Picture of Choosing The Resistor To Use With LEDs
This question gets asked every day in Answers and the Forums: What resistor do I use with my LEDs? So I've put together several different ways to figure it out.

Lets get right to it:
Each of the steps do the same thing. Step 1 is the simplest and we go downhill from there.

No mater what way you choose you must first know these three things:

  • Supply voltage This is how much power you're putting into the circuit. Batteries and wall warts will have the output voltage printed on them somewhere. If you're using multiple batteries*, add the voltage together.
  • LED Voltage Sometimes "Forward Voltage" but usually just abbreviated "V".
  • LED Current Sometimes "Forward Current". This is listed in milliamps or "mA".

Both of these last two can be found on the packaging for your LEDs or on your supplier's web site. If they list a range ("20-30mA") pick a value in the middle (25 in this case). Here are some typical values, but use your own values to be sure you don't burn out your LEDs!:

Red LED: 2V 15mA
Green LED: 2.1V 20mA
Blue LED: 3.2V 25mA
While LED: 3.2V 25mA

Okay, lets get started!

* Batteries in series.

Introduction photo credits:
LED photo by Luisanto.
Resistor photo by oskay.
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Step 1: The Web Way

Picture of The Web Way
The easiest way is to use one of the online calculators provided below.

Just click on one and enter the info from the previous step and you're set! You only need to go to one.

The LED Center (For single LEDs)

The LED Center (For arrays of LEDs)

LED (For single or arrays of LEDs)

LED (For single or arrays of LEDs)

Step 2: The Retro Way

Picture of The Retro Way
Go to Evil Mad Scientist Labs web page at this link and print and make your own slide rule-like calculator.

PDF, assembly and usage instructions are all on the page linked above.

It's pretty nifty and ends up being about business card size so you can keep one in that box with the rest of your LEDs.
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Zack_Cantrell3 months ago
Okay, so I have an SMD LED... VF=11v-12v
Supply voltage of 12v

This seems to be an ignorant question but I would like to know that if the voltage drop of my LED is 11v-12v then by Ohms law should I or should I not need a resistor.. At most maybe a 1ohm
dhaneshg4 months ago

i want use Led bulb in real electric circuit for indication at 230 volt. how much resistance is require to protect led???? plz reply

Lets say you use 3.5V 25mA LED.

(230v - 3.5v) / (0.025A) = 9060ohms.

230v is not a game. Do not do it yourself.

dadibom oronnadiv3 months ago
And the watts the resistor would have to be rated for would be at 230*0.025 i guess?

RIP dhaneshg

LMFAO this had my rolling.

if i use small stepdown transformer and zener diode to control voltage for LED than??

dhaneshg4 months ago

but sir at the time of Festivals we are use LED bulb series to decorate that time normal volt is 230volt naa? i open a series light and found a Bride circuit, but couldn't understand.. so plz tell me how to build it?

CharlesD14 months ago

After using the "wizzard" I'm still not sure if everything is correct. From a 12vdc power supply, I plan to power a 15 led array wired in parallel. The numbers from the retailer are as follows: Power = 0.5w, Working Voltage = 3.2-3.5, Current = 150ma. The wizzard calculator red flagged one of the lines. I am looking to get the correct resistance value for this array. Any help would be great. Thanks guys.

Can anyone define the difference between voltage and current in a simple way?

Voltage is pressure. Current is flow.

Voltage is how excited the electrons want to move through a wire. Current is how many electrons are actually moving through a wire.

If a nightclub had a popular band playing that night, Voltage would be how excited the patrons would be to get in they would be close together at the door waiting. A Resistor would be the size of the bouncer at the door. If he is small and weak he would let everybody in as quick as they wanted. A big bouncer would let a single line in, one at a time. Current would be how fast the nightclub fills up.

Hope that helps :P

AshwillP Joxman2k6 months ago

Not really true hey, but I really laughed at how awesome you described it. Remember current has to keep on flowing, so if you say current would be how fast the nightclub fills up, doesn't it mean that the circuit will be sort circuited? Or am I maybe confused. It is more that a resistor can only allow a certain amount of current to pass before burning, meaning that the club is the resistor and not the bouncer. Smaller club equals less current at a time flowing through, where a larger club would let more current flow through at the same time. This has all have to happen in a loop, when one goes in one goes at, or the club will be over capasity.

It is very easy budy, Follow the simple steps here ...
How to calculate the value of resistor for LED’s (with different types of LED’s circuits)

or simply use this online calculator:
Required Value of Resistor for LED’s Circuit Calculator

this is a great teaching method , hope some professor know this technik

Thanks franko :) Most schools just teach so you can pass tests, which is basically memorizing knowledge. Understanding comes from relating what is taught to something you already understand, by way of relationships between the parts. Understanding comes from asking questions. In order to ask questions you need to be curious. Wisdom and excellence comes from asking the right questions.

Using the above example:

Wire gauge would be how big the doors are. The larger the door the less you can fit on the front of the bar. 12 gauge wires are big, where as 22 gauge wires are small so you can fit more.

milliampere-hour (mAh) would be how long the line is.

The LED would be the band.

A capacitor would be the bouncer letting in say 10 customers at a time after 10 customers leave.

A diode would be the exit that locks after you leave.

Wattage would be the heat that is produced as customers go through the door.

... it's not a perfect senario but it explains the relationships well. you could also use a restaurant, football stadium, outdoor concert, even driving to work in the city.


Voltage is a difference in electrical charge between two points and the de driving force for the current which is the resulting flow of charge. It is easier to understand by looking at analogues in either liquid flows or mechanics.

In a liquid flow through a pipe our Voltage analogue would be the pressure difference between the two ends of our pipe and the current is the same as the volume flow of liquid through the pipe. If you fill your pipe with sand or a wad of fabric you have increased the resistance and you will have to apply a higher pressure to get the same flow.

In mechanics a simple comparisson can be made with a block on an inclined board: the heigth difference between the two sides is our driving force (like voltage in electrics) and the resulting speed of the block sliding down the plane is similar again to current (though in electrics you will have many small blocks sliding down the same plane in a row). Our resistance in this case is the friction between the block and the board, if we reduce the friction by using a smoother surface like glass the block will slide down faster.

Voltage is a "potential" so it describes how much the electrons want to get from + (positive) to - (negative). At really high voltages, the electrons will even jump an air gap (create a spark) to get from one side to the other. Current is a measure of how many electrons are in motion when a circuit is powered and running. The more electrons that are in motion, the more physical work they can do (like starting a car or melting a fuse). An open switch has no current going through it, since the electrons are not able to move through it, but it might have a voltage across it. Too much voltage across a switch can let the electrons spark and jump through it anyway. If you close a switch that has a voltage across it (switch it from off to on), you let the electrons flow through it and suddenly, there is a current. months ago

Actually ,i cut all the leds into singles for use with different projects,and discovered that these little leds are very adaptable ,i have been running a green single led on 2 double aa batteries non stop for 5 days straight and still as bright as first day ,i also made a solar powered orange led lantern with one 1.2 volt rechargable battery .the led doesn't require a resistor unless you are using them in strings or a/c , they are negative power capable ,and that part gets tricky .so before you use them on a project ,test led with 3 volts max d/c to see which way the current flows ,and mark the positive side . now you have endless supply of lights ,for projects .one thousand leds uses less power than a 40 watt light bulb.

they take a certain amount of power before they light up .so its just math after that and basically only when the are used as an a/c string.or higher voltage. if using as a power on light and want to use a resistor then a .025 m/a 6 volt would be perfect the 6 volt ac fuse is like i said in the power plug which comes apart. im working on a cordless phone rechargeable battery pack as another power source.cordless phones are also very inexpensive at thrift you have lots of Christmas leds and a rechargeable phone power supply for maybe 10$ .plus the resistor u need may be inside the phone .free..and not have to search for it. months ago

For you guys looking for resisters most are built in the led ,its actually a 5 or 6 volt fuse located in both plugs off a string of lights ,different total amounts of leds

of led strings give you different fuses and resisters .check a 10 led string compared to a 35 led string and use whichever fits your project also cutting off male end and using 2 AA batterys well as a power supply with no more than 20 MA depending on total combined led voltage, 78 to 90 volts string is recomended.

Grathio (author) months ago

This advice only applies to commercial strings or strips of LEDS. If you're using individual LED components, you must look at the specifications and see what the current and voltage draw are for the LED, and choose a proper matching resistor.

mshafin11 months ago

it says while instead of white

if i get it right...amps are enabling the charge to it doesnt matter how many amps are created by power supply?
Grathio (author)  Martythebest4 years ago
Right. As long as there are enough amps you're good. Too many amps is not a problem in any way.
...Unless you are operating a "power LED" in which case too many amps will blow your LED... which is why such LEDs require a driver, not just a simple resistor.

hello KDS4444, i have been looking for this answer on the internet for a while now. so what you are saying is that when using "high power LEDs" we actually have to watch out for the mA output of the power source, not just the output V? for example if i have 2 LEDs @ 2.5V and 350mA connected in series to a 5V 700mA, will it actually hurt them? it should be a 5V 350mA power source? also, if they were to match up perfectly, why do we still have to use a resistor? can anyone please answer these questions. love and peace..!

so a 7.2 VDC 3000 mAh RC car battery all i need to worry about is that 7.2 VDC
nreed41 year ago
What if you want to use leds of different forward voltages?
foggy34 nreed41 year ago

just use the approperiate resistor for Each led.

nreed4 foggy341 year ago


N3em4 years ago
Thank you so much its helps alot ,but i have qustion

how can i bulid led curcit like this
foggy34 N3em1 year ago


merlin131 year ago
Great tutorial for a beginner like me, thanks a lot! I just have one question. Most of the calculators I've seen (and the one you mentioned) calculate the resistor using the LED forward current. So for a 3.3V supply, using an LED with 2.0V voltage and 20mA forward current, the resistor comes up to be 65ohm. Perfect!

But if I want to use this LED with a Raspberry Pi let's say, the max current on pins is 4mA. If I use this resistor, I risk damaging the Raspberry Pi pin, right? So instead I calculate the resistor using 4mA as the current which gives me 330ohm resistor. This is the right value I need to use, is this correct?
Thanks for all your help and again, thanks for the tutorial!
jcook321 year ago
Ok for the project I'm doing the LED calculator says i need a . I found LED's on eBay that come with resistor, but the resistors they have to pick from are 4.5V, 5V, 6V, 9V or 12V resistors can I use any of those? or do I still need to buy the 120 ohm resistors
Grathio (author)  jcook321 year ago
What they're saying in the listing is if you tell them what voltage you're running the LEDs, they'll calculate the correct resistor and send it along -- You don't need to calculate.

However I would verify that they send appropriate resistors once they arrive.
Using math is often the fastest way. Simple algebra. It usually takes me longer to find a datasheet or some other information than it does to find & solve for R in Ohm's Law. R= V/I boom that simple.
This is just an application of ohms law.
V=IR (Voltage = Current times Resistance)
if the supply voltage is 5 the forward voltage is 2.5 and the forward current is 25 mA or .025 amps.
R=V/I so R= (5-2.5)/.025 so R=2.5/.025 which is 100 ohms. It is simple to use and more effective than an online calculator. There is a good site that explains this:
scroll down to the section labeled LED problems or you can look at the earlier sections to help.
tutdude983 years ago
thanks its help me :D
Vick Jr5 years ago
So the supply current (like those listed on wall warts) makes absolutly no difference? It won't hurt to use a supply voltage with a very high current, as long as the resistor is calculated using the supply voltage, LED voltage, and LED current?
Grathio (author)  Vick Jr5 years ago
Yes, that's correct.  You can perhaps think of Amps as "available power".  If you're not using it, it doesn't cause any trouble.

You need to make sure you have enough amps to supply everything you're powering, so if you're powering t12 LED's that pull 30 milliamps each you need a power supply with at least (12x30mA) 0.36 Amps. More than that will not cause any problems.

It's the same with the (somewhat) common task of buying a replacement computer power supply.  You can use pretty much any power supply as long as the voltage is the same and it has at least as many amps.

It's like using rope to climb down a cliff.  If you run short of rope (amps) you're in big trouble.  So you might as well bring at least as much as you need.
Gaoh Grathio4 years ago
that's it!
just the answer for my problem.
the current of the power supply always confused me, cause i thought it was the source for LED to burnt.
thanks, this is helpful.
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