LED Resistor Formula
A series resistor is commonly used to limit current through an LED. The resistor receives the difference between the supply voltage and the LED's forward voltage.
Multiple LEDs in Series
When multiple LEDs are connected in series, their forward voltages add together.
where N is the number of LEDs in series.
Example: One 5 V LED Circuit
Suppose a red LED has a forward voltage of 2.0 V and the target current is 20 mA.
R = 150 Ω
The calculated resistance is therefore 150 Ω.
LED Resistor Power Calculation
The resistor also dissipates electrical power as heat. Its power should be considered when selecting a physical resistor.
Example
With a 5 V supply, 2 V LED and 20 mA current:
PR = 3 V × 0.020 A
PR = 0.06 W
The theoretical dissipation is approximately 0.06 W. A resistor with a suitable power rating and practical thermal margin should be selected rather than choosing a rating exactly equal to the calculated dissipation.
Choosing a Standard Resistor Value
The calculated resistance does not always correspond to a resistor value that is available in a particular resistor series. Common preferred-value series include E12, E24 and E96.
| Calculated | Series | Example Standard Value | Effect |
|---|---|---|---|
| 150 Ω | E12 | 150 Ω | 20 mA nominal |
| 347 Ω | E24 | 360 Ω | Slightly lower current |
| 487 Ω | E24 | 510 Ω | Slightly lower current |
| 91 Ω | E24 | 91 Ω | Approximately calculated value |
If the exact calculated value is unavailable, selecting a slightly higher standard resistance generally reduces LED current. This can provide useful current margin, but the resulting brightness and circuit requirements should still be checked.
Typical LED Forward Voltage
LED forward voltage varies with semiconductor material, color, operating current, temperature and individual device construction. The values below are useful starting points, but the manufacturer's electrical specifications should be used for a particular LED.
| LED Type / Color | Typical Forward Voltage |
|---|---|
| Infrared | ≈ 1.1–1.5 V |
| Red | ≈ 1.8–2.2 V |
| Orange | ≈ 1.9–2.2 V |
| Yellow | ≈ 2.0–2.4 V |
| Green | ≈ 2.0–3.2 V |
| Blue | ≈ 2.8–3.6 V |
| White | ≈ 2.8–3.6 V |
Multiple LEDs in Series
When LEDs are connected in series, the forward voltage of each LED contributes to the total voltage drop.
Two LEDs with 2 V forward voltage each require approximately 4 V total LED voltage.
Three 2 V LEDs require approximately 6 V total LED voltage.
Two white LEDs with 3.2 V forward voltage require approximately 6.4 V.
Example: Two LEDs From 12 V
Two LEDs with a forward voltage of 2 V each are connected in series to a 12 V supply and operated at 20 mA.
R = 400 Ω
A nearby standard resistor such as 430 Ω would produce a lower current than the exact 400 Ω calculation.
LEDs in Series vs. Parallel
Series LEDs
LEDs in series carry the same current and their forward voltages add together. A single appropriately sized series resistor can be used with the string when the supply voltage provides sufficient headroom.
Parallel LEDs
Identical-looking LEDs can have different forward-voltage characteristics. Connecting multiple LEDs directly in parallel with one shared resistor can therefore result in unequal current distribution.
For independently driven parallel LEDs, a separate current-limiting resistor for each LED branch is commonly used so each branch has defined current limiting.
Supply Voltage Headroom
The resistor requires some voltage across it to regulate the LED current. If the supply voltage is too close to the LED's total forward voltage, there may not be enough voltage available across the resistor.
For a simple resistor-limited circuit, increasing the supply voltage increases the voltage across the resistor and therefore increases the required resistance for a given target current.
Worked LED Resistor Examples
R = (5 − 2) / 0.02
R = (9 − 2) / 0.01
R = (12 − 3.2) / 0.02
R = (24 − 2) / 0.01
LED Resistor Calculator Applications
Indicator LEDs
Calculate current-limiting resistors for status and power indicator LEDs on electronic circuits and PCBs.
Microcontroller Projects
Estimate series resistors for LEDs controlled by development boards, digital outputs and embedded systems.
Breadboard Circuits
Quickly calculate practical resistor values for prototype LED circuits.
Electronics Design
Estimate LED current, resistor dissipation and voltage headroom during circuit design.
Practical LED Resistor Design Notes
- Use the manufacturer's forward voltage when available. Typical color-based values are only approximations.
- Check maximum LED current. The calculated current should be appropriate for the specific LED.
- Check resistor power. The resistor must safely dissipate the calculated power.
- Allow practical margin. Component tolerances, supply variation and temperature can affect the actual operating point.
- Consider the driver. Microcontroller GPIO pins and other outputs have their own voltage, current and power limits.
- For high-power LEDs, use the manufacturer's recommended current-control method. A simple resistor may waste significant power and provide poorer current regulation than a dedicated constant-current driver.
Related Electronics Calculators
LED Resistor Calculator FAQ
How do you calculate an LED resistor?
The basic LED series resistor formula is R = (Vsupply − Vforward) / ILED. Supply voltage and LED forward voltage are measured in volts, while LED current is measured in amps.
What resistor do I need for a 5V LED?
The required resistor depends on the LED forward voltage and desired current. For example, with a 2.0 V LED at 20 mA, R = (5 − 2) / 0.02 = 150 ohms.
What resistor do I need for a 12V LED?
The resistor depends on the LED forward voltage and target current. For example, a 2.0 V LED operated at 20 mA from 12 V requires approximately 500 ohms. A nearby standard resistor value can then be selected.
Why does an LED need a resistor?
A resistor limits current through the LED. Without appropriate current limiting, an LED connected directly to a voltage source can draw excessive current and may be damaged.
What is LED forward voltage?
Forward voltage is the voltage drop across an LED when it is conducting at a specified current. It varies with LED color, semiconductor material, current, temperature and the specific device.
What LED forward voltage should I use?
Use the forward voltage specified by the LED manufacturer at the intended operating current when available. Typical values are approximately 1.8–2.2 V for many red LEDs, around 2–2.4 V for many green or yellow LEDs, and around 2.8–3.6 V for many blue and white LEDs.
How do I calculate the resistor power rating?
The resistor power is P = I²R, which is also equal to the voltage across the resistor multiplied by its current. Choose a resistor with an appropriate power rating and practical thermal margin.
Can I use a standard resistor value instead of the calculated value?
Yes. In many applications a standard resistor value is selected. Using a slightly higher resistance generally reduces LED current, while a lower resistance increases current. The actual current should remain within the LED and circuit specifications.
Can one resistor be used for multiple LEDs?
A single resistor can sometimes be used with LEDs in series when their total forward voltage is compatible with the supply voltage. Parallel LEDs generally should not share one resistor because small forward-voltage differences can cause unequal current sharing.