Tip: For multiple parallel LED strings, calculate one resistor per string.
Why you need a resistor for an LED
LEDs are current-driven devices. Unlike a basic incandescent bulb, an LED can draw too much current if connected directly to a voltage source. A small series resistor controls that current and protects the LED from overheating and failure.
This LED light resistor calculator helps you quickly find the right resistor value and a practical resistor power rating so your circuit is safe and reliable.
LED resistor formula
Core equation
R = (Vs - (Vf × N)) / I
- R = resistor value in ohms (Ω)
- Vs = supply voltage
- Vf = forward voltage of one LED
- N = number of LEDs in series
- I = desired LED current in amps (A)
Resistor power is estimated with: P = I² × R. In real projects, choose a resistor with a power rating at least 2× the calculated power for better heat margin.
How to use this calculator
- Enter your source voltage (for example, 5V, 9V, or 12V).
- Enter LED forward voltage (typical red is around 2.0V, white around 3.2V).
- Enter desired current in mA (common indicator LEDs use 5–20mA).
- Enter how many LEDs are placed in series with the resistor.
- Click Calculate Resistor to see exact and recommended values.
Typical LED forward voltages
| LED Color | Typical Forward Voltage (Vf) |
|---|---|
| Red | 1.8V – 2.2V |
| Yellow / Orange | 2.0V – 2.2V |
| Green (standard) | 2.0V – 2.4V |
| Blue | 3.0V – 3.4V |
| White | 3.0V – 3.4V |
Series vs. parallel LED wiring
Series chain
In series, voltages add up. One resistor is used for the chain. This is efficient and keeps current equal through each LED in the same string.
Parallel strings
For parallel wiring, each string should have its own resistor. Avoid sharing one resistor across different parallel branches; current can become uneven and damage LEDs.
Common mistakes to avoid
- Using supply voltage lower than total LED forward voltage.
- Forgetting to convert mA to A in manual calculations.
- Choosing the resistor wattage too close to the minimum.
- Using a single resistor for multiple parallel LED branches.
Quick example
If you have a 12V supply, one red LED at 2.0V, and target 20mA:
- Voltage across resistor = 12 - 2 = 10V
- Current = 20mA = 0.02A
- R = 10 / 0.02 = 500Ω
- Nearest common higher E12 value: 560Ω
Using 560Ω slightly reduces current, which is usually safer and extends LED life.
Final notes
This calculator is ideal for indicator LEDs and basic low-power LED projects. For high-power LEDs, always use a proper constant-current driver instead of a simple resistor-only method.