LED Series Resistor Calculator
Every LED needs a current-limiting resistor or it burns out. Enter the supply voltage, the LED's forward voltage and its desired current — get the correct resistor value, the nearest standard value, and the power it must handle.
How to use this calculator
- Enter the supply voltage powering the circuit.
- Enter the LED's forward voltage and desired current (from its datasheet; 20 mA is typical).
- Use the nearest standard value shown — always round up to stay safe.
Formula used
The resistor drops the excess voltage the LED can't take. Forward voltage varies by colour (~1.8–2.2 V red, ~3.0–3.4 V blue/white). Standard current is 20 mA for indicator LEDs; the resistor also needs a power rating above (Vsupply − Vf) × I.
Example calculation
5 V supply, 2.0 V red LED, 20 mA: R = (5 − 2) ÷ 0.02 = 150 Ω exactly — a standard value.
It dissipates (3 V × 0.02 A) = 60 mW, so any 1/4 W resistor is fine.
Why an LED needs a resistor
LEDs are current-driven devices with almost no internal resistance once they're conducting — connect one directly across a supply above its forward voltage and the current shoots up uncontrolled, destroying it in an instant. The series resistor exists to limit that current, dropping the difference between the supply and the LED's forward voltage while setting the current via Ohm's law.
Two practical points: always round the resistor value up, since a slightly higher resistance means slightly less current — a touch dimmer but safe, whereas rounding down risks over-driving the LED. And check the resistor's power rating: it must dissipate the dropped voltage times the current as heat, which for typical indicator LEDs is tiny (a 1/4 W resistor suffices) but grows if the supply voltage is much higher than the LED needs.
Why use this calculator?
- Get the exact and nearest-standard resistor for any LED and supply.
- See the required power rating so the resistor doesn't cook.
- Rounds up automatically to protect the LED.
Frequently asked questions
What resistor do I need for an LED?
Subtract the LED's forward voltage from the supply, then divide by the LED current. For a 2 V red LED at 20 mA on 5 V: (5−2)/0.02 = 150 Ω. Enter your values above for any LED.
What happens without a current-limiting resistor?
The LED draws excessive current and burns out almost immediately, because it has very little resistance of its own to limit the current. The resistor is essential unless the LED has built-in current limiting (some do).
What forward voltage does my LED have?
It depends on colour: roughly 1.8–2.2 V for red/yellow, 3.0–3.4 V for blue/white/green. The exact figure is in the LED's datasheet. Using the right value matters — guessing too low makes the resistor undersized.