LED Resistor Calculator
Enter the supply voltage, the LED forward voltage and the current you want. You get the exact value and the next standard resistors up.
Typical forward voltages: red 1.8 to 2.2 V, green 2.0 to 3.2 V, blue and white 2.8 to 3.4 V. Check the data sheet for your LED.
Worked examples
Red LED on 5 V
5 V supply, 2 V LED, 20 mA
150 ohms
(5 - 2) / 0.02.
Green LED on 9 V
9 V supply, 2.1 V LED, 20 mA
345 ohms, so 360 ohms (E24) or 390 ohms (E12)
(9 - 2.1) / 0.02.
About LED Resistor Calculator
An LED needs its current limited, or it will draw too much and burn out within seconds. The standard fix is a resistor in series. This calculator works out the value from your supply voltage, the LED's forward voltage and the current you want, then rounds up to the next standard resistor so the current stays at or below your target.
You can put several LEDs in series on one resistor by entering their number, and the page adds up their forward voltages. It shows the exact value, the next value in the E24 and E12 series, the current each of those would give, and the power the resistor will have to dissipate with a suggested rating at twice that. It also warns you when the supply cannot light the number of LEDs you chose.
How to use LED Resistor Calculator
- Enter the supply voltage, for example 5 volts from a USB port or 12 volts from a battery.
- Enter the forward voltage and the current for the LED, from its data sheet, such as 2 volts and 20 milliamps.
- If you have several LEDs in series, enter how many.
- Read the exact resistance, the standard values to buy and the current each would give.
Common uses
- Wiring an indicator LED to a 5 volt or 12 volt supply
- Choosing a resistor for a string of LEDs in series
- Dimming an LED by running it at a lower current
- Learning how to protect a diode in a first electronics project
Frequently asked questions
What is the LED resistor formula?
The resistance equals the supply voltage minus the total LED forward voltage, divided by the current you want. For 5 volts, a 2 volt LED and 20 milliamps that is 150 ohms.
Why round up to a standard value?
Resistors are sold in preferred values, so the exact answer is often not available. Rounding up gives slightly less current than the target, which is safer for the LED and still bright.
What forward voltage should I use?
It depends on the colour and type. Red is usually 1.8 to 2.2 volts, green 2.0 to 3.2, and blue and white 2.8 to 3.4. The data sheet gives the exact figure.
Can I wire LEDs in parallel on one resistor?
It is better not to. Small differences between LEDs make them share the current unevenly. Use one resistor per LED, or put the LEDs in series.