LED Resistor Calculator
Enter supply voltage, LED forward voltage and current to get the series resistor and the nearest standard value.
Disclaimer: This calculator is provided for general informational and educational purposes only, on an “as is” basis and without any warranty of accuracy or fitness for a particular purpose. Results may contain errors — always verify independently before relying on them in real designs. PartAndStock accepts no liability for any loss or damage arising from use of this tool, including when embedded on third-party sites.
How to use
- 1Enter the supply voltage (e.g. 5 V or 12 V).
- 2Enter the LED forward voltage (see the color table below; red ≈ 2 V).
- 3Enter the LED current in mA (typically 20 mA).
- 4The required resistor, nearest standard value and power are calculated instantly.
How it works
Calculate the current-limiting series resistor needed to safely drive an LED, including power dissipation and the nearest E-series value.
Why a series resistor is needed
LEDs are driven by current, not voltage: once past their forward voltage, their current rises very quickly. An LED connected straight to a voltage source draws excessive current and burns out. A series resistor takes up the difference between the supply voltage and the LED's forward voltage, limiting the current to a safe level.
Choosing the right resistor power
The resistor dissipates power as heat: P = (V_supply − V_forward) × I. At small currents this is usually under 1/4 W, but for safety choose a resistor rated at least twice the calculated power. Using the nearest standard (E-series) value is also a practical approach.
Worked examples
- 5 V supply, red LED (V_forward = 2 V), 20 mA → R = (5 − 2) / 0.02 = 150 Ω, power = 0.06 W
- 12 V supply, white LED (V_forward = 3.2 V), 20 mA → R = (12 − 3.2) / 0.02 = 440 Ω (nearest standard 470 Ω)
Typical LED Forward Voltages (V_forward)
| LED color | Typical V_forward |
|---|---|
| Infrared (IR) | 1.2 – 1.6 V |
| Red | 1.8 – 2.2 V |
| Yellow / Orange | 2.0 – 2.2 V |
| Green | 2.0 – 3.0 V |
| Blue / White | 3.0 – 3.4 V |