LED Resistor Calculator
Series resistor for any LED — colour presets, LEDs in series, next-up E24 value, actual current and required wattage.
If Vs − n·Vf is small (< ~1 V) the current becomes very sensitive to Vf spread between parts and temperature — drop one LED from the chain or use a proper current-regulating driver. For mains or high-power LEDs, a resistor wastes too much heat: use a constant-current driver instead.
LED Resistor Calculator computes the series resistor that sets an LED's current, snapped to a standard E24 value — free, instant and private in your browser. Beginners wiring their first LED and engineers adding indicators to any rail use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About LED Resistor Calculator
LED Resistor Calculator computes the series resistor that sets an LED's current, snapped to a standard E24 value using the standard engineering relation: R = (Vs − n·Vf)/If, then P = I²R for the wattage. Worked live: a red LED (2.0 V) on 5 V at 20 mA wants 150 Ω — the E24 step up from the exact 150 Ω. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use LED Resistor Calculator
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use LED Resistor Calculator?
- ✓Implements the real formula — R = (Vs − n·Vf)/If, then P = I²R for the wattage — with the substitution shown, not a black box
- ✓Built for beginners wiring their first LED and engineers adding indicators to any rail
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate led resistor?+
The series resistor that sets an LED's current, snapped to a standard E24 value follows R = (Vs − n·Vf)/If, then P = I²R for the wattage. For example, a red LED (2.0 V) on 5 V at 20 mA wants 150 Ω — the E24 step up from the exact 150 Ω. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
Can I skip the resistor if the supply is close to the LED voltage?+
No — an LED is not a resistor; below Vf almost no current flows and just above it current rises almost vertically. With only a small (Vs − Vf) margin the current becomes wildly sensitive to temperature and part spread. Use a resistor or a proper current source, always.
How bright is 20 mA vs 5 mA really?+
Modern indicator LEDs are clearly visible at 1–5 mA; the 20 mA habit dates from older parts. Brightness is roughly linear in current, but perception is logarithmic — quartering the current looks only somewhat dimmer and quarters the power.
Is the LED Resistor Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
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