150 Ω
- Brown
- Green
- Brown
- Gold
Current: 20 mA
Heat in the resistor: 60 mW, so use a ¼ W resistor or bigger.
LED resistor calculator
Pick your supply, the LED colour and the current. You get the resistor to buy, its colour bands and the power it needs.
Resistor to use150 Ω
The formula gives 150 Ω; the next standard value up is safer. 3 V is left for the resistor.
Quick picks
150 Ω colour bands
150 Ω
- Brown
- Green
- Brown
- Gold
Current: 20 mA
Heat in the resistor: 60 mW, so use a ¼ W resistor or bigger.
- Exact value from the formula
- 150 Ω
- Power in each LED
- 40 mW
How do I work out an LED resistor?
R = (supply voltage − LED voltage) ÷ current
The LED keeps its forward voltage; the resistor takes the rest. Ohm's law then gives the resistance for the current you want. With LEDs in series, add up their voltages first.
- Take the LED voltage away from the supply. 5 V − 2 V for a red LED leaves 3 V for the resistor.
- Divide by the current in amps. 20 mA is 0.02 A, so 3 ÷ 0.02 = 150 Ω.
- Pick the next standard value up. If the answer is not a value you can buy, the next one up is the safe choice.
- Check the power. 3 V × 0.02 A = 0.06 W of heat, well inside a ¼ W resistor.
What voltage does each LED colour need?
Typical figures for standard 3 mm and 5 mm LEDs at about 20 mA. Your LED's datasheet has the exact one.
| LED colour | Typical forward voltage | Usual range |
|---|---|---|
| Red | 2.0 V | 1.8 to 2.2 V |
| Orange | 2.0 V | 1.9 to 2.2 V |
| Yellow | 2.1 V | 1.9 to 2.2 V |
| Yellow-green | 2.1 V | 1.9 to 2.4 V |
| Bright green | 3.0 V | 2.8 to 3.3 V |
| Blue | 3.0 V | 2.8 to 3.3 V |
| White | 3.0 V | 2.8 to 3.3 V |
What resistor for a common supply?
One LED at 20 mA, with the next E12 value up (the values in every starter kit) and its colour bands.
| Supply | LED | Exact | Resistor (E12) | Colour bands | Heat |
|---|---|---|---|---|---|
| 3.3 V | Red (2.0 V) | 65 Ω | 68 Ω | BlueGrayBlackGold | 25 mW |
| 3.3 V | Blue or white (3.0 V) | 15 Ω | 15 Ω | BrownGreenBlackGold | 6 mW |
| 5 V | Red (2.0 V) | 150 Ω | 150 Ω | BrownGreenBrownGold | 60 mW |
| 5 V | Blue or white (3.0 V) | 100 Ω | 100 Ω | BrownBlackBrownGold | 40 mW |
| 9 V | Red (2.0 V) | 350 Ω | 390 Ω | OrangeWhiteBrownGold | 130 mW |
| 9 V | Blue or white (3.0 V) | 300 Ω | 330 Ω | OrangeOrangeBrownGold | 110 mW |
| 12 V | Red (2.0 V) | 500 Ω | 560 Ω | GreenBlueBrownGold | 180 mW |
| 12 V | Blue or white (3.0 V) | 450 Ω | 470 Ω | YellowVioletBrownGold | 170 mW |
Can I see a few worked out?
Worked out by the same code as the calculator above.
150 Ω
(5 V − 1 × 2 V) ÷ 20 mA = 150 Ω. Bands: Brown, Green, Brown, Gold.
20 mA · 60 mW of heat330 Ω
(9 V − 1 × 3 V) ÷ 20 mA = 300 Ω. Bands: Orange, Orange, Brown, Gold.
18.2 mA · 110 mW of heat150 Ω
(3.3 V − 1 × 2 V) ÷ 10 mA = 130 Ω. Bands: Brown, Green, Brown, Gold.
8.67 mA · 11 mW of heat150 Ω
(12 V − 3 × 3 V) ÷ 20 mA = 150 Ω. Bands: Brown, Green, Brown, Gold.
20 mA · 60 mW of heatWhat do people often get wrong?
No resistor at all
The LED may flash once and die. Every LED needs something to limit its current.
One resistor for LEDs in parallel
The LEDs never share evenly. Give each LED its own resistor.
LED the wrong way round
It simply stays dark. The long leg goes to plus.
Too small a resistor rating
On 12 V or more, the resistor can make enough heat to need ½ W. The calculator tells you.
Short answers to common questions
What resistor do I need for an LED on 5 V?
For a red LED (about 2 V) at 20 mA: (5 − 2) ÷ 0.02 = 150 Ω, a standard value. For a blue or white LED (about 3 V): (5 − 3) ÷ 0.02 = 100 Ω. A normal ¼ W resistor is enough for both.
Why does an LED need a resistor?
An LED holds a nearly fixed voltage across itself. Above that voltage its current shoots up, and only the resistor holds it down. Without one, the LED can burn out in a moment.
Why pick the next standard value up and not the nearest?
A slightly larger resistor gives slightly less current, which keeps the LED and the resistor within their limits. The light looks just as bright to the eye.
What current should I use?
A standard 5 mm LED is usually rated about 20 mA. 10 to 15 mA is still bright and lasts longer, and modern LEDs are clearly lit at 2 to 5 mA, which suits indicator lights and battery projects.
What does forward voltage mean?
It is the voltage an LED keeps across itself while it is lit. It depends mostly on the colour: about 2 V for red, orange and yellow, and about 3 V for blue, white and bright green. The datasheet gives the exact figure.
Can several LEDs share one resistor?
In series, yes: add their forward voltages and use one resistor for the string (set LEDs in series in the calculator). In parallel, no: give each LED its own resistor, or one LED takes most of the current.
What wattage should the resistor be?
At least twice the heat it makes. The calculator works out the heat (current × the voltage across the resistor) and suggests the smallest common rating with that margin. For most single LEDs on 5 V or 9 V, a ¼ W resistor is fine.
Which way round does the LED go?
The longer leg (anode) goes towards the plus side, the shorter leg next to the flat edge (cathode) towards minus. The resistor can go on either side of the LED.