LED Resistor Calculator
Enter your supply voltage, LED forward voltage and LED current to calculate the resistor value, nearest standard value and power dissipation.
Reviewed by the ToolNestr Editorial Team — July 2026
How LED resistor values are calculated
LEDs are current-driven devices and require a current-limiting resistor to prevent damage. The resistor value is calculated using Ohm's Law: R = (Vs - Vf) / If, where Vs is the supply voltage, Vf is the LED's forward voltage, and If is the desired forward current in amperes.
For multiple LEDs in series, the total forward voltage is the sum of each LED's Vf, and the same current flows through all of them. The resistor power dissipation is calculated as P = If² × R, and the chosen resistor should have a power rating at least twice this value for safety.
Worked example
Standard resistor values (E12 series)
The E12 series provides 12 resistance values per decade with \u00b110% tolerance. Always round up to the next standard value for LED current limiting.
Hobbyist
Add LEDs to your DIY projects without worrying about burning them out \u2014 get the right resistor value every time.
Electronics Student
Learn the relationship between supply voltage, forward voltage, current and resistance in practical LED circuits.
Lighting Designer
Plan LED arrays and displays with confidence, knowing every LED string has the correct current-limiting resistor.
Arduino User
Quickly find the right resistor for LEDs connected to your microcontroller pins and avoid damaging your board.
| LED color | Forward voltage range | Typical V\u2092 | Resistor for 12V, 20mA |
|---|---|---|---|
| Red | 1.8 \u2013 2.2 V | 2.0 V | 500 \u03a9 |
| Yellow | 2.0 \u2013 2.4 V | 2.2 V | 490 \u03a9 |
| Green | 2.0 \u2013 3.0 V | 2.5 V | 475 \u03a9 |
| Blue | 3.0 \u2013 3.6 V | 3.3 V | 435 \u03a9 |
| White | 3.0 \u2013 3.6 V | 3.3 V | 435 \u03a9 |
| UV | 3.3 \u2013 3.8 V | 3.5 V | 425 \u03a9 |
How to use the LED Resistor Calculator
Enter supply voltage
Type your power supply voltage (e.g., 5V for Arduino, 12V for automotive, 3.3V for ESP32). The supply must exceed the LED forward voltage.
Enter LED specs
Input the LED forward voltage (check datasheet or use typical values from the table above) and the desired current in mA.
Select nearest resistor
The calculator shows the exact resistor value and the nearest standard E12/E24 value. Always round up to the next standard value for safety.
Tips for LED resistor selection
Always round up to the next standard value
If your calculation gives 475 \u03a9 and the nearest E12 values are 470 \u03a9 and 560 \u03a9, choose 560 \u03a9. The slightly lower current protects the LED with no visible brightness difference.
Use a higher wattage resistor than calculated
Choose a resistor with at least 2\u00d7 the calculated power dissipation. A resistor running at its exact power rating gets very hot and may fail over time. A 1/2W resistor is a safe choice for most indicator LEDs.
Combine LEDs in series when possible
Connecting LEDs in series with one resistor is more efficient than parallel with separate resistors. The calculator supports series strings \u2014 just enter the number of LEDs. Ensure total forward voltage is less than supply voltage.
Check the datasheet for exact values
LED forward voltage and maximum current vary by manufacturer and even between batches. Always consult the datasheet for the specific LED you are using, and design with a safety margin below the maximum ratings.
Why LEDs need resistors
LEDs are current-driven devices, not voltage-driven. Unlike incandescent bulbs which have built-in resistance, an LED\u2019s resistance drops sharply once its forward voltage is reached. Without a current-limiting resistor, the LED would draw unlimited current and destroy itself almost instantly. The resistor sets the operating point and ensures the LED receives exactly the current it was designed for.
Calculating for series strings
For multiple LEDs in series, the total forward voltage is the sum of each LED\u2019s V\u2092. The resistor drops the remaining voltage: V\u1d04\u1d47\u1d42\u1d43\u1d34\u1d35\u1d34 = V\u209b\u2055 - N \u00d7 V\u2092. The same current flows through all LEDs, making series connections more efficient than parallel. The calculator handles this automatically \u2014 just enter the number of LEDs in series.
Power dissipation
The resistor dissipates power as heat according to P = I\u00b2R. For a 560 \u03a9 resistor at 20 mA: P = 0.02\u00b2 \u00d7 560 = 0.224 W. A standard 1/4W resistor would be marginal \u2014 a 1/2W resistor is safer. Higher supply voltages or currents may require 1W or larger resistors. Always check the resistor\u2019s power rating before finalizing your design.
Standard resistor series
Resistors are manufactured in standard values defined by the E-series. E12 (\u00b110% tolerance) has 12 values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. E24 (\u00b15% tolerance) adds 12 intermediate values. E48 and E96 offer even finer resolution for precision applications. For LED current limiting, E12 values are usually sufficient.
Frequently asked questions
Why do LEDs need a current-limiting resistor?
LEDs have a very low internal resistance and will draw excessive current if connected directly to a voltage source. The resistor limits current to a safe level determined by the LED’s forward voltage and desired operating current.
How do I calculate the resistor value for an LED?
Use the formula: R = (Vₛ⁕ - Vₒ) / I, where Vₛ⁕ is the supply voltage, Vₒ is the LED forward voltage, and I is the desired current in amps. For example, R = (12V - 2V) / 0.02A = 500 Ω.
What happens if I use a resistor with too low a value?
Too low a resistor allows excessive current, which makes the LED brighter but dramatically shortens its lifespan. The LED may overheat, change color, or fail permanently within minutes or hours.
Can I connect multiple LEDs in series?
Yes. The same current flows through all LEDs in series. The resistor value becomes R = (Vₛ⁕ - N × Vₒ) / I, where N is the number of LEDs. The total forward voltage must not exceed the supply voltage.
What are the standard resistor values (E12 series)?
The E12 series includes: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and multiples of these by powers of 10. Always round up to the next standard value to keep current below the LED’s maximum rating.
How do I choose the resistor power rating?
Calculate power as P = I² × R (or P = Vᴄᵇᵂᵃᴴᴵᴴ × I). Choose a resistor with at least 2× the calculated power dissipation. A 1/4W (0.25W) resistor is fine for most small indicator LEDs at low currents.
What are typical LED forward voltages?
Forward voltage varies by color: Red ~1.8–2.2V, Yellow/Orange ~2.0–2.4V, Green ~2.0–3.0V, Blue ~3.0–3.6V, White ~3.0–3.6V, UV ~3.3–3.8V. Always check the datasheet for exact values.
What is the typical current for an indicator LED?
Standard indicator LEDs run at 10–20 mA. High-brightness LEDs may need 20–50 mA, and power LEDs can require 350–1500 mA. For longest life, run LEDs at around 15 mA rather than their absolute maximum.
Can I connect LEDs in parallel with one resistor?
This is not recommended because minor differences in forward voltage cause uneven current sharing. One LED may draw most of the current and fail. Use a separate resistor for each LED or connect them in series.
What does “forward voltage” mean?
Forward voltage (Vₒ) is the voltage drop across the LED when it is conducting current. It varies by the LED’s semiconductor material (color) and is specified in the datasheet. The supply voltage must be higher than Vₒ for the LED to light.
Sources & references
This tool uses standard formulas and reference values from:
- • NFPA 70 — National Electrical Code (NEC), conduit fill, box fill and conductor ampacity. nfpa.org
- • IPC-2221, Generic Standard on Printed Board Design (trace width / current). ipc.org
- • NIST reference constants and unit definitions; IEEE standards where applicable. nist.gov
For educational and preliminary use. Verify against full engineering calculations and the governing standard before any real design.