LED Resistor Calculator
Series resistor, nearest E-series value and power for one or more LEDs.
What it does
An LED must have a series resistor to limit its current. The resistor drops whatever voltage is left after the LED’s forward voltage: R = (Vs − n·Vf) / I. The calculator then rounds up to a standard E-series value.
Where it's used
Indicator LEDs on circuit boards, Arduino / ESP32 projects, and strings of LEDs wired in series from one supply.
How to use it
- Enter the supply voltage.
- Choose the LED colour (it sets a typical forward voltage) or pick Custom and type Vf from the datasheet.
- Enter the current you want — 5–20 mA is typical for indicator LEDs.
- Use the standard resistor value and check its power rating.
The formula explained
- Vs
- supply voltage
- Vf
- forward voltage of one LED (depends on colour)
- n
- number of LEDs in series
- I
- LED current you want
- R
- series resistor
- P = I²R
- heat in the resistor
Worked example
Three white LEDs in series from a 12 V supply at 15 mA.
Inputs:
- Supply voltage: 12 V
- LED colour / forward voltage: White (≈3.2 V)
- LED current: 15m A
- LEDs in series: 3
- Standard series: E24
The calculator gives:
- Standard resistor (E24, next higher): 160 Ω
- Exact resistance: 160 Ω
- Actual LED current: 15 mA
Tips and common mistakes
- Always round the resistor up: a smaller value means more current and a shorter LED life.
- If n × Vf is close to the supply voltage the LEDs will be dim or uneven — use fewer LEDs per string.
- Give every parallel string its own resistor; LEDs sharing one resistor in parallel do not share current evenly.
Questions people ask
What resistor do I need for one red LED on 5 V?
With a red LED (about 2 V) at 20 mA: R = (5 − 2) / 0.02 = 150 Ω, which is a standard value. The resistor turns 60 mW into heat, so an ordinary 1/8 W or 1/4 W part is fine.
What forward voltage do different LEDs have?
Roughly 1.8–2.2 V for red, orange, yellow and classic green, and 2.8–3.4 V for blue, white and true green. Infrared is lower, about 1.2–1.5 V. The datasheet value is best, because it also changes with current and temperature.
Can I connect an LED without a resistor?
Not to a voltage source. An LED’s current rises very steeply with voltage, so a small change in supply voltage or temperature can destroy it. Use a resistor, or a constant-current LED driver for high-power LEDs.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.