How is the LED resistor calculated?

R = (Vsupply − Vforward) / Iforward, with current in amps. The resistor drops the difference between the supply and the LED's forward voltage at the chosen current.

Why pick the next-higher standard resistor?

Choosing the next-higher E12 value keeps the actual current at or below your target, protecting the LED. A lower value would push more current through it.

What forward voltage should I use for my LED?

Typical forward voltages are about 1.8–2.2 V for red, 2.0–2.4 V for yellow, 2.0–3.0 V for green, and 3.0–3.4 V for blue and white. Check your LED's datasheet for the exact figure at your target current.

What current should an LED run at?

Standard indicator LEDs are usually run at around 20 mA, though many are perfectly bright at 5–10 mA. Never exceed the maximum forward current on the datasheet, or the LED will overheat and fail.

What power rating does the resistor need?

The tool shows the power the resistor dissipates, P = (Vsupply − Vf) × I. Pick a resistor rated comfortably above that — a 1/4 W resistor is fine for most indicator LEDs, but check when running higher currents or larger voltage drops.

What are E12 standard values?

E12 is the common series of preferred resistor values (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their decade multiples). The calculator rounds up to the nearest E12 value so the current stays at or below your target.

LED Resistor Calculator

Q: Is anything uploaded?

No — the calculation runs entirely in your browser, nothing is sent anywhere.

To drive an LED safely you add a series (current-limiting) resistor that drops the difference between the supply voltage and the LED’s forward voltage at your chosen current. This calculator gives the exact resistor, the nearest real-world part and the power it must handle — for hobbyists, makers and anyone wiring an LED to a battery, microcontroller pin or power rail.

How it works

An LED has a roughly fixed forward voltage (Vf). The resistor absorbs the leftover voltage at the current you want:

R = (V_supply − V_f) / I — with current I in amps.

The calculator also computes:

Power in the resistor: P = (V_supply − V_f) × I
Nearest standard part: it rounds up to the next E12 value so the real current never exceeds your target (a smaller resistor would push more current through the LED).
Actual current with that standard resistor: I = (V_supply − V_f) / R.

If the supply voltage is not greater than the forward voltage, no resistor can drive the LED and the tool flags it.

Example

A red LED (Vf = 2.0 V) on a 5 V supply at 20 mA:

R = (5 − 2.0) / 0.020 = 3.0 / 0.020 = 150 Ω

Quantity	Value
Exact resistor	150 Ω
Nearest E12 part	150 Ω
Resistor power	3.0 × 0.020 = 0.06 W
Actual current	20 mA

A 150 Ω resistor is itself an E12 value, so the actual current lands exactly on the 20 mA target. The calculation runs entirely in your browser — nothing is sent anywhere.