LED Resistor Calculator
Calculate the correct current-limiting resistor for any LED circuit. Enter supply voltage, LED forward voltage, desired current and number of LEDs in series — get resistor value and power dissipation instantly.
Last updated: May 2026
Enter supply voltage, LED Vf, current and LED count to calculate.
R = (Vs − N × Vf) ÷ If · P = If² × R
Device presets
LED current limiting: why it matters
LEDs are current-controlled devices. Without a resistor (or constant-current driver), the forward voltage drop is almost flat — a small rise in supply voltage causes a large rise in current, quickly destroying the LED. The resistor drops the excess voltage as heat, keeping current constant at the rated value.
The formula: R = (Vs − N × Vf) ÷ If. The numerator is the voltage left for the resistor after the LEDs claim their share. Divide by the target current to get resistance.
Common LED forward voltage by colour
| Colour | Typical Vf | Max If |
|---|---|---|
| Infrared | 1.2–1.8 V | 50–100 mA |
| Red | 1.8–2.2 V | 20–30 mA |
| Orange / Yellow | 2.0–2.2 V | 20 mA |
| Green (standard) | 2.0–2.4 V | 20 mA |
| Blue / White | 3.0–3.4 V | 20–30 mA |
| UV (400 nm) | 3.4–3.8 V | 20 mA |
Frequently Asked Questions
How much current should I use for a standard indicator LED?
Most 3 mm and 5 mm through-hole LEDs are rated at 20 mA continuous maximum. For a clearly visible indicator in a typical room, 5–10 mA is sufficient and extends LED life. For SMD LEDs (0402, 0603, 0805), check the datasheet — maximum current is often 10–20 mA depending on package and thermal pad. Always use a value well below the absolute maximum rating, not at it.
What happens if the resistor value is lower than calculated?
Current increases above the target. For example, if the calculation gives 150 Ω and you use 100 Ω, current rises by 50%. The LED may survive this at first but typically shows accelerated lumen depreciation (dimming over time). Sustained overcurrent above 120% of the rated maximum can destroy the LED within hours. Round up to the nearest standard E12 or E24 value, not down.
Can an Arduino or Raspberry Pi GPIO drive an LED directly?
Yes, with a resistor in series. Arduino GPIO pins are rated at 40 mA absolute max but 20 mA recommended per pin. Raspberry Pi GPIO pins are limited to 16 mA per pin (lower than Arduino). Use 8–10 mA for RPi GPIO-driven LEDs to stay comfortably within limits. The resistor is always required — never connect an LED pin-to-pin without one.
What resistor wattage rating do I need?
Calculate P = If² × R (in watts). Standard leaded resistors come in 1/8 W (0.125 W), 1/4 W (0.25 W), 1/2 W, 1 W and 2 W. Use a component rated at least 2× the calculated dissipation for safe continuous operation. A 20 mA circuit with a 150 Ω resistor dissipates 0.06 W — a standard 1/4 W resistor is more than adequate.
How do I wire multiple LEDs in parallel vs series?
Series (end-to-end): the forward voltages add up. R = (Vs − N×Vf) ÷ If — this calculator handles it. Advantage: one resistor per string. Disadvantage: if one LED fails open, all go out. Parallel: each LED gets its own resistor. Advantage: independent failure. Disadvantage: more components and slightly higher total current. For three or more identical LEDs on a fixed supply, series is usually more power-efficient.