Reference chart

LED forward voltage by colour

An LED needs a minimum voltage across it before it lights: its forward voltage. It depends mostly on the colour. This chart gives the value for each colour, the values the lessons' simulator uses, and how many LEDs you can string in series on common supplies.

Forward voltage by colour

The LearnEE simulator uses one value per colour, at 20 mA. Real LEDs vary by maker and by how much current flows, so the third column gives the usual range at 20 mA.

Colour	Simulator	Real parts (typical)	Wavelength
Infrared	not in the parts bin	1.2–1.6 V	850 or 940 nm
Red	2.0 V	1.8–2.2 V	620–630 nm
Orange	not in the parts bin	1.9–2.2 V	600–610 nm
Yellow	2.1 V	1.9–2.2 V	585–595 nm
Green (yellow-green)	2.2 V	1.9–2.4 V	565–570 nm
Green (bright, true green)	not in the parts bin	2.8–3.6 V	520–530 nm
Blue	3.2 V	2.8–3.4 V	460–470 nm
White	3.2 V	2.8–3.4 V	a blue LED plus a phosphor
Ultraviolet	not in the parts bin	3.0–3.6 V	395–405 nm

Green comes in two kinds. Older yellow-green LEDs drop about 2 V, like the simulator's. The brighter, true-green ones are made like blue LEDs and drop about 3 V. Check the datasheet for your exact part.

How much current?

Current	What you get
1–2 mA	A dim glow, fine for a power light
5–10 mA	Plenty for most indicator LEDs
20 mA	The usual rating for standard 3 mm and 5 mm LEDs
Over 30 mA	Too much for most small LEDs: they run hot and die young

In the simulator, an LED counts as lit from 0.5 mA, is overdriven above 30 mA and burns out at 50 mA. High-power LEDs, like the ones in torches, take hundreds of milliamps and need a heatsink, so follow their datasheet instead.

How many in series?

Add up the forward voltages of the string and keep at least 1 V for the resistor. Each cell gives the most LEDs that fit, and the resistor for 20 mA rounded up to the next E12 value:

Colour	5 V	9 V	12 V
Red (2.0 V)	2, with 56 Ω	4, with 56 Ω	5, with 100 Ω
Yellow (2.1 V)	1, with 150 Ω	3, with 150 Ω	5, with 82 Ω
Green (2.2 V)	1, with 150 Ω	3, with 120 Ω	5, with 56 Ω
Blue or white (3.2 V)	1, with 100 Ω	2, with 150 Ω	3, with 120 Ω

Two yellow or green LEDs on 5 V leave less than a volt for the resistor. They'll light, but small changes in the supply swing the current a lot. A 9 V battery also sags as it empties, so a string that only just fits will dim early.

Working out the resistor

Add up the drops: n × V_LED, where n is the number of LEDs.
Subtract from the supply to get the resistor's voltage.
Divide by the current you want, in amps.
Round up to the next standard value.

R = (12 V − 3 × 3.2 V) ÷ 0.02 A = 2.4 V ÷ 0.02 A = 120 Ω

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Ohm's law” is lesson 5 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 4 before it first.

Or start from the very first lesson, no account needed

Work it out

LED resistor calculatorThe resistor for one LED or a string, rounded to a value you can buy.

The ideas behind it

Questions people ask

What is the forward voltage of a red LED?

About 2 V: most red indicator LEDs drop 1.8 to 2.2 V at 20 mA. Check the datasheet for your exact part, or measure it with a multimeter's diode setting.

Why do blue and white LEDs need more voltage than red?

Bluer light carries more energy per photon, and the LED's voltage has to supply it. Blue and white LEDs drop about 3 V, red ones about 2 V. A white LED is a blue one with a yellow phosphor coat, so it needs a blue LED's voltage.

How much current does an LED need?

Standard 3 mm and 5 mm LEDs are rated for about 20 mA, and many are bright enough at 5 to 10 mA. Always set the current with a series resistor. LEDs on their own will draw far more than they can survive.

How many LEDs can I put in series on 12 V?

Add up their forward voltages and keep at least a volt for the resistor. That's five red LEDs (10 V), five yellow or green (2.1 or 2.2 V each) or three blue or white (9.6 V).