LED Resistor Calculator
Find the correct current-limiting resistor for any LED circuit.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
Can't find what you need?
Request a ToolHow to Use the LED Resistor Calculator
Every LED needs a current-limiting resistor to prevent it from drawing too much current and burning out. This calculator finds the correct resistor value using Ohm's Law and then rounds to the nearest E24 standard resistor series value you can actually buy.
- Select your LED type. Choose from common LED colors with pre-filled forward voltages, or select Custom to enter any forward voltage manually.
- Enter supply voltage. This is the voltage of your power source: 3.3 V for many microcontrollers, 5 V for USB, 9 V or 12 V for typical bench supplies.
- Set desired current. Most standard indicator LEDs run at 20 mA. High-brightness LEDs may use 10 mA or less. Check your LED datasheet for the rated forward current.
- Read the results. The calculator shows the exact resistance needed, the nearest E24 series value to buy, and the power the resistor will dissipate.
About LED Resistor Calculations
The formula is: R = (Vs - Vf) / I, where Vs is supply voltage, Vf is LED forward voltage, and I is current in amps. For example, running a red LED (Vf = 2.0 V) from 5 V at 20 mA: R = (5 - 2.0) / 0.02 = 150 Ω. The E24 series nearest to 150 Ω is exactly 150 Ω.
Power dissipation in the resistor is P = (Vs - Vf)² / R. Always use a resistor rated for at least twice the calculated power. For most single-LED circuits the power is well under 100 mW, so a standard 1/4 W resistor works fine.
Frequently Asked Questions
What is the forward voltage of a standard LED?
Forward voltage varies by LED color due to differences in semiconductor materials. Red and yellow LEDs typically have a forward voltage of around 2.0 V. Green, blue, and white LEDs are usually 3.0 to 3.4 V. Infrared LEDs are often 1.2 to 1.5 V. Always check your LED's datasheet for the exact value at your intended operating current.
What is the E24 resistor series?
The E24 series is a standard set of 24 preferred resistor values per decade (e.g., 10, 11, 12, 13, 15, 16, 18, 20... up to 91, then repeating with 100, 110...). These are the values most commonly stocked by electronics suppliers. Using the nearest E24 value means you can find the resistor at any parts store while keeping the current within a few percent of your target.
Can I use one resistor for multiple LEDs in series?
Yes. For LEDs in series, subtract the sum of all forward voltages from the supply voltage, then divide by the current. For three red LEDs (Vf = 2.0 V each) from a 12 V supply at 20 mA: R = (12 - 6.0) / 0.02 = 300 Ω. Make sure the supply voltage is high enough to exceed the sum of all forward voltages, with at least 1 to 2 V remaining for the resistor.
What wattage resistor do I need for an LED circuit?
For most single-LED circuits running at 20 mA, the power dissipation is well under 100 mW, and a standard 1/4 W (0.25 W) resistor is more than adequate. This calculator shows the exact power in milliwatts using the nearest standard resistor value. As a rule, choose a resistor rated at least twice the calculated dissipation to run cool and extend its life.