LED Resistor Calculator
By Santaji GadepatilinUpdated October 27, 2026
LEDs need their current limited, or they burn out within seconds. A series resistor is the simplest way to do it — read about the LED resistor calculation and the LED working principle.
How to use the LED resistor calculator
Enter the supply voltage, the LED’s forward voltage (from the datasheet or the table below), the number of LEDs in series and the current. You get the resistor value and the power it will dissipate.
LED resistor formula
R = (Vs − n × Vf) ÷ If · P = If² × R
Typical forward voltages
| LED colour | Forward voltage |
|---|---|
| Infrared | 1.2–1.5 V |
| Red | 1.8–2.2 V |
| Orange, yellow | 2.0–2.2 V |
| Green (standard) | 2.0–2.4 V |
| Green (bright), blue, white | 2.8–3.4 V |
| UV | 3.1–4.0 V |
Standard 5 mm indicator LEDs are usually rated 20 mA; many work well at 5–10 mA.
Resistor values at 20 mA
| Supply | Red (2.0 V) | White/blue (3.2 V) |
|---|---|---|
| 3.3 V | 65 Ω | 5 Ω |
| 5 V | 150 Ω | 90 Ω |
| 9 V | 350 Ω | 290 Ω |
| 12 V | 500 Ω | 440 Ω |
| 24 V | 1100 Ω | 1040 Ω |
Standard values
The nearest E12 values are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82 (× 1, 10, 100, 1k…). Round up to the next value. See the resistor colour code calculator.
High-power LEDs
For power LEDs (350 mA and above) a resistor wastes too much energy and does not compensate for temperature. Use a constant-current LED driver instead.
Indicator LEDs on 24 V DC panels
For a 24 V PLC panel indicator at 10 mA with a 2 V LED: R = 22 ÷ 0.01 = 2.2 kΩ, dissipating 0.22 W — use a 0.5 W resistor.
Frequently asked questions
How do I calculate an LED resistor?
R = (supply voltage − LED forward voltage × number of LEDs) ÷ LED current. A red LED (2 V) at 20 mA on 5 V needs (5 − 2) ÷ 0.02 = 150 Ω.
What if the calculated value is not a standard resistor?
Choose the next higher standard value. It slightly reduces current and brightness but is safer for the LED.
What resistor wattage do I need?
At least twice the calculated power. For most single indicator LEDs, a 0.25 W resistor is plenty.
Can I put LEDs in parallel with one resistor?
It is not recommended — small differences in forward voltage make one LED take most of the current. Give each LED or string its own resistor.