Loading Calculator...
Please wait a moment
Please wait a moment
See LED voltage chart below
Typical: 20mA for 5mm LEDs
R = (Vs - Vf) ÷ I
P = I² × R
Where:
R = (Vs - (Vf × n)) ÷ I
Where n = number of LEDs. Total forward voltage is the sum of all LED voltages. Current remains the same through all LEDs.
| LED Color | Forward Voltage (V) | Typical Current (mA) |
|---|---|---|
| Infrared | 1.2-1.5V | 20-100mA |
| Red | 1.8-2.2V | 20mA |
| Orange | 2.0-2.2V | 20mA |
| Yellow | 2.0-2.4V | 20mA |
| Green (Standard) | 2.0-2.4V | 20mA |
| Green (Pure/Bright) | 3.0-3.4V | 20mA |
| Blue | 3.0-3.4V | 20mA |
| White | 3.0-3.6V | 20-30mA |
| UV (Ultraviolet) | 3.0-4.0V | 20mA |
Values are typical for standard 5mm through-hole LEDs. Check your LED's datasheet for exact specifications.
An LED resistor is a current-limiting resistor placed in series with a light-emitting diode (LED) to control the amount of current flowing through it. LEDs are semiconductor devices with a characteristic called forward voltage (Vf) -- the minimum voltage needed to turn them on. Once that threshold is reached, an LED's internal resistance drops dramatically, and without an external resistor the current would spike uncontrollably, destroying the LED almost instantly. The resistor absorbs the difference between the supply voltage and the LED's forward voltage, converting that excess energy into heat while maintaining a safe, steady current through the LED. The required resistance is calculated using Ohm's law: R = (Vs - Vf) / If, where Vs is the supply voltage, Vf is the LED forward voltage, and If is the desired forward current (typically 20 mA for standard indicator LEDs). Choosing the correct resistor value and wattage rating is fundamental to every LED circuit, from simple indicator lights to complex LED arrays.
Identify your power source voltage. Common sources include 3.3V (microcontrollers), 5V (USB/Arduino), 9V (battery), 12V (automotive/LED strips), and 24V (industrial). The supply voltage must be higher than the LED's forward voltage.
Check the LED datasheet for its forward voltage and recommended forward current. Standard 5mm LEDs typically operate at 20 mA. Red LEDs have a Vf around 1.8-2.2V, while blue and white LEDs are 3.0-3.6V. High-power LEDs may require 350 mA or more.
Subtract the LED forward voltage from the supply voltage, then divide by the desired current in amps. For example, with a 12V supply and a red LED (Vf = 2.0V, If = 20 mA): R = (12 - 2) / 0.020 = 500 ohms. Round up to the next standard resistor value (510 ohms).
Calculate power dissipated: P = (Vs - Vf) x If. Using our example: P = 10V x 0.020A = 0.2W. Choose a resistor rated for at least double the calculated wattage for reliability. A 1/2W resistor would be appropriate here. Standard 1/4W resistors work for most single-LED indicator circuits.
Without a current-limiting resistor, an LED connected directly to a voltage source will draw excessive current and burn out within milliseconds. The resistor acts as a protective barrier, ensuring the current stays within the LED's safe operating range and extending its lifespan to the rated 50,000+ hours.
LED brightness is directly proportional to the current flowing through it. By selecting a specific resistor value, you can precisely control the LED current and therefore its brightness. A higher resistance reduces current and dims the LED, while a lower resistance increases current and brightness up to the maximum rated value.
An LED without a resistor behaves almost like a short circuit once forward voltage is exceeded, drawing dangerously high current from the power supply. This can damage microcontroller I/O pins, drain batteries rapidly, or blow fuses. The resistor protects both the LED and the circuit driving it.
| LED Color | Forward Voltage (Vf) | Typical Current (mA) | Wavelength (nm) | Resistor for 5V Supply |
|---|---|---|---|---|
| Infrared | 1.2 - 1.5V | 20 - 100 | 850 - 940 | 175 - 190 ohms |
| Red | 1.8 - 2.2V | 20 | 620 - 645 | 140 - 160 ohms |
| Orange | 2.0 - 2.2V | 20 | 590 - 620 | 140 - 150 ohms |
| Yellow | 2.0 - 2.4V | 20 | 570 - 590 | 130 - 150 ohms |
| Green (Standard) | 2.0 - 2.4V | 20 | 520 - 570 | 130 - 150 ohms |
| Green (Pure/Bright) | 3.0 - 3.4V | 20 | 495 - 520 | 80 - 100 ohms |
| Blue | 3.0 - 3.4V | 20 | 460 - 490 | 80 - 100 ohms |
| White | 3.0 - 3.6V | 20 - 30 | Broad spectrum | 47 - 100 ohms |
| UV (Ultraviolet) | 3.0 - 4.0V | 20 | 380 - 420 | 50 - 100 ohms |
Resistor values calculated at 20 mA using R = (5V - Vf) / 0.020A, rounded to nearest standard E24 value.
Yes, when LEDs are wired in series the same current flows through all of them. Calculate the total forward voltage by adding the Vf of each LED, then use R = (Vs - total Vf) / If. Ensure the supply voltage is at least 2V higher than the total forward voltage for the resistor to regulate properly.
Calculate the power dissipated by the resistor using P = I squared times R, or equivalently P = (Vs - Vf) times I. Choose a resistor rated for at least twice the calculated power. For most single-LED indicator circuits at 20 mA, a standard 1/4 watt resistor is sufficient. High-power LED circuits may require 1W or 2W resistors with adequate heat sinking.
It does not matter. The resistor can be connected to either the anode (positive) or cathode (negative) side of the LED. In a series circuit, the same current flows through all components regardless of their order. Place the resistor wherever it is most convenient for your circuit layout.
Each parallel LED should have its own individual resistor. Do not share a single resistor among parallel LEDs, because slight differences in forward voltage between LEDs will cause unequal current distribution. The LED with the lowest Vf will hog most of the current and may burn out while the others remain dim.
Microcontroller GPIO pins can source a limited current, typically 20-25 mA for Arduino or 12 mA for Raspberry Pi. You still need a resistor. For a 5V Arduino pin driving a red LED (2.0V Vf) at 15 mA: R = (5 - 2) / 0.015 = 200 ohms. Use 220 ohms as the nearest standard value. For higher-current LEDs, use a transistor or MOSFET driver.
Calculate resistance from voltage and current using Ohm's law. Includes series and parallel resistance calculations for multi-resistor circuits.
Calculate voltage, current, resistance, and power using Ohm's law. The foundational relationship behind every LED resistor calculation.
Calculate voltage drop across wires and resistors. Useful for understanding how supply voltage is distributed in LED circuits with long wire runs.