How is the LED series resistor calculated?

The resistor must drop the difference between the supply voltage and the LED forward voltage at the desired current. The formula is R = (V_supply - V_f) / I, where I is in amps. For example, a 5 V supply, a red LED with Vf = 2.0 V and a target current of 20 mA gives R = (5 - 2.0) / 0.020 = 150 ohms.

Why round up to the next E12 or E24 value?

Resistors only come in standard preferred values. Using the next higher value means the actual current will be at or below your target — a lower resistor would push more current through the LED, risking early failure or exceeding the maximum forward current. The calculator shows the actual current you will get with the chosen standard part.

What is the LED forward voltage?

The forward voltage (Vf) is the voltage that appears across the LED when it conducts at its rated current. It is set by the semiconductor material: red and yellow LEDs are typically 1.8–2.2 V; green is 2.0–3.0 V; blue, white and UV are usually 3.0–3.6 V. Always check the LED datasheet for the exact value at your target current.

What resistor power rating do I need?

The resistor dissipates P = (V_supply - V_f) x I watts. A standard rule of thumb is to derate to 50 %, meaning you should use a resistor rated at least twice the calculated dissipation. For most 20 mA indicator LEDs on a 5 V supply the power is well under 100 mW, so a 1/4 W (250 mW) resistor is more than adequate. The calculator displays the recommended rating based on this 2x derating rule.

How do I wire multiple LEDs with one resistor?

LEDs in series share the same current; their forward voltages add together and one resistor handles the whole string. LEDs in parallel with a single shared resistor have imbalanced currents because of manufacturing spread in Vf — a small Vf difference causes one LED to hog current while others starve. The correct approach for parallel LEDs is one resistor per branch. The parallel mode in this calculator treats all LEDs as identical for a rough estimate only.

Can I use this for a 3.3 V microcontroller GPIO pin?

Yes. Enter 3.3 V as the supply. Bear in mind that a GPIO pin usually sources 8–20 mA maximum; set your target current below that limit. For blue or white LEDs (Vf around 3.2–3.4 V) there may be barely any headroom on a 3.3 V supply — in that case drive the LED from a 5 V rail via a transistor or MOSFET.

LED Resistor Calculator

Every LED needs a current-limiting series resistor to survive. Without one, the LED draws as much current as the supply can deliver, overheating and burning out within seconds. This calculator gives you the exact resistor value, the nearest real-world standard part from the E12 or E24 preferred-value series, and the power the resistor will dissipate — so you can choose the right component the first time.

How the calculation works

An LED has an approximately fixed forward voltage (V_f) set by its semiconductor material. The resistor absorbs whatever voltage is left over between the supply and the LED, and limits the current to a safe level:

R = (V_supply - V_f) / I_f

where I_f is the forward current in amps. All three quantities appear on the LED’s datasheet; the supply voltage comes from your circuit.

The tool then computes:

Exact resistor — the mathematically precise value from the formula above.
Nearest standard part — the next higher E12 or E24 value. Rounding up means the real current is always at or below your target; rounding down would exceed it.
Actual current with the chosen standard resistor: I_actual = (V_supply - V_f) / R_standard.
Power in the resistor: P = (V_supply - V_f) x I_actual (watts). This is heat you need to budget for and handle safely.
Recommended power rating: the first standard resistor wattage at or above twice the dissipated power, applying the common 50 % derating rule for long-term reliability.

Multi-LED configurations

Series string: the LED forward voltages add together (V_f_total = n x V_f), while the current through each LED is the same. One resistor controls the whole string: R = (V_supply - n x V_f) / I. Make sure V_supply is comfortably above n x V_f — typically at least 1–2 V of headroom.

Parallel branches: ideally each branch gets its own resistor. A single shared resistor is unreliable because tiny differences in Vf between physically identical LEDs cause current to pile into the LED with the lowest Vf. The parallel estimate in this tool assumes all LEDs are identical and is provided for rough checking only.

Worked example

A blue LED (V_f = 3.2 V) running from a 9 V battery at 20 mA:

Quantity	Value
Voltage headroom	9 - 3.2 = 5.8 V
Exact resistor	5.8 / 0.020 = 290 Ω
Nearest E12 value	330 Ω
Actual current	5.8 / 330 = 17.6 mA
Resistor power	5.8 x 0.0176 = 102 mW
Recommended rating	1/4 W (250 mW)

Three blue LEDs wired in series from the same 9 V battery: combined V_f = 9.6 V — this exceeds the supply voltage, so the string will not work. Switch to two in series (V_f_total = 6.4 V, headroom = 2.6 V) and the resistor becomes 2.6 / 0.020 = 130 Ω, nearest E12 value 150 Ω.

Standard value series

Resistors are manufactured in preferred-value series so that the full resistance range is covered with the minimum number of distinct values. E12 has 12 values per decade (10 %, 5 mm through-hole packs), suitable for most hobby work. E24 doubles that to 24 values per decade (5 % tolerance), used in precision or surface-mount designs where finer steps matter. Both series repeat by multiplying the base values by powers of ten: 100 Ω, 1 kΩ, 10 kΩ and so on are all the same “10” base value at different decades.

Every figure is calculated locally in your browser — nothing is uploaded or stored.