An LED resistor calculator with standard values
An LED must be driven by a controlled current, and the cheapest way to do that is a series resistor. This LED resistor calculator works out the resistance from your supply voltage, the LED forward voltage and the current you want, then rounds it to a real E12, E24 or E96 resistor value, shows the current you will actually get and the power the resistor has to dissipate.
It handles a single LED, several LEDs in one series string, several LEDs in parallel with a resistor each, and the shortcut of one shared resistor. A table also shows the same LEDs on other common supplies, so you can see the answer for 3.3 V, 5 V, 9 V, 12 V and 24 V at once.
The numbers are textbook Ohm’s-law arithmetic. Forward voltage changes with colour, current and temperature, so take it from the datasheet of the LED you are using, and treat the result as a design starting point to confirm on the bench.
How to use the LED Resistor Calculator
- Enter the supply voltage, the LED forward voltage and the current in milliamps.
- Set the number of LEDs and how they are wired: all in series, in parallel with a resistor each, or parallel with one shared resistor.
- Pick the resistor series you can buy: E24 is the usual hobby choice, E96 gives closer values.
- Read the standard value, the exact value, the actual LED current and the minimum power rating.
LED Resistor Calculator features
Standard values
Rounds up to the next E12, E24 or E96 value so the current never exceeds your target.
Series and parallel
Strings of LEDs, one resistor per LED, or one shared resistor, each with its own formula.
Wattage
Shows resistor dissipation and a rating with a 2× margin.
Other supplies
A table for 3.3, 5, 9, 12 and 24 V using the same LED settings.
The LED resistor formula
The resistor must drop the part of the supply that the LEDs do not use, at the LED current: R = (Vs − n × Vf) ÷ I. Example: a 12 V supply, a red LED of 2 V and 20 mA gives R = (12 − 2) ÷ 0.02 = 500 Ω. The next standard E24 value is 510 Ω, which gives 19.6 mA, and the resistor dissipates I² × R = 0.196 W, so a 0.5 W part leaves a safe margin.
For LEDs in series the forward voltages add, so three 2 V LEDs on 12 V leave 6 V across a 300 Ω resistor. For LEDs in parallel each needs its own resistor, because forward voltages differ slightly and the lowest one would otherwise hog the current. The supply must exceed the total forward voltage with some headroom, otherwise the LEDs will not reach full brightness and the calculator reports an error.
| LED colour (typical) | Forward voltage at 20 mA |
|---|---|
| Red | 1.8 – 2.2 V |
| Yellow / amber | 2.0 – 2.2 V |
| Green | 2.0 – 3.0 V |
| Blue / white | 2.8 – 3.4 V |
Choosing a value and a rating
- Rounding up is the safe direction: more resistance means slightly less current and a slightly dimmer LED, never an overload.
- A resistor’s power rating should be at least double the calculated dissipation to keep it cool.
- Driving an LED from a battery that sags, or from a supply that varies, changes the current; a constant-current driver is better when brightness matters.
- High-power LEDs are normally driven by a constant-current driver, not a resistor.
LED Resistor Calculator FAQ
How do I calculate the resistor for an LED?
Subtract the LED forward voltage from the supply voltage and divide by the LED current in amps. For 5 V, a 2 V LED and 20 mA that is (5 − 2) ÷ 0.02 = 150 Ω.
What resistor do I need for an LED on 12 V?
For a 2 V LED at 20 mA, 500 Ω, so use a 510 Ω E24 resistor. For a 3.2 V white LED at 20 mA it is 440 Ω, so use 470 Ω.
Can I put several LEDs on one resistor?
Yes in series, if the supply covers the sum of the forward voltages. In parallel it works but is not recommended, because unequal LEDs share the current unevenly.
What wattage resistor do I need?
Calculate I² × R and choose a rating at least twice that. Most indicator LEDs need only a 0.25 W resistor on 5 V.
Why does the calculator round up?
The next higher resistor value keeps the current at or below your target. The next value down is also shown, with the current it would give.
What if my supply is lower than the forward voltage?
Then the LED will not light properly. Use fewer LEDs in series, or a higher supply.