LED Resistor Calculator
Find the current-limiting resistor for an LED — plus its nearest E12 size and power rating.
Nearest E12 value: 150 Ω · resistor power: 0.06 W. Pick a resistor rated above the dissipated power (e.g. a ¼ W part for 0.06 W).
The series resistor for an LED is R = (Vsupply − Vled) ÷ Iled. Driving a 2.0 V red LED at 20 mA from a 5 V supply needs R = (5 − 2.0) ÷ 0.02 = 150 Ω, which is already a standard E12 value. The resistor dissipates P = 3 × 0.02 = 0.06 W.
Why an LED needs a series resistor
An LED is a diode, not a resistor: past its forward voltage the current rises almost vertically, so connecting it straight across a supply lets the current run away and destroys it. A series resistor absorbs the leftover voltage — the difference between your supply and the LED’s forward drop — and pins the current to a safe value. The same idea, splitting a voltage across resistors, drives the voltage divider and follows straight from Ohm’s law.
Vsupply = supply voltage (V), Vled = LED forward voltage (V), Iled = LED current (A). Enter current in mA; the tool divides by 1000.
Worked example
Drive a red LED (forward voltage 2.0 V) at 20 mA from a 5 V supply:
- 1 Find the voltage across the resistor. Subtract the LED forward voltage from the supply: 5 V − 2.0 V = 3.0 V.
- 2 Convert the current to amps. Divide milliamps by 1000: 20 mA ÷ 1000 = 0.02 A.
- 3 Divide to get the resistance. R = 3.0 V ÷ 0.02 A = 150 Ω.
- 4 Round up to a standard value. 150 Ω is already an E12 value; otherwise pick the next standard size up.
- 5 Check the power rating. P = 3.0 V × 0.02 A = 0.06 W — well within a common ¼ W resistor.
Typical LED forward voltages by colour
Approximate ranges — always confirm against your LED’s datasheet, as they vary by chemistry and current.
| LED colour | Typical forward voltage |
|---|---|
| Red | ≈ 1.8–2.2 V |
| Yellow | ≈ 2.1 V |
| Green | ≈ 2.2 V |
| Blue / white | ≈ 3.0–3.4 V |
Rounding and power rating
Round up, not down. The exact figure rarely lands on a stocked value, so pick the next standard size up. A larger resistor means slightly less current — a dimmer but safer LED. Rounding down pushes more current through and can shorten the LED’s life.
Check the wattage. The resistor turns the leftover voltage into heat: P = (Vsupply − Vled) × Iled. Most through-hole resistors are rated ¼ W (0.25 W). At higher supply voltages or currents the dissipation climbs, so choose a resistor comfortably above the calculated power.