward voltage (color presets included: red 2.0 V, yellow 2.1 V, green/white 3.2 V, blue 3.3 V), the target current, and how many LEDs are in series. You get the exact resistance R = (Vs − n·Vf) ÷ If, the nearest standard E24 value rounded up (the safe direction), the actual current with that resistor, and a power rating suggestion of at least twice the dissipation.
Common uses
- Wire an indicator LED to a 5 V or 12 V supply without frying it
- Run three LEDs in series from 12 V with one resistor
- Pick a resistor wattage that won't run hot
Example
Input
12 V supply · 3 red LEDs (2.0 V) · 20 mA
Output
300 Ω exact → 300 Ω E24 · 0.12 W → use ¼ W
Limitations
- Forward voltage varies between individual LEDs and with temperature — for tight current control use a constant-current driver instead.