ElectronicsApril 12, 2026

LED Resistor Calculator Guide: How to Choose the Right Resistor

Q: How do I calculate the resistor value for an LED?

Use the formula: R = (V_supply − V_forward) / I_LED. Subtract the LED's forward voltage from the supply voltage, then divide by the desired current (typically 20mA for standard LEDs). For a 5V supply and red LED (2V forward, 20mA): R = (5 − 2) / 0.020 = 150 ohms.

Q: What happens if I don't use a resistor with an LED?

Without a current-limiting resistor, the LED will draw excessive current and burn out almost immediately. LEDs have very low internal resistance, so even a small voltage above the forward voltage causes a large current spike. The LED may appear to work briefly before failing, or it may fail instantly depending on the power supply.

Q: What is the forward voltage of different LED colors?

Red LEDs: 1.8-2.2V. Orange: 2.0-2.2V. Yellow: 2.0-2.2V. Green: 2.0-3.5V (varies by type). Blue: 3.0-3.5V. White: 3.0-3.5V. UV: 3.3-3.8V. Infrared: 1.2-1.5V. Always check the datasheet for exact values, as forward voltage varies by manufacturer.

Q: Can I use one resistor for multiple LEDs in series?

Yes. For LEDs in series, add up all the forward voltages and use the formula: R = (V_supply − sum of V_forward) / I_LED. The supply voltage must exceed the total forward voltage. For three red LEDs (2V each) on 12V: R = (12 − 6) / 0.020 = 300 ohms. All LEDs share the same current, so they must be the same type.

Q: What wattage resistor do I need for an LED?

Calculate power dissipation: P = I² × R, or P = (V_supply − V_forward) × I. For a 150-ohm resistor at 20mA: P = 0.020² × 150 = 0.06W. A standard 1/4W (0.25W) resistor handles this easily. As a safety margin, choose a resistor rated for at least twice the calculated power dissipation.

By The hakaru Team·Last updated March 2026

Quick Answer

*Formula: R = (V_supply − V_forward) / I_LED.
*Standard LED current: 20 mA (0.020 A). High-brightness LEDs vary.
*Forward voltage: Red ~2V, Green ~3V, Blue/White ~3.3V.
*Always use a resistor. Without one, the LED burns out instantly.

Use our free LED Resistor Calculator →

Why LEDs Need Resistors

An LED is a diode. Once the voltage across it reaches the forward voltage threshold, its internal resistance drops dramatically and current surges. Without something to limit that current, the LED draws as much as the power supply can deliver and destroys itself — often in under a second.

A resistor in series with the LED limits current to a safe value. It’s the most basic protection circuit in electronics, and it’s where every beginner should start.

The Formula: Ohm’s Law Applied

The resistor must drop the difference between the supply voltage and the LED’s forward voltage. The current through the resistor (and LED, since they’re in series) is the LED’s rated current.

R = (V_supply − V_forward) / I_LED

Example: 9V battery, white LED (3.2V forward), 20mA target current:

R = (9 − 3.2) / 0.020 = 5.8 / 0.020 = 290&ohm;. The nearest standard value is 300&ohm; (or 330&ohm; for a slight margin). Using 330&ohm; gives: I = 5.8 / 330 = 17.6mA. Slightly dimmer but safer.

Forward Voltage by LED Color

LED Color	Forward Voltage	Typical Current
Infrared	1.2–1.5V	20–50 mA
Red	1.8–2.2V	20 mA
Orange / Yellow	2.0–2.2V	20 mA
Green	2.0–3.5V	20 mA
Blue	3.0–3.5V	20 mA
White	3.0–3.5V	20 mA
UV	3.3–3.8V	20 mA

The variation exists because forward voltage depends on the semiconductor material. Red LEDs use AlGaAs or GaAsP (lower bandgap). Blue and white LEDs use InGaN (higher bandgap). Always check the datasheet — “green” LEDs can range from 2V to 3.5V depending on the technology.

Series LED Circuits

In a series circuit, all LEDs share the same current but their voltages add up. One resistor protects the entire chain. The formula becomes:

R = (V_supply − (V_f1 + V_f2 + V_f3 + ...)) / I_LED

The supply voltage must exceed the total forward voltage. Three blue LEDs (3.3V each) need at least 9.9V plus some headroom for the resistor. A 12V supply works: R = (12 − 9.9) / 0.020 = 105&ohm;, use 120&ohm;.

Parallel LED Circuits

In parallel, each LED should have its own resistor. Even LEDs of the same type have slightly different forward voltages, and without individual resistors, the LED with the lowest forward voltage hogs the current and burns out first. Then the next one takes over. Domino failure.

Calculate each branch independently using the standard formula. Total supply current = I_LED × number of LEDs.

Resistor Power Rating

The resistor converts excess voltage to heat. Calculate dissipated power: P = (V_supply − V_forward) × I_LED. For most indicator LEDs at 20mA, this is well under 0.1W. Standard 1/4W resistors handle it easily. For high-power LEDs drawing 350mA or more, power dissipation matters and you may need 1W or 2W resistors.

Calculate the perfect resistor for your LED circuit

Use our free LED Resistor Calculator →

Frequently Asked Questions

How do I calculate the resistor value for an LED?

R = (V_supply − V_forward) / I_LED. For a 5V supply and red LED (2V, 20mA): R = 3 / 0.020 = 150 ohms.

What happens if I don’t use a resistor with an LED?

The LED draws excessive current and burns out almost immediately. LEDs have very low internal resistance above the forward voltage threshold.

What is the forward voltage of different LED colors?

Red: 1.8–2.2V. Green: 2.0–3.5V. Blue/White: 3.0–3.5V. Always check the datasheet for exact values.

Can I use one resistor for multiple LEDs in series?

Yes. Add up all forward voltages: R = (V_supply − total V_forward) / I_LED. Supply voltage must exceed total forward voltage. LEDs must be the same type.

What wattage resistor do I need for an LED?

P = (V_supply − V_forward) × I_LED. For most 20mA LEDs, this is under 0.1W. A standard 1/4W resistor works fine. Choose 2× the calculated power for safety margin.