LED Resistor Calculator
Find the series resistor for an LED from supply voltage, forward voltage and current — with the nearest standard value, power dissipation and a circuit diagram.
510 Ω
exact 490 Ω · use ≥ ½ W resistor · dissipates 0.196 W · 9.8 V across the resistor
Why an LED needs a resistor
An LED is a diode, not a resistor: above its forward voltage the current rises almost vertically with voltage. Connect one straight across a supply and a tiny over-voltage means a huge over-current — the LED burns out in moments.
A series resistor fixes this by absorbing the difference between the supply and the LED's forward voltage, setting a defined current. From Ohm's law: R = (V_supply − n · V_f) / I_f, where n is the number of LEDs in series.
Typical forward voltages
| LED colour | Typical V_f | Typical I_f |
|---|---|---|
| Infrared | 1.2 – 1.6 V | 20 – 50 mA |
| Red | 1.8 – 2.2 V | 20 mA |
| Yellow / Green | 2.0 – 2.4 V | 20 mA |
| Blue / White | 3.0 – 3.4 V | 20 mA |
| UV | 3.3 – 3.8 V | 20 mA |
Choosing the real resistor
The exact ohm value rarely exists as a part, so pick the nearest standard (E-series) value. Rounding up gives slightly less current and a longer-lived, slightly dimmer LED — almost always the safe choice. Then check the power rating: the resistor dissipates P = (V_supply − n · V_f) · I_f, and a real resistor should be rated at roughly twice that.
Series vs. parallel LEDs
- Series: one resistor for the whole string and identical current through every LED — but the supply must exceed the sum of all forward voltages.
- Parallel: never share a single resistor across parallel LEDs — small V_f differences make one hog the current. Give each LED its own resistor.
- Headroom: keep at least ~20% of the supply across the resistor; too little and the current becomes very sensitive to V_f and temperature.