How to Choose an LED Resistor
A current-limiting resistor keeps an ordinary indicator LED within a chosen operating current when it is used with a fixed-voltage supply. This guide shows the calculation, explains why the LED data sheet matters, and works through three examples. The numbers below are illustrations, not universal ratings for LEDs of a particular colour.
The three inputs
- Supply voltage (VS): the voltage across the complete LED-and-resistor branch.
- LED forward voltage (VF): the approximate drop across the LED at the intended current. Find the range in the specific LED data sheet; it varies with device and current.
- Target LED current (I): a value in amperes chosen within the LED’s specified limits and any limit of the circuit driving it. Brighter is not always better.
For one LED and one resistor in series, the resistor must drop the difference between supply and LED voltage. Ohm’s law then gives R = (VS − VF) ÷ I. Convert milliamps to amps before calculating: 10 mA = 0.010 A. If the supply voltage does not exceed the LED’s forward voltage by a useful margin, this simple method cannot regulate the current reliably.
Worked example: 5 V, 2 V LED, 10 mA
Assume a regulated 5 V supply, an LED data-sheet value of 2.0 V at the chosen operating point, and a target of 10 mA. The resistor sees 5.0 − 2.0 = 3.0 V. Its calculated resistance is 3.0 ÷ 0.010 = 300 Ω. Choosing the next higher common value, 330 Ω, gives an approximate current of 3.0 ÷ 330 = 9.1 mA. This is an estimate because the LED forward voltage and supply voltage can vary.
Also check the resistor’s power: P = (VS − VF) × I. At the estimated 9.1 mA it dissipates about 3.0 × 0.0091 = 0.027 W. A 0.25 W part has ample margin for this illustrative case, subject to the real operating conditions and component ratings.
Two more calculation examples
| Assumed supply | Assumed LED VF | Target | Calculated R | Example choice | Approximate current |
|---|---|---|---|---|---|
| 3.3 V | 2.0 V | 5 mA | 260 Ω | 270 Ω | 4.8 mA |
| 5.0 V | 3.2 V | 10 mA | 180 Ω | 180 Ω | 10 mA |
The 2.0 V and 3.2 V figures in the table are assumptions for practice. Replace them with the data for the actual LED, and check the worst-case supply and forward-voltage ranges. A higher resistor normally reduces current; a lower one increases it.
Common mistakes
- Using colour as a specification: two LEDs that look alike can have different forward voltages and current ratings. Use the part number and data sheet.
- Forgetting resistor power: a resistance value alone is not enough. Calculate dissipation and choose a suitable rated part with margin.
- Assuming a controller pin is a power supply: if an LED is controlled by a microcontroller, check that board’s output-voltage and pin-current limits separately. The example 5 V supply calculations do not certify a particular GPIO pin.
- Putting parallel LEDs on one resistor: their currents may not divide equally. Each branch needs its own appropriate current limiting arrangement.
- Applying this to high-power LEDs: LED strips and high-power emitters may need a dedicated current driver rather than this simple indicator-LED calculation.
Quick checklist
- Identify the exact LED and its forward-voltage and current data.
- Use the voltage of the actual source or driver and check its limits.
- Calculate R, then select a standard value that keeps current within the chosen limit.
- Calculate resistor power and consider the supply and LED tolerances.
For a refresher on the formula, read our Ohm’s law guide. If you are new to prototyping, the breadboard guide explains the layout before you work with components.
Further reading
Adafruit’s LED forward-voltage explanation and DigiKey’s LED resistor selection guide provide additional examples. Check the manufacturer data sheet for the LED you actually use.
