LED Series Resistor Calculator
Size the current-limiting resistor for an LED or series string from supply voltage, forward voltage and current — with the nearest E24 value.
Resistor
150 Ω
Nearest E24: 150 Ω
Actual current
20 mA
Resistor power
60 mW
Min. rating
1/8 W
Total LED drop
2 V
R = (Vsupply − total Vf) / I. The nearest E24 value rounds up so current stays at or below target. Use 2× the dissipated power as the minimum resistor rating.
How to use
- Enter the supply voltage, the LED forward voltage (from its datasheet), and the desired current in mA.
- Set how many LEDs are wired in series (their forward voltages add).
- Read the exact resistor, the nearest E24 value, the resulting current and the minimum resistor wattage.
About this tool
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An LED needs a series resistor to limit its current; without one it draws whatever the supply allows and burns out. The resistor value is R = (Vsupply − Vforward) / I, where Vforward is the LED's forward voltage drop and I is the target current. This calculator handles a single LED or several in series (their forward voltages add up), then gives you the exact resistor, the nearest standard E24 value at or above it, the actual current that standard value produces, the power the resistor dissipates, and the minimum safe power rating (with 2× headroom). If the LED string needs more voltage than the supply provides, it tells you instead of returning a nonsensical value.
Frequently asked questions
What resistor do I need for an LED?
R = (Vsupply − Vforward) / I. For a typical red LED (Vf ≈ 2 V, I = 20 mA) on 5 V, that is (5 − 2) / 0.02 = 150 Ω.
What is the LED forward voltage?
The voltage an LED drops when conducting — roughly 1.8–2.2 V for red, 2–2.4 V for yellow/green, and 3–3.4 V for blue/white. Use the exact figure from the LED's datasheet for best accuracy.
Why is the suggested resistor larger than the calculated one?
Standard resistors come in fixed E24 steps. The tool rounds up to the nearest standard value so the current stays at or just below your target rather than exceeding it.
What power rating should the resistor have?
The resistor dissipates P = (Vsupply − Vforward) × I. Pick a rating at least double that for margin; the tool shows the minimum standard wattage (1/8 W, 1/4 W, etc.).
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