LED Series Resistor Calculator

Calculate the correct series resistor for your LED in seconds. Enter the supply voltage, LED forward voltage, and desired current to get the resistor value, the nearest standard value, and the power rating needed. Protect your LED and get the brightness you want.

Frequently Asked Questions

Why does an LED need a series resistor?
An LED has very low resistance once it starts conducting. Without a resistor, the current rises rapidly until the LED overheats and burns out. A series resistor limits the current to a safe value.

What formula does the LED series resistor calculator use?
R = (Vsupply − Vf) ÷ If
Here Vsupply is the source voltage, Vf is the LED forward voltage, and If is the desired LED current.

What is the forward voltage (Vf) of an LED?
It is the voltage drop across the LED when it is lit. Typical values are red 1.8–2.2 V, yellow 2.0–2.2 V, green 2.0–3.0 V, and blue or white 3.0–3.4 V. Always check the datasheet for the exact value.

What current should I use for my LED?
Most standard 5 mm LEDs work well at 10–20 mA. Using 5–10 mA still gives good brightness with longer life and lower power consumption. Never exceed the maximum forward current in the datasheet.

What if the calculated resistor value is not a standard value?
Choose the next higher standard value from the E12 or E24 series. A slightly higher resistance gives slightly lower current, which is safer for the LED.

How do I calculate the resistor power rating?
P = (Vsupply − Vf) × If, which is the same as If² × R. Choose a resistor rated at least twice the calculated power. For most small LEDs, a 1/4 W resistor is enough.

What happens if the resistor value is too low?
The LED draws too much current, runs hot, loses brightness over time, and may fail quickly.

What happens if the resistor value is too high?
The LED will be dim, or it may not light at all if the current is too small. It is safe, but the brightness is reduced.

Can I connect multiple LEDs in series with one resistor?
Yes. Add the forward voltages of all LEDs and use the total as Vf in the formula. The supply voltage must be higher than the total forward voltage. All LEDs share the same current.

Can I connect LEDs in parallel with one resistor?
It is not recommended. Small differences in forward voltage cause uneven current sharing, and one LED may take most of the current. Use a separate resistor for each LED.

How do I calculate the resistor for a 5 V supply?
Example: red LED, Vf = 2 V, If = 20 mA.
R = (5 − 2) ÷ 0.02 = 150 Ω.
A 150 Ω resistor works. The power is 3 × 0.02 = 0.06 W, so a 1/4 W resistor is fine.

How do I calculate the resistor for a 3.3 V supply?
Example: red LED, Vf = 2 V, If = 10 mA.
R = (3.3 − 2) ÷ 0.01 = 130 Ω. The nearest standard value is 130 Ω or 150 Ω.
Blue and white LEDs have Vf close to 3.0–3.4 V, so they may not work properly on 3.3 V.

Can I power an LED directly from a microcontroller pin?
Yes, if the current stays within the pin limit, usually 10–20 mA depending on the microcontroller. Always use a series resistor, and use a transistor or MOSFET driver for higher current or many LEDs.

Does the resistor go on the positive or the negative side of the LED?
Either side works, because the resistor is in series and the same current flows through both. Placing it on the positive side is the common practice.

Does LED color affect the resistor value?
Yes, indirectly. Different colors have different forward voltages, so the voltage across the resistor changes, which changes the resistor value for the same current.

Why is my LED brightness different from the calculated value?
Forward voltage varies with temperature, current, and manufacturing tolerance. Resistor tolerance and supply voltage changes also affect the current. Small differences are normal.

Can I use this calculator for high-power LEDs?
For high-power LEDs (1 W and above), a resistor wastes a lot of power as heat. A constant-current LED driver is the better choice. The calculator works for low-power indicator and signal LEDs.

Can I use this calculator for AC supply?
No. LEDs need DC. For AC, use a rectifier circuit or a proper LED driver with the correct protection.

Is the series resistor enough to protect the LED from reverse voltage?
No. The resistor limits current only. Most LEDs tolerate only about 5 V in reverse, so add a protection diode if reverse voltage is possible.