LED Resistor Calculator
Calculate the current-limiting resistor for LEDs: exact ohms, nearest standard E24 value, and power rating.
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LED Resistor Calculator
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.
Frequently asked questions
How do I calculate the resistor for an LED?
Subtract the total LED forward voltage from the supply voltage and divide by the desired current: R = (Vs − n·Vf) ÷ If. One red LED (2.0 V) on 12 V at 20 mA needs (12 − 2) ÷ 0.02 = 500 Ω, rounded up to the 510 Ω standard value.
What is LED forward voltage?
It is the voltage an LED drops across itself while conducting, set by the semiconductor's color chemistry: about 2.0 V for red, 2.1 V for yellow, 3.2 V for green and white, and 3.3 V for blue. The calculator's color presets fill in these typical values.
Why round up to the next standard resistor value?
Resistors come in standard E24 steps, and rounding up slightly lowers the current — the safe direction for the LED. Rounding down would overdrive it, so the calculator picks the smallest E24 value at or above the exact resistance and shows the actual resulting current.
What wattage resistor do I need for an LED circuit?
At least twice the actual dissipation (P = V² ÷ R across the resistor), which is the margin this calculator recommends. A red LED on 5 V at 20 mA dissipates only about 0.06 W, but a single LED dropping 10 V from a 12 V supply burns nearly 0.2 W and warrants a half-watt part.
Does this calculator work offline without sending my values anywhere?
Yes. Everything is computed in your browser with JavaScript — no voltages or circuit details are transmitted, and once the page has loaded the math works without a connection.