💡 LED Resistor Calculator

An LED (light-emitting diode) is a semiconductor device that only conducts electricity in one direction and emits light when current passes through it. Unlike a traditional incandescent bulb, an LED does not behave like a resistor: once its forward voltage is reached, a tiny increase in voltage produces a huge increase in current. Without a resistor to tame that current, the LED draws more than it can handle and burns out almost instantly.

The current-limiting resistor sits in series with the LED and drops the excess voltage, keeping the current at a safe, predictable level. It is the single most important component in any LED circuit — whether you are wiring a single indicator light, an Arduino project, or a strip of addressable LEDs.

If you are combining several building blocks on one board, our parallel resistance calculator, voltage divider calculator and Ohm's law guide cover the other pieces you will likely need.

The table below lists the typical forward voltage of common LEDs. Use the value that matches the colour of your LED — or, better, the figure printed in its datasheet.

The resistor value comes straight from Ohm's law. The voltage that has to be absorbed by the resistor is the supply voltage minus the LED's forward voltage: V_R = V_s − V_f. Dividing that by the desired current gives the resistance:

R = (V_s − V_f) / I

where I is the current in amperes (so a current entered in milliamps must be divided by 1000). For example, a red LED (V_f ≈ 2 V) running from a 5 V supply at 15 mA needs R = (5 − 2) / 0.015 ≈ 200 Ω.

The power dissipated by the resistor is P = V_R × I. That tells you which wattage rating to buy. As a safety rule, choose a resistor rated for at least twice the power it will actually dissipate — so a resistor that only dissipates 45 mW is comfortably run on a standard ¼ W (250 mW) part.

Because resistors only come in standard values, the calculator also reports the nearest E12 series value (the most common resistor family: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 × 10ⁿ). For a one-off hobby build you can use the nearest value; for anything safety- or efficiency-critical, round up to the next standard value so the LED stays below its maximum current.

Resistor tolerance gives you extra slack. A typical ±5% carbon-film part means a 220 Ω resistor is really somewhere between 209 and 231 Ω, which shifts the current by only a few milliamps. So the calculated value is a guide rather than a strict law — choose the nearest standard part and the circuit will almost always work perfectly. Surface-mount (SMD) resistors follow the same maths but use the E24 or E96 series, which gives you finer values if a through-hole part is slightly off.

The table below shows ready-made resistor values for the most common hobby supplies and LED colours, assuming a target current of 15 mA. If your supply or LED differs, just use the calculator above.

1. Note the supply voltage of your circuit (for example 5 V from a USB rail or 3.3 V from a microcontroller pin).
2. Look up the forward voltage (Vf) for your LED colour in the table above, or from the component datasheet.
3. Decide the target current — 10 to 20 mA is the usual safe range for a small indicator LED.
4. Enter the three values and press Calculate to get the resistance, power and nearest standard value.
5. Pick the nearest standard resistor; if you are between two values, choose the higher one and confirm its wattage is at least double the dissipated power.

What happens if I connect an LED without a resistor?

The LED behaves almost like a short circuit once lit, so current shoots up until the chip overheats and fails. A 5 V source can destroy a small LED in seconds. Always include a series resistor (or use a constant-current driver for high-power LEDs).

Can I use one resistor for several LEDs in parallel?

Not safely. LEDs in parallel do not share current evenly — one will hog the current and burn out, then the next, in a chain failure. Give each LED its own resistor, or wire them in series so they share the same current.

How do I choose the resistor wattage?

Multiply the voltage across the resistor by the current to get watts, then pick a rating at least twice that figure. Most indicator LEDs dissipate well under 100 mW, so a standard ¼ W resistor is plenty.

My LED is dim — is the resistor wrong?

A resistor larger than needed simply lowers the current, making the LED dimmer but safe. If it is too dim, reduce the resistance toward the calculated value (or the next lower standard value) while staying above the minimum safe current.

Does the resistor value change with battery type?

The formula uses whatever supply voltage you enter. As a battery drains its voltage drops, so the current falls slightly — the LED just gets a bit dimmer rather than failing, which is why a resistor-based circuit is forgiving.

For multiple LEDs of the same colour, wire them in series and calculate the resistor for the combined forward voltage (add the V_f values). Series wiring shares one current and wastes less power than separate resistors.

If you need to dim or brighten an LED under software control, keep the fixed resistor for safety and vary the brightness with PWM (pulse-width modulation) from a microcontroller — never by lowering the series resistance at runtime.

High-power LEDs (1 W and up) get hot enough that their forward voltage shifts with temperature. For those, skip the plain resistor and use a dedicated constant-current LED driver.

Keep a handful of common values (220 Ω, 330 Ω, 470 Ω, 1 kΩ) in your parts box — they cover the vast majority of 5 V and 3.3 V indicator-LED builds.

When you move from a breadboard to a finished product, swap the through-hole resistor for an SMD part of the same value and wattage. The maths is identical; only the package changes, which saves space on a crowded board.