LED Series Resistor Calculator - Ohm Value & Power

How to use this tool

1. Enter the supply voltage (Vs), per-LED forward voltage (Vf), target current (If), and number of LEDs in the series string.
2. Choose E12 or E24; the calculated resistance, rounded-up value, actual current, power, and suggested resistor rating update as you type.

LED resistor formula

R = (Vs − N × Vf) / If

• Vs - supply voltage
• Vf - LED forward voltage (typically 1.8–3.5 V depending on colour)
• If - desired forward current (typically 10–30 mA)
• N - number of LEDs in one series string

Typical LED forward voltages

• Infrared: 1.2–1.5 V
• Red: 1.8–2.2 V
• Yellow / Orange: 2.0–2.4 V
• Green: 2.0–3.5 V
• Blue / White: 2.8–3.5 V
• Ultraviolet: 3.2–3.8 V

Circuit diagram

The resistor is placed in series between the power supply and the LED’s anode (positive leg):

   Vs (+)
    │
   [R] ← calculated resistor
    │
   [LED] -> anode (+) - cathode (−)
    │
   GND (−)

Current flows from Vs through the resistor, through the LED from anode to cathode, and back to ground. The resistor absorbs the “excess” voltage (Vs − Vf) and limits current to the safe If value. Without this resistor, the LED would draw uncontrolled current and burn out in milliseconds.

Standard resistor values (E12 and E24 series)

Resistors are sold in preferred values arranged in E-series. Choose E12 for a compact set of common values or E24 for finer steps. This calculator always rounds up to the first value at or above the calculated resistance, so the estimated current stays at or below the target. For example, a calculated 290 Ω becomes 330 Ω in E12 and 300 Ω in E24; rounding down could drive more current than intended. Values repeat by decade, so 10, 100, and 1,000 Ω are all in both series.

Multiple LEDs in series

When LEDs are wired in series, their forward voltages add up. Enter the number of LEDs in the string and use their total forward voltage in the calculation:

R = (Vs − ΣVf) / If

Example: 3 red LEDs in series (Vf = 2.0 V each) on a 12 V supply at 20 mA: R = (12 − 6) / 0.020 = 300 Ω. Your supply voltage must exceed the sum of all forward voltages; if Vs ≤ ΣVf, no current can flow regardless of resistor value.

Choose a resistor power rating

The resistor converts the voltage it drops into heat. Its dissipation is the voltage across the resistor multiplied by the actual current after rounding to a standard value. The calculator suggests the next common power rating that is at least twice that dissipation; for example, a resistor dissipating 60 mW calls for at least a ⅛ W part by this simple 2× rule. A larger margin may be appropriate in a warm enclosure or where ventilation is poor, so check the component datasheet and its derating curve before building a circuit.

PWM dimming

To dim an LED, pulse-width modulation (PWM) is strongly preferable to reducing the supply voltage or increasing the series resistance. At a lower fixed current, an LED's color temperature shifts and efficiency drops. With PWM, the LED is switched fully on and off many times per second - the duty cycle (on-time ÷ period) controls perceived brightness while the LED always operates at its rated current when on. This maintains consistent color temperature and maximizes efficiency across the full brightness range.