What forward voltage should I use?

Red and yellow LEDs are typically 1.8–2.2 V. Green, blue, and white LEDs are usually 3.0–3.5 V. Check your LED's datasheet for the exact value.

What current should I use?

Standard 5 mm through-hole LEDs are rated for 20 mA maximum. A common value for a bright but conservative drive is 10–15 mA.

Should I round up or down to the nearest standard resistor value?

Round up to the next E12/E24 standard value to keep current at or below the rated maximum. Rounding down would exceed the LED's current rating.

LED Series Resistor Calculator

Calculate the correct series resistor for an LED. Enter supply voltage, LED forward voltage and desired current to get resistance in ohms and power dissipation.

By AbraCalc Editorial Team · Reviewed by AbraCalc Methodology Review · Last reviewed: 2026-06-25

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How to use this tool

Enter supply voltage (vs), led forward voltage (vf) and led current in the fields above.
Results update instantly as you type — or click Calculate.
Read your series resistance and the full breakdown beneath it.

An LED needs a series resistor to limit current. The formula is R = (Vs − Vf) / I, where Vs is supply voltage, Vf is the LED forward voltage drop, and I is the desired current in amps. Power dissipated by the resistor is P = (Vs − Vf) × I.

Formula

Series Resistance: R = (V_s − V_f) / I

Power Dissipated: P = (V_s − V_f) × I

Where V_s is supply voltage (V), V_f is LED forward voltage (V), and I is LED current (A, converted from mA input).

How it works

An LED requires a fixed forward voltage to conduct; any excess supply voltage must be dropped across a series resistor to limit current to the desired value. Ohm's law applied across that voltage drop gives the required resistance.

Power dissipated in the resistor is the product of the voltage drop and the current — energy that becomes heat, not light. The calculation assumes a steady DC supply and a single LED; for strings of LEDs the combined forward voltages must be summed before using this formula.

Worked example

Worked example — 5 V supply, 2 V LED, 20 mA

Inputs: V_s = 5 V, V_f = 2.0 V, I = 20 mA = 0.02 A.
Voltage drop across resistor = 5 − 2 = 3 V.
R = 3 V ÷ 0.02 A = 150 Ω.
P = 3 V × 0.02 A = 0.06 W.

Series Resistance: 150 Ω | Power Dissipated: 0.06 W

Key terms

Forward Voltage (V_f): The voltage drop across an LED when it is conducting in the forward direction. Typical values range from about 1.8 V (red) to 3.5 V (blue/white).
Forward Current: The current flowing through an LED in normal operation. Most standard LEDs are rated for 20 mA; exceeding this shortens LED life.
Series Resistor: A resistor placed in series with an LED to limit current and drop the surplus supply voltage, preventing the LED from burning out.
Power Dissipation: The rate at which energy is converted to heat in the resistor, measured in watts. Choose a resistor rated for at least twice the calculated dissipation.
Voltage Drop: The difference between supply voltage and LED forward voltage (V_s − V_f) that must be absorbed by the series resistor.

Frequently asked questions

What forward voltage should I use?: Red and yellow LEDs are typically 1.8–2.2 V. Green, blue, and white LEDs are usually 3.0–3.5 V. Check your LED's datasheet for the exact value.
What current should I use?: Standard 5 mm through-hole LEDs are rated for 20 mA maximum. A common value for a bright but conservative drive is 10–15 mA.
Should I round up or down to the nearest standard resistor value?: Round up to the next E12/E24 standard value to keep current at or below the rated maximum. Rounding down would exceed the LED's current rating.

References & sources

Wikipedia: LED circuit