LED Resistor Calculator
Calculate the current-limiting resistor for one or more LEDs in series. Results include the exact resistance, nearest E24 standard value, and power dissipation.
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How the LED Resistor Calculator Works
An LED requires a current-limiting resistor to prevent damage from excessive current. Unlike a regular resistor, an LED has a non-linear V-I characteristic: once the forward voltage threshold is crossed, small voltage increases cause exponential current increases. A series resistor linearizes this relationship and sets the operating current.
Core Formula
R = (Vsupply − N × Vforward) / Iforward
Where N is the number of LEDs in series. The voltage across the resistor is the supply minus the total LED forward voltages. The resistor limits current to the desired operating point.
E24 Standard Resistor Series
The E24 series has 24 resistor values per decade (e.g., 100Ω to 910Ω) spaced approximately 10% apart. Since exact calculated values are rarely in the catalog, the calculator finds the nearest E24 value and shows what actual current flows through the LED with that resistor. Rounding up (higher resistance) reduces brightness slightly; rounding down increases it.
- E24 values per decade: 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91
- E12 and E6 are subsets of E24 (fewer choices per decade)
- For precision, E96 series offers 1% tolerance resistors with 96 values per decade
Typical LED Forward Voltages
| Color | Vf (V) | Typical If |
|---|---|---|
| Red | 1.8–2.2 | 20 mA |
| Yellow / Orange | 1.9–2.2 | 20 mA |
| Green | 2.0–3.5 | 20 mA |
| Blue | 2.8–3.5 | 20 mA |
| White | 3.0–3.5 | 20 mA |
| IR (850nm) | 1.2–1.6 | 100 mA |
Design Example: Arduino 5V → Blue LED
- Vsupply = 5V, Vf = 3.0V, If = 20mA
- R = (5 − 3.0) / 0.020 = 100 Ω
- 100Ω is E24 standard — no rounding needed
- Power = (5−3.0) × 0.020 = 40 mW (use 1/8W or higher rated resistor)
Frequently Asked Questions
Do I need a resistor if my LED driver already limits current?
No. Constant-current LED drivers (e.g., BC3587, AL8861) regulate current directly without a resistor. Series resistors are only needed for simple voltage-source drive circuits like GPIO pins and bench power supplies.
What happens if I skip the resistor?
Without a resistor, the LED draws unlimited current from the supply (limited only by internal wiring resistance). This typically destroys the LED in milliseconds and may also damage the GPIO pin or power supply. Always use a current-limiting resistor or a current-regulated driver.
Can I put LEDs in parallel instead of series?
Parallel LEDs are discouraged because slight Vf differences cause unequal current sharing — one LED hogs most of the current and fails. For multiple LEDs at equal brightness, use series strings with individual resistors, or a constant-current driver with matched LED forward voltages.