LED Resistor Calculator
Instantly calculate the correct current-limiting resistor value, power rating, and colour code for any LED circuit — single or series.
Standard 5mm LED
Red / Yellow: ~2.0 V · Blue / White: ~3.3 V · All @ 20 mA
Required Resistor
How to Use the LED Resistor Calculator
Follow these four steps to find the right current-limiting resistor for your LED circuit in seconds.
- 01
Set your supply voltage (Vs)
Enter the total voltage your power source provides — 3.3 V for a microcontroller pin, 5 V for USB, 9 V or 12 V for a DC adapter.
- 02
Enter the LED forward voltage (Vf)
Check your LED's datasheet or use a colour preset above. Red and yellow LEDs typically drop 2.0–2.2 V; blue, white, and UV LEDs drop 3.0–3.5 V.
- 03
Choose the forward current (If)
Most standard 5mm LEDs are rated at 20 mA. High-brightness or power LEDs may specify 30–100 mA — always check the datasheet.
- 04
Read the result
The calculator instantly displays the exact resistance, the nearest E24 standard value, minimum power rating, and the corresponding 4-band colour code.
LED Resistor Formula Explained
Ohm's Law applied to an LED circuit: the resistor must absorb the voltage difference between the supply and the LED's forward voltage. Dividing that voltage drop by the desired current gives the required resistance.
The power formula tells you how much heat the resistor will dissipate. Always choose a resistor rated for at least twice this figure — a 2× derating factor ensures safe operation and a long component life.
For a series LED string, replace Vf with Vf × n (where n = number of LEDs). The calculator handles this automatically when you switch to Series mode.
Common LED Resistor Values Reference Table
Use this table as a quick reference for standard 5mm LEDs at 20 mA. Values shown are the nearest E24 resistor.
| LED Colour | Vf (V) | If (mA) | R @ 5 V | R @ 9 V | R @ 12 V |
|---|---|---|---|---|---|
| Red | 1.8 – 2.2 | 20 | 150 Ω | 350 Ω | 496 Ω |
| Orange | 2.0 – 2.2 | 20 | 150 Ω | 340 Ω | 490 Ω |
| Yellow | 2.0 – 2.2 | 20 | 150 Ω | 340 Ω | 490 Ω |
| Green | 2.0 – 2.4 | 20 | 130 Ω | 330 Ω | 480 Ω |
| Blue | 3.0 – 3.5 | 20 | 75 Ω | 270 Ω | 420 Ω |
| White | 3.0 – 3.5 | 20 | 75 Ω | 270 Ω | 420 Ω |
| UV | 3.0 – 3.5 | 20 | 75 Ω | 270 Ω | 420 Ω |
| Infrared | 1.0 – 1.5 | 20 | 175 Ω | 375 Ω | 525 Ω |
* Approximate E24 standard values. Forward voltage varies by manufacturer and LED construction. Always verify against the component datasheet.
Common LED Resistor Mistakes
Skipping the resistor entirely
Direct connection to a voltage source forces unlimited current. The LED will burn out in milliseconds, often taking a microcontroller GPIO pin with it.
Ignoring power rating
A resistor with the right ohm value can still burn out if it cannot handle the heat. Always apply a 2× safety derating — a 0.1 W calculation needs a ¼ W part.
Parallel LEDs with one resistor
Tiny Vf differences cause current imbalance. The brightest LED draws the most current, overheats first, and the cascade continues until all fail.
Using nominal instead of actual Vf
LED datasheets quote a typical Vf. Junction temperature and manufacturing spread can shift this by ±0.2 V. Measure with a multimeter when precision matters.
Wiring LEDs in Series vs. Parallel
Frequently Asked Questions
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