LED Resistor Calculator
Current-limiting resistor value for an LED circuit.
Enter your values
Enter the supply voltage in V.
Enter the led forward voltage in V.
Enter the desired led current in mA.
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
- 350 Ω
- Resistor power rating
- 0.28 W (2x safety margin)
- Actual power dissipated
- 0.14 W
Quick answer
Resistor R = (supply voltage - LED forward voltage) / desired current.
R = (Vs - Vf) / I
At a glance
| What it does | Current-limiting resistor value for an LED circuit. |
|---|---|
| Category | Engineering |
| Inputs needed | Supply voltage, LED forward voltage, Desired LED current |
| Main output | Resistor value |
| Formula | R = (Vs - Vf) / I |
| Cost | Free — no sign-up, no download |
How to use the LED Resistor Calculator
- 1Enter your supply voltage and the LED's forward voltage (check the datasheet, typically 1.8-3.3V).
- 2Enter the desired LED current, commonly 20mA for standard indicator LEDs.
- 3Round up to the nearest standard resistor value and pick a power rating with margin.
Inputs explained
- Supply voltage(V)
- Enter the supply voltage in V.
- LED forward voltage(V)
- Enter the led forward voltage in V.
- Desired LED current(mA)
- Enter the desired led current in mA.
Worked example
Using the values the calculator loads with:
Inputs
- Supply voltage9 V
- LED forward voltage2 V
- Desired LED current20 mA
Results
- Resistor value350 Ω
- Resistor power rating0.28 W (2x safety margin)
- Actual power dissipated0.14 W
Frequently asked questions
What if I use the wrong resistor value?
Too low burns out the LED quickly; too high makes it dim or fail to light.
Why double the power rating?
It keeps the resistor running cool, extending its life and improving reliability.
What do I need to enter into the LED Resistor Calculator?
Just 3 values: supply voltage, led forward voltage, desired led current. 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?
Resistor R = (supply voltage - LED forward voltage) / desired current. It applies the formula R = (Vs - 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.