What is LED forward voltage?

Forward voltage (Vf) is the voltage drop across the LED when it conducts current. It depends on the LED color: red/yellow ≈ 1.8–2.2 V, green/blue/white ≈ 2.8–3.5 V. Check your LED's datasheet for the exact value.

What current should I use for a standard LED?

Most 5 mm through-hole LEDs are rated for 20 mA maximum. Running them at 10–15 mA gives good brightness with a longer lifespan. High-brightness and SMD LEDs may have different ratings — always check the datasheet.

Why does the result show a standard resistor value?

Resistors are manufactured in standard series (E12, E24, E48, etc.). The calculator finds the nearest E24 value that is equal to or greater than the calculated resistance, which keeps current at or below your target — the safe direction.

What power rating resistor should I use?

The calculator shows the minimum power dissipated in the resistor. Always choose a resistor rated at least 2× that value for a safety margin. Common ratings: 1/8 W, 1/4 W, 1/2 W, 1 W.

Can I wire multiple LEDs with one resistor?

Yes — for LEDs in series, subtract the combined forward voltages from the supply voltage before calculating. For LEDs in parallel, each LED should have its own resistor for even current sharing.

What if supply voltage equals or is less than forward voltage?

The circuit cannot work — the supply cannot forward-bias the LED. You need a higher supply voltage or a boost converter.

LED Resistor Calculator

Calculate the correct series resistor value for an LED circuit given supply voltage, LED forward voltage, and desired current.

Circuit Parameters

Supply Voltage (V_s)

Common values: 3.3 V, 5 V, 9 V, 12 V

LED Forward Voltage (V_f)

Quick:

LED Current (I_f)

Typical range: 5–30 mA. Check your LED datasheet.

Results

Fill in the values above and click Calculate.

Formula Used

R = (V_s − V_f) ÷ I_f

P = I_f² × R

Where R is resistance in ohms, P is power in watts, I_f in amps.

Summary

This tool calculates the series resistor required to safely limit current through an LED. By applying Ohm's Law to the voltage drop across the resistor (supply voltage minus LED forward voltage), it finds the exact resistance and minimum power rating needed. Use it any time you wire an LED to a power source without a dedicated driver.

How it works

Enter the supply voltage of your circuit (e.g., 5 V for USB, 12 V for automotive).
Enter the LED forward voltage — found on your LED's datasheet (typically 1.8–3.5 V).
Enter the desired LED current in milliamps (common values: 10–20 mA for standard LEDs).
The calculator applies Ohm's Law: R = (Vs - Vf) / If.
It then recommends the nearest standard E24 resistor value and minimum power rating.

Use cases

Selecting a resistor before soldering an LED to a breadboard or PCB.
Verifying an existing circuit design before powering it on.
Quickly comparing resistor needs for 3.3 V, 5 V, and 12 V supply rails.
Teaching electronics — visualizing Ohm's Law in a real-world LED context.
Protecting high-brightness LEDs running at lower currents to extend lifespan.
Calculating resistors for multiple LED colors with different forward voltages.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu