About the LED Resistor Calculator
An LED must have a resistor in series, otherwise it takes too much current and burns out. This calculator finds the resistor from the supply voltage, the type of LED and the current you want. It rounds up to the next standard E12 value, tells you the actual current with that resistor and the power the resistor must handle. It also covers several LEDs in series. Type the supply voltage, the LED colour, the current and the number of LEDs and the LED resistor appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the LED Resistor Calculator
- 1Enter the supply voltage, the LED colour, the current and the number of LEDs.
- 2Pick the units or options from the menus if the LED resistor tool offers them.
- 3Read the LED resistor in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the LED resistor
Anyone building a torch, a model, a dashboard indicator or a project on a breadboard needs this sum. It saves a trip through a datasheet, since most LEDs need 10 to 20 mA and the typical forward voltage depends only on the colour.
The tool rounds up on purpose: a slightly larger resistor gives a slightly dimmer LED and a longer life. Lower the current to 5 or 10 mA for indicator lights, which still look bright on modern LEDs.
How the LED resistor is worked out
Resistor = (supply voltage − total LED voltage) ÷ current. The LED voltage is about 1.8 V for red, 2.0 V for yellow, 2.1 V for green, 3.0 to 3.2 V for blue and white. A red LED on a 9 V supply at 20 mA needs (9 − 1.8) ÷ 0.02 = 360 Ω. The next standard value up is 390 Ω, which gives 18.5 mA and dissipates 0.156 W. For three white LEDs in series on 12 V the answer is 120 Ω.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Five LED colours plus custom
- Rounds up to E12
- Several LEDs in series
Mistakes to avoid with LED resistor
If the supply is lower than the LED voltage the LED will not light, and the tool says so. With several LEDs in series, add their voltages. Do not put LEDs in parallel on one resistor; give each branch its own resistor, because their voltages differ slightly.
On phone and computer
On a phone, type into the boxes one after another and read the LED resistor underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the LED resistor move. The led resistor calculator works the same on both. Once you have a value, read the colour bands with Resistor Color Code Calculator, check the current with Ohm's Law Calculator, and use Resistor Calculator for series and parallel combinations. Voltage Divider Calculator helps when you need a different supply level.
Features
- Round up, not down
- Add LED voltages in series
- Check power rating
- Clear messages when the LED resistor inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate LED resistor?
Resistor = (supply voltage − total LED voltage) ÷ current. The LED voltage is about 1.8 V for red, 2.0 V for yellow, 2.1 V for green, 3.0 to 3.2 V for blue and white. A red LED on a 9 V supply at 20 mA needs (9 − 1.8) ÷ 0.02 = 360 Ω. The next standard value up is 390 Ω, which gives 18.5 mA and dissipates 0.156 W. For three white LEDs in series on 12 V the answer is 120 Ω.
What is the formula for LED resistor?
Resistor = (supply voltage − total LED voltage) ÷ current. The LED voltage is about 1.8 V for red, 2.0 V for yellow, 2.1 V for green, 3.0 to 3.2 V for blue and white. The led resistor calculator applies it exactly and rounds only the displayed answer to six significant figures.
Why do I need a resistor with an LED?
An LED has almost no resistance once it conducts, so the current would rise until it fails. The resistor sets a safe current.
What current should I use?
20 mA is a common maximum for standard 5 mm LEDs; 10 mA is plenty for most indicators. Check the datasheet for high-power types.
What power rating should the resistor have?
Choose at least double the calculated power. A quarter-watt resistor is fine for most small LEDs.
Is this led resistor calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.