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LED Series Resistor Calculator

Size the series resistor for one or more LEDs, with a safe standard value, real current and power.

Calculator

Series resistor
150 Ω ideal 150 Ω
Actual LED current
20 mA
Resistor power
60 mW
Voltage across resistor
3 V

How LED Series Resistor Calculator works

An LED is a diode, not a resistor. Above its forward voltage the current rises almost vertically with applied voltage, so connecting one directly across a supply lets current run away until something fails. A series resistor turns that unstable arrangement into a predictable one by absorbing the difference between the supply and the LED’s forward drop.

The resistor value follows directly from Ohm’s law applied to that difference: R = (Vsupply − Vforward) / Iforward. With several LEDs in series the forward voltages add, so the resistor drops what remains — which also means the supply must exceed the total forward voltage with margin left over, or the string will not light at all.

Forward voltage depends strongly on the emitted colour, because it is set by the semiconductor bandgap. Red and infrared sit around 1.8–2.2 V, yellow and green around 2.0–2.4 V, and blue, white, and ultraviolet around 3.0–3.4 V. Those are typical figures: real parts vary between batches, which is why the calculation gives an approximate operating point rather than an exact one.

That variation is also why LEDs should never be paralleled on a single resistor. The device with the lowest forward voltage takes a disproportionate share of the current, runs hotter, drops further, and takes more still. Each parallel branch needs its own resistor.

Reference

  • R = (Vsupply − Vforward) / Iforward
  • Series string: R = (Vsupply − n × Vforward) / Iforward
  • Resistor dissipation: P = (Vsupply − Vforward) × Iforward
  • Typical Vf — red 1.8–2.2 V, green 2.0–2.4 V, blue/white 3.0–3.4 V

How to use this calculator

  1. Enter the supply voltage

    Use the actual rail the LED will run from — 5 V from USB, 3.3 V from a regulator, 12 V from an adapter.

  2. Enter forward voltage and current

    Take both from the LED datasheet. If you only know the colour, the typical ranges above are a reasonable starting point.

  3. Set the number in series

    For a string, enter how many LEDs share the resistor. Their forward voltages add.

  4. Take the standard value

    Use the suggested next-higher E-series resistor and check the resulting real current and resistor dissipation.

Worked examples

A red indicator on 5 V

Given
5 V supply, Vf 2.0 V, target 20 mA
Result
R = 150 Ω exactly; use 150 Ω, dissipating 60 mW

A standard value lands exactly here. Most indicators are perfectly visible at 5 mA, which would call for 620 Ω instead.

A white LED on 3.3 V

Given
3.3 V supply, Vf 3.2 V, target 20 mA
Result
R = 5 Ω — and the design is unworkable

Only 0.1 V of headroom means normal Vf variation swings the current wildly. Use a boost converter or a constant-current driver instead.

Three LEDs in series on 12 V

Given
12 V, three white LEDs at 3.2 V, 20 mA
Result
R = 120 Ω, dissipating 48 mW

Series wiring shares one current through all three, which keeps their brightness matched — unlike a parallel arrangement.

When to use it

  • Sizing the resistor for a panel or status indicator LED.
  • Designing a series LED string for a light bar or backlight from a fixed supply.
  • Reducing the current of an over-bright indicator without changing the part.
  • Checking that a resistor’s power rating is adequate on a higher-voltage rail.
  • Confirming a microcontroller pin can source the current directly, or needs a transistor.

Things to watch out for

  • Never connect an LED across a supply without current limiting. Even a brief connection can destroy the junction.
  • Do not put LEDs in parallel on one resistor. Forward-voltage mismatch makes the current share unequal and the imbalance grows as the hotter device drops further.
  • Check your microcontroller’s per-pin and total port current limits. Twenty milliamps is often the absolute maximum per pin and well below what the whole port can sustain simultaneously.
  • Forward voltage falls as the junction heats, so current creeps up during operation. A series resistor damps this; a constant-current driver eliminates it.

Frequently asked questions

What resistor do I need for an LED?

R = (supply voltage − LED forward voltage) / desired current. For a typical red LED at 20 mA on 5 V that is (5 − 2) / 0.02 = 150 Ω. Round up to the next standard value, which errs toward less current and a longer-lived LED.

What happens without a resistor?

Current is limited only by the LED’s tiny internal resistance and the supply’s capability, so it rises until the junction overheats. The LED either fails immediately or degrades quickly, and it can take the driving pin with it.

Can I use one resistor for several LEDs?

Only if they are wired in series, sharing the same current. Never share one resistor across parallel LEDs — small forward-voltage differences make one device hog the current, and thermal feedback makes the imbalance worse over time.

How do I know my LED’s forward voltage?

From its datasheet. Failing that, colour is a good guide: red around 2.0 V, green around 2.2 V, blue and white around 3.2 V. You can also measure it with a multimeter’s diode test function.

Why is my LED dimmer than expected?

Usually because the standard resistor value you fitted is higher than the calculated one, or the supply sags under load. Check the actual voltage across the resistor and divide by its resistance to see the true current.

Does the resistor go before or after the LED?

Either — it is a series circuit, so the same current flows through both regardless of order. Placement is a layout convenience, not an electrical decision.

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