LED Resistor Calculator
Series resistor for any LED.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Resistor value
- 495 Ω
- Nearest E12 value
- 500 Ω
- Power dissipated
- 0.198 W
- Rating needed
- 0.25 W
Quick answer
An LED's series resistor is R = (supply voltage − total LED forward voltage) ÷ current. The resistor drops the leftover voltage and sets the current. A red LED (2 V) on a 5 V supply at 20 mA needs (5 − 2) ÷ 0.020 = 150 Ω, dissipating 0.06 W, so a standard 1/4 W resistor is fine.
What the LED Resistor calculator does
This calculator works out the current-limiting resistor for a single LED or several identical LEDs wired in series, the power that resistor turns into heat, and whether a common 1/4 W part is enough. It is the same calculation sometimes called an LED current-limiting resistor calculator.
LEDs have almost no internal resistance once they turn on, so connecting one straight across a supply lets current rise until it fails. A series resistor fixes the current at a safe value. It is the standard approach for indicator LEDs, LED strips' individual segments and hobby projects on 3.3 V, 5 V, 9 V and 12 V supplies; high-power LEDs (350 mA and up) are better driven by a constant-current driver.
The math is Ohm's law applied to the voltage left over after the LEDs. Once you have a value, the resistor color code calculator helps you pick the right part from a mixed bag.
The formula
R = (Vs − n × Vf) ÷ I P = (Vs − n × Vf) × I
| Symbol | Meaning | Unit |
|---|---|---|
| R | Series resistor value | Ω |
| Vs | Supply voltage | V |
| n | Number of identical LEDs in series in the string | — |
| Vf | Forward voltage of one LED at the chosen current, from its datasheet | V |
| I | LED current (enter in mA; divide by 1,000 for amps) | mA |
| P | Power dissipated in the resistor | W |
This is Ohm's law (R = V ÷ I) applied to the voltage across the resistor. It assumes a steady DC supply and that n × Vf is less than Vs — if the LED string needs more voltage than the supply provides, the LEDs will not light properly and you need fewer LEDs per string. Because forward voltage varies from part to part and with temperature, leave at least 1–2 V across the resistor so it, not the LED, controls the current.
Worked examples
Red LED on an Arduino 5 V pin
A standard 5 mm red indicator LED (about 2 V) driven at 20 mA from a 5 V supply.
Inputs
- Supply voltage5 V
- LED forward voltage2 V
- LED current20 mA
- LEDs in series1
Result
- Resistor value150 Ω
- Nearest E12 value150 Ω
- Power dissipated0.06 W
- Rating needed0.25 W
150 Ω is itself a standard value, and the resistor dissipates only 0.06 W. Many modern LEDs are bright enough at 5–10 mA, so 330 Ω is also a common choice for an indicator.
Three white LEDs on 12 V
Three white LEDs (about 3.2 V each) in series on a 12 V supply at 20 mA, as in a small light bar.
Inputs
- Supply voltage12 V
- LED forward voltage3.2 V
- LED current20 mA
- LEDs in series3
Result
- Resistor value120 Ω
- Nearest E12 value120 Ω
- Power dissipated0.048 W
- Rating needed0.25 W
The string uses 9.6 V, leaving 2.4 V for a 120 Ω resistor. Stringing LEDs in series wastes far less power than giving each its own resistor, but a fourth white LED (12.8 V) would exceed the supply.
Low-current LED on a 3.3 V microcontroller
A red LED (2.1 V) on a 3.3 V GPIO pin, limited to 10 mA to stay within the pin's current rating.
Inputs
- Supply voltage3.3 V
- LED forward voltage2.1 V
- LED current10 mA
- LEDs in series1
Result
- Resistor value120 Ω
- Nearest E12 value120 Ω
- Power dissipated0.012 W
- Rating needed0.25 W
Only 1.2 V is left for the resistor, so small variations in forward voltage change the current noticeably. Blue and white LEDs (about 3 V or more) are a poor match for 3.3 V supplies.
Typical LED forward voltage by color
Use the datasheet value at your chosen current if you have it. Otherwise these typical figures for standard indicator LEDs are a reasonable starting point.
| LED color | Typical Vf | Notes |
|---|---|---|
| Infrared | 1.2 – 1.6 V | Used in remotes and sensors |
| Red | 1.8 – 2.2 V | Lowest Vf of visible colors |
| Orange / yellow | 2.0 – 2.2 V | Similar to red |
| Yellow-green (older type) | 2.0 – 2.3 V | Dimmer, older chemistry |
| True green | 2.9 – 3.4 V | InGaN type, much brighter |
| Blue | 2.9 – 3.4 V | InGaN type |
| White | 2.9 – 3.4 V | Blue LED with a phosphor |
| UV (around 400 nm) | 3.1 – 3.6 V | Check datasheet |
Choosing a standard resistor and wattage
Calculated values rarely match a resistor you can buy. Round up to the next standard value so the current ends up slightly below your target, never above it. The E12 series, the most commonly stocked range, repeats these values in every decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82 (so 120 Ω, 1.5 kΩ and so on). For example, a calculated 693 Ω becomes 820 Ω from the E12 range.
For wattage, a good practice is to pick a resistor rated for at least twice the calculated dissipation so it runs cool. Under about 0.12 W a 1/4 W part is fine; a figure like 0.6 W calls for a 1 W or 2 W resistor, or a higher-efficiency design.
If you cannot find the exact value, two resistors in series or parallel can get close — see the series/parallel resistor calculator.
Wiring several LEDs: series, parallel and strips
- Series string, one resistor: most efficient, as shown in the 12 V example. Keep the total forward voltage at least 1–2 V below the supply.
- Parallel LEDs need one resistor each. Sharing a single resistor between parallel LEDs lets the one with the lowest Vf hog the current and burn out.
- Several strings: split a large number of LEDs into equal series strings, each with its own resistor, all fed from the same supply.
- Battery power: voltage sags as a battery discharges, so brightness falls. Pair this with the battery runtime calculator to see how long the LEDs will last.
How to use the LED Resistor Calculator
- 1Enter the supply voltage your circuit actually delivers (measure a wall adapter under load if unsure).
- 2Enter the LED forward voltage from its datasheet, or use the color table on this page.
- 3Enter the LED current in milliamps — 20 mA for most 5 mm indicator LEDs, less for modern high-efficiency parts.
- 4Enter how many LEDs share one resistor in a series string, then round the result up to a standard resistor and check the wattage.
Frequently asked questions
What resistor do I need for an LED on 12 V?
For one red LED (2 V) at 20 mA, (12 − 2) ÷ 0.02 = 500 Ω, so use 560 Ω, the next E12 value up. For one white LED (3.2 V) it is 440 Ω, so use 470 Ω.
What happens if I use an LED without a resistor?
Current is limited only by the supply and wiring, so it usually rises far above the LED's rating and destroys it within seconds. Coin-cell LED throwies get away with it only because the cell's internal resistance limits current.
Can I use a higher resistor value than calculated?
Yes. A higher value lowers the current and brightness, which is safe. A lower value raises the current and can shorten the LED's life.
Does it matter which side of the LED the resistor goes on?
No. In a series circuit the same current flows through every part, so the resistor can be on the anode or cathode side.
What wattage resistor do I need for an LED?
Multiply the voltage across the resistor by the current, then choose a rating at least double that. Most single indicator LEDs on 5–12 V need only a 1/4 W resistor.
References
- The Art of Electronics (3rd edition), Horowitz and Hill — Cambridge University Press
- IEC 60063: Preferred number series for resistors and capacitors — International Electrotechnical Commission (IEC)
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
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