What is an LED series resistor calculator?
LEDs are current-driven devices. To prevent damage and control brightness, you almost always need a series resistor between the power supply and the LED string. This calculator helps you find:
- The required series resistance value
- The nearest standard resistor value
- The power rating the resistor must safely handle
By entering the supply voltage, LED forward voltage and desired current, the calculator solves the simple but critical sizing equation for you.
Basic LED series resistor formula
For one or more LEDs connected in series, the resistor value is:
R = (VS − VF,total) / ILED
- VS – supply voltage
- VF,total – sum of LED forward voltages in the string
- ILED – desired LED current
The power dissipated in the resistor is:
PR = ILED2 × R
or equivalently:
PR = (VS − VF,total) × ILED
Always choose a resistor with a power rating comfortably above the calculated value (typically at least a 2× safety margin).
Typical LED forward voltage by color
Forward voltage (VF) depends on color, LED technology and operating current. The table below shows typical ranges for standard indicator LEDs at nominal current:
| LED Color | Typical VF Range (V) |
|---|---|
| Red | 1.8 to 2.1 |
| Amber | 2.0 to 2.2 |
| Orange | 1.9 to 2.2 |
| Yellow | 1.9 to 2.2 |
| Green | 2.0 to 3.1 |
| Blue | 3.0 to 3.7 |
| White | 3.0 to 3.4 |
Datasheets for specific LED models should always be preferred, but when they are not available, these ranges are a practical starting point for estimation.
Design examples
Example 1 – single red LED from 5 V
- VS = 5 V
- VF ≈ 2.0 V (red LED)
- ILED = 15 mA
R = (5 − 2.0) / 0.015 ≈ 200 Ω
Choose the nearest standard value, e.g. 220 Ω to reduce current slightly and improve safety margin.
Example 2 – three white LEDs from 24 V
- VS = 24 V
- VF ≈ 3.2 V each → VF,total ≈ 9.6 V
- ILED = 20 mA
R = (24 − 9.6) / 0.02 = 720 Ω
Choose 750 Ω or 820 Ω as a standard value.
Resistor power: P ≈ (24 − 9.6) × 0.02 ≈ 0.288 W → use at least a 0.5 W resistor.
Practical tips for LED resistor selection
- Use series strings, not large parallel groups: For multiple LEDs, it is safer to have several series strings, each with its own resistor, instead of one resistor feeding many parallel LEDs.
- Check supply tolerances: Higher-than-nominal supply voltage increases LED current; choose resistor values with this in mind.
- Consider environment: In high-temperature enclosures or industrial panels, derate resistor power more aggressively.
- Brightness vs lifetime: Running LEDs below their maximum rated current improves reliability and can still provide sufficient brightness.
This calculator accelerates the design of status indicators, panel lights, and low-voltage illumination in industrial control systems, PLC panels, and embedded electronics.
