How do I calculate an LED resistor?

Use R = (Vs - Vf) ÷ I for one LED. For multiple LEDs in series, use R = (Vs - Vf × N) ÷ I.

Why does an LED need a resistor?

An LED is a current-driven device. A small voltage change can cause a large current change, so a resistor is used to limit current from a voltage source.

What LED current should I use?

Small indicator LEDs are often designed around 5 mA to 20 mA, but the correct current comes from the LED datasheet and brightness requirement.

Can I connect LEDs in parallel with one resistor?

It is better to use one resistor per branch. Direct parallel LEDs can share current unevenly because forward voltage varies from LED to LED.

What resistor wattage should I choose?

Calculate resistor power using I²R and choose a resistor rating above the result. A 2× margin is a common starting point for small circuits.

What happens if supply voltage is lower than LED voltage?

The LED may not turn on properly. You need enough supply voltage to cover the total LED forward voltage plus resistor voltage drop.

Can I use this for high-power LEDs?

For high-power LEDs, a constant-current LED driver is usually better than a simple resistor because current and heat must be controlled more carefully.

Does LED forward voltage change?

Yes. Forward voltage changes with LED type, current, temperature, and manufacturing variation.

Engineering

LED Resistor Calculator

Calculate the current-limiting resistor for one LED, series LEDs, or parallel LED branches. Estimate resistor value, resistor wattage, LED power, and total circuit current.

LED color / forward voltage preset

Supply voltage (V)

LED forward voltage Vf

LED current (mA per branch)

LEDs in series per branch

Parallel branches

Series resistor per branch

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Minimum resistor wattage—

Total circuit current—

LED voltage total per branch—

Total LED power—

LED resistor formula

An LED normally needs a current-limiting resistor when connected to a voltage source. The resistor drops the extra voltage and limits current through the LED.

R = (Vs - Vf × N) ÷ I Resistor power = I² × R Total current = branch current × number of parallel branches

Use one resistor per parallel branch. Avoid putting LEDs directly in parallel with only one shared resistor, because small forward-voltage differences can make one branch take more current than the others.

Worked example

Supply voltage = 5 V White LED Vf = 3.2 V LED current = 20 mA R = (5 - 3.2) ÷ 0.020 R = 90 Ω

A nearby standard value such as 91 Ω or 100 Ω is normally selected. A slightly higher resistor value reduces LED current and brightness but usually improves reliability.

Common LED forward voltages

LED type	Typical Vf
Red / orange	1.8–2.2 V
Yellow / green	2.0–2.4 V
Blue / white	3.0–3.4 V
Infrared	1.1–1.5 V

Trusted references

For background on LED current limiting and forward-voltage behavior, see Mouser – calculating LED current limiting resistor and Vishay – example SMD LED datasheet with forward voltage curves.

Common questions

Use R = (Vs - Vf) ÷ I for one LED. For multiple LEDs in series, use R = (Vs - Vf × N) ÷ I.
An LED is a current-driven device. A small voltage change can cause a large current change, so a resistor is used to limit current from a voltage source.
Small indicator LEDs are often designed around 5 mA to 20 mA, but the correct current comes from the LED datasheet and brightness requirement.
It is better to use one resistor per branch. Direct parallel LEDs can share current unevenly because forward voltage varies from LED to LED.
Calculate resistor power using I²R and choose a resistor rating above the result. A 2× margin is a common starting point for small circuits.
The LED may not turn on properly. You need enough supply voltage to cover the total LED forward voltage plus resistor voltage drop.
For high-power LEDs, a constant-current LED driver is usually better than a simple resistor because current and heat must be controlled more carefully.
Yes. Forward voltage changes with LED type, current, temperature, and manufacturing variation.