LED Resistor Calculator
Calculate the correct series resistor for one LED string. If you have multiple strings in parallel, use one resistor per string.
Tip: Typical LED currents are 5-20mA for indicator LEDs. High-power LEDs often need a constant-current driver instead of a resistor.
How this LED resistor calculator works
LEDs are current-driven components. That means you do not usually connect them directly to a power source without current limiting. A series resistor is the simplest way to limit current in low-power LED projects like breadboards, Arduino circuits, panel indicators, and hobby electronics.
This calculator uses the classic formula:
R = (Vs - Vf_total) / I
- R = resistor value in ohms
- Vs = supply voltage
- Vf_total = total forward voltage of LEDs in series
- I = desired LED current in amps
Step-by-step method
1) Add up LED forward voltage
If you place LEDs in series, their forward voltages add. For example, three white LEDs at 3.0V each give a total of around 9.0V.
2) Subtract from supply voltage
The remaining voltage appears across the resistor. This is the voltage that sets LED current.
3) Divide by target current
Convert milliamps to amps, then divide. If your result is not a standard resistor value, choose the nearest standard value (usually equal or slightly higher to stay safe).
4) Check resistor power
Resistors dissipate heat. Use P = I²R and choose a resistor wattage with margin (at least 2x is a good practice).
Quick examples
| Supply | LED Setup | Target Current | Ideal Resistor | Practical Choice |
|---|---|---|---|---|
| 5V USB | 1 red LED (2.0V) | 20mA | 150Ω | 150Ω, 1/4W |
| 9V battery | 1 green LED (2.2V) | 15mA | 453Ω | 470Ω, 1/4W |
| 12V supply | 3 white LEDs (3.0V each) | 20mA | 150Ω | 150Ω, 1/2W margin |
Common mistakes to avoid
- No resistor: LEDs can fail quickly due to overcurrent.
- One resistor for parallel LEDs: each parallel string should have its own resistor.
- Ignoring power rating: resistor may run hot if wattage is too low.
- Using typical Vf only: LED forward voltage changes with current, color, and temperature.
- Too little voltage headroom: if supply is close to LED total Vf, current becomes unstable.
When to use a constant-current driver instead
For high-power LEDs, long LED strips, or precision brightness control, a dedicated constant-current driver is better than a resistor. Drivers improve efficiency, reduce thermal drift, and protect LEDs more effectively.
FAQ
Can I use a bigger resistor than calculated?
Yes. A higher resistor lowers current and brightness, and usually increases LED life.
Can I use a smaller resistor?
Only with caution. Smaller resistance increases current, which can overheat or damage the LED.
Why does the calculator show “not enough voltage”?
Your supply voltage is less than (or equal to) the total LED forward voltage, leaving no usable voltage for the resistor. Reduce LEDs in series or raise supply voltage.
What resistor tolerance should I choose?
For most indicator projects, 5% is fine. If you need tighter current control, use 1% resistors and verify LED datasheet values.
Final tip
For reliable LED circuits: calculate current, choose a standard resistor, then confirm power dissipation with margin. This calculator gives you all of those values in one place so you can build faster and safer.