LED Resistor Calculator
This LED resistor calculator finds the series resistor an LED needs from the supply voltage, the LED forward voltage, and the LED current. Enter the three values and the LED resistor calculator returns the resistor value, the nearest standard size, and the power it dissipates. It is the LED and resistor calculator you reach for before wiring an LED, so you pick a current-limiting resistor that protects the part.
Results
The LED Resistor Calculator is part of our Electronics collection. Browse more electronics to size, convert, and check your work alongside this tool.
What the LED resistor calculator does
An LED draws as much current as it can unless something limits it, and that something is a series resistor. This LED resistor calculator sizes that resistor so the LED runs at its rated current instead of burning out. Enter the supply voltage, the LED forward voltage, and the target current, and the LED and resistor calculator returns the resistor value plus the nearest standard part. It is the fastest way to calculate LED resistor value before you build a circuit.
How the LED resistor calculator sizes the resistor
The resistor drops the voltage the LED does not use, at the current you choose, which is Ohm's law:
The LED resistor calculator takes the supply voltage, subtracts the LED forward voltage, and divides by the current in amps. It also finds the power the resistor must handle, P = (Vsupply − Vforward) × I, so you pick a resistor rated above it. This is the current-limiting resistor calculator for LEDs in its simplest, most reliable form.
LED resistor calculator example
A 12 V supply, a red LED with a 2 V forward voltage, run at 20 mA. The LED resistor calculator works out:
The resistor dissipates about 0.2 W, so a quarter-watt part is marginal and a half-watt is safer. Enter your own values and the LED and resistor calculator returns the size and power to calculate LED resistor value in one step.
How to use the led and resistor calculator
Enter the supply voltage feeding the circuit, the forward voltage of your LED from its datasheet, and the current you want in milliamps. The LED resistor calculator subtracts the forward voltage and divides by the current to size the resistor. For a 12 V strip or panel, a 12v led resistor calculator uses the same steps; the led light resistor calculator only needs the correct forward voltage for your LED color.
Reading the LED resistor calculator result
The exact resistor value rarely matches a stock part, so the LED resistor calculator gives the nearest standard size. Rounding up slightly lowers the current and is the safe direction, since it protects the LED at a tiny cost in brightness. Check the power figure too: a resistor run near its rating gets hot, so pick one rated at least double. When you calculate LED current for a design, staying near the LED datasheet value keeps it bright without shortening its life.
When to use the LED resistor calculator
- Wiring a single LED with a current-limiting resistor calculator for LEDs
- Driving a 12 V LED with a 12v led resistor calculator
- Sizing resistors for strings with an led series resistor calculator
- Checking brightness by choosing where to calculate LED current
Common LED resistor calculator mistakes
- No resistor at all. Connecting an LED straight to a supply destroys it; always size a current-limiting resistor.
- Ignoring power. The LED resistor calculator shows the watts; a resistor run near its rating overheats.
- Wrong forward voltage. Blue and white LEDs need a higher forward voltage than red, so use the datasheet to calculate LED resistor value.
Tips for accurate results
- Take the LED forward voltage and current from the datasheet before using the LED resistor calculator.
- Round up to the nearest standard resistor for a safe current.
- Pick a resistor rated at least double the power shown.
Why an LED needs a current limiting resistor calculator for LEDs
An LED is not like a bulb; its current rises steeply once it passes the forward voltage, so connecting it straight across a supply lets the current run away and destroys it in an instant. The series resistor is what holds the current at a safe level, which is why a current-limiting resistor calculator for LEDs is the first tool you use. The resistor takes the difference between the supply and the LED forward voltage and, by Ohm's law, that voltage across the resistor sets the current. Change the supply and the current changes, which is why a 12v led resistor calculator gives a different value than a 5 V one for the same LED.
Rounding and the led series resistance calculator
Two practical points decide a good design. First, standard resistor values are spaced apart, so the LED resistor calculator rounds to the nearest stock part; round up to be safe, since a slightly larger resistor means slightly less current and a longer LED life. Second, watch the power rating: the resistor turns the surplus voltage into heat, and on a high supply that can exceed a quarter-watt part. For several LEDs, wiring them in series and sizing one resistor for the summed forward voltage is more efficient than one resistor each, while an led parallel resistor calculator handles the case where each branch needs its own limit. Either way, calculate LED current for the whole arrangement before you commit.
LED resistor chart
Series resistor for common LED colors at 20 mA on a 12 V supply, an led resistor chart from the calculator:
| LED color | Forward voltage | Resistor at 12 V, 20 mA |
|---|---|---|
| Red | 2.0 V | 500 Ω (470 Ω std) |
| Yellow | 2.1 V | 495 Ω (470 Ω std) |
| Green | 2.2 V | 490 Ω (470 Ω std) |
| Blue | 3.2 V | 440 Ω (470 Ω std) |
| White | 3.4 V | 430 Ω (430 Ω std) |
LED Resistor Calculator FAQ
How do I calculate an LED resistor value?
Subtract the LED forward voltage from the supply voltage and divide by the current. The LED resistor calculator does this and gives the nearest standard size to calculate LED resistor value quickly.
What current should I use?
Most indicator LEDs run at 10 to 20 mA; check the datasheet. Enter it and the LED resistor calculator sizes the current-limiting resistor to match.
Can I use it for 12 V LEDs?
Yes. Enter 12 V as the supply and the correct forward voltage, and the 12v led resistor calculator returns the series resistor for that circuit.
What about LEDs in series?
Add the forward voltages of the LEDs in the string, then size one resistor for the total. An led series resistor calculator uses this combined forward voltage.