LED Resistor Calculator
Series resistor for any LED.
Enter your values
Enter the supply voltage in V.
Enter the led forward voltage in V.
Enter the led current in mA.
Enter the leds in series you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Resistor value
- 495 Ω
- Nearest E12 value
- 500 Ω
- Power dissipated
- 0.198 W
- Rating needed
- 0.25 W
Quick answer
The series resistor drops the difference between supply voltage and LED forward voltage.
R = (Vsupply − Vf) ÷ I
At a glance
| What it does | Series resistor for any LED. |
|---|---|
| Category | Engineering |
| Inputs needed | Supply voltage, LED forward voltage, LED current, LEDs in series |
| Main output | Resistor value |
| Formula | R = (Vsupply − Vf) ÷ I |
| Cost | Free — no sign-up, no download |
How to use the LED Resistor Calculator
- 1Enter the supply voltage.
- 2Enter LED forward voltage and desired current.
- 3Choose the next standard resistor above the result.
Inputs explained
- Supply voltage(V)
- Enter the supply voltage in V.
- LED forward voltage(V)
- Enter the led forward voltage in V.
- LED current(mA)
- Enter the led current in mA.
- LEDs in series
- Enter the leds in series you are working with.
Worked example
Using the values the calculator loads with:
Inputs
- Supply voltage12 V
- LED forward voltage2.1 V
- LED current20 mA
- LEDs in series1
Results
- Resistor value495 Ω
- Nearest E12 value500 Ω
- Power dissipated0.198 W
- Rating needed0.25 W
Frequently asked questions
What is typical Vf?
About 2 V for red, 3.2 V for blue and white.
Can I skip the resistor?
No — LEDs have no internal current limiting and will burn out.
What do I need to enter into the LED Resistor Calculator?
Just 4 values: supply voltage, led forward voltage, led current, leds in series. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the LED Resistor Calculator work out the answer?
The series resistor drops the difference between supply voltage and LED forward voltage. It applies the formula R = (Vsupply − Vf) ÷ I and shows the working so you can check each step by hand.
Is the LED Resistor Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.