LED Series Resistor Calculator
Use this tool to calculate the resistor value needed to safely power one LED string. If you have multiple parallel strings, use one resistor per string.
R = (Vs - (Vf × N)) / I
where Vs = supply voltage, Vf = LED forward voltage, N = number of LEDs in series, I = LED current in amps.
Why an LED resistor matters
LEDs are current-driven devices. Even a small increase in voltage can cause a large increase in current, which can overheat and permanently damage the LED. A resistor limits current and keeps the LED in a safe operating range.
If you are wiring LEDs to batteries, microcontrollers, wall adapters, or bench supplies, this is one of the most important design steps. A simple resistor dramatically improves reliability and LED life.
How this LED resistor calculator works
Step 1: Determine voltage left for the resistor
First, subtract total LED forward voltage from supply voltage:
- Total LED voltage = Vf × number of LEDs in series
- Resistor voltage drop = Vs − total LED voltage
Step 2: Convert current from mA to amps
Most LED datasheets and hobby examples use milliamps. Ohm's law uses amps. For example, 20 mA = 0.020 A.
Step 3: Apply Ohm's law
Resistor value is:
R = V / I
where V is the resistor voltage drop and I is LED current in amps.
Step 4: Check resistor power
Any resistor converts electrical energy to heat. Power dissipated in the resistor is:
P = V × I
A good rule is to choose a resistor rated for at least 2× the calculated power for cooler operation and better durability.
Typical LED forward voltages
| LED Color/Type | Typical Forward Voltage (Vf) | Common Current |
|---|---|---|
| Red | 1.8V – 2.2V | 10–20 mA |
| Yellow/Amber | 2.0V – 2.2V | 10–20 mA |
| Green (old style) | 2.0V – 2.4V | 10–20 mA |
| Green/Blue/White (modern) | 2.8V – 3.6V | 10–30 mA |
| High-power LEDs | Varies widely | 350 mA+ |
Example calculations
Example 1: 5V USB source, one red LED
- Vs = 5V
- Vf = 2.0V
- I = 20 mA = 0.020A
- R = (5 - 2) / 0.020 = 150Ω
A standard 150Ω resistor is perfect.
Example 2: 12V source, three white LEDs in series
- Vs = 12V
- Vf = 3.2V each, N = 3 → total LED drop = 9.6V
- Voltage across resistor = 12 - 9.6 = 2.4V
- I = 20mA = 0.020A
- R = 2.4 / 0.020 = 120Ω
Nearest common value: 120Ω. Power is 2.4 × 0.020 = 0.048W, so 1/4W resistor is comfortably safe.
Common mistakes to avoid
- No resistor at all: LEDs can fail quickly without current limiting.
- One resistor for many parallel LEDs: current can split unevenly. Use one resistor per branch.
- Ignoring worst-case Vf: real LEDs vary by part and temperature.
- Underrated resistor wattage: resistors running hot drift and fail sooner.
- Forgetting supply variation: a "12V" adapter may output more with light load.
Practical design tips
- If brightness is too high, increase resistor value slightly to reduce current.
- For indicator LEDs, 5–10 mA is often bright enough and improves lifetime.
- When battery-powered, designing for lower current can significantly extend runtime.
- If efficiency is critical, consider a constant-current LED driver instead of a resistor.
Final takeaway
A resistor is the easiest way to protect LEDs in low- to medium-power projects. Enter your supply voltage, LED forward voltage, series count, and target current into the calculator above. You'll get a safe resistor value, a nearest standard part, and a recommended power rating in seconds.