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LED Series Resistor Calculator

Enter your supply voltage, the LED's forward voltage and current, and how many LEDs are wired in series to get the resistor value that keeps the LED at its rated current.

led resistorcurrent limiting resistorled forward voltage
LED Series Resistor

Find the current-limiting resistor for one or more LEDs on a fixed supply.

Required resistance
350 Ω
Nearest standard value (E12)
390 Ω
Resistor power dissipation
125.6 mW
Suggested wattage rating
1/4 W is fine

Current-limiting resistor

R = (V_supply − n·V_f) / I_f

Subtract the combined forward voltage of every LED in the series string from the supply voltage, then divide by the LED's rated forward current to get the resistor that absorbs the difference.

How to use

  1. Check the LED's datasheet for forward voltage (V_f) and forward current (I_f) — common defaults are 2V and 20mA for a standard red LED.
  2. Enter your supply voltage and how many LEDs are wired in series (not parallel).
  3. Use the nearest standard resistor value at or above the calculated result, and check the suggested wattage before soldering it in.

Example

Input: Supply = 9V, LED V_f = 2V, I_f = 20mA, 1 LED

Output: R ≈ 350 Ω → nearest standard value 390 Ω

Student-friendly breakdown

This walkthrough emphasizes the most searched ideas for LED Series Resistor Calculator: led resistor calculator, led current limiting resistor calculator, resistor for led calculator, led resistor calculator series. Start with the formula above, then follow the guided steps to double-check your work. For quick revision, highlight the givens, plug into the equation, and finish by verifying your units.

Need more support? Use the links below to open the long-form guide, browse additional examples, or hop into adjacent calculators within the same topic — each one is a quick way to double-check your work or handle a related question without starting from scratch.

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Deep dive & study plan

LED Series Resistor Calculator: Finds the current-limiting resistor for LEDs on a fixed supply voltage. It's built around led resistor, current limiting resistor, led forward voltage, so you can go from a raw question to a checked answer without switching tools.

The math behind it: Subtract the combined forward voltage of every LED in the series string from the supply voltage, then divide by the LED's rated forward current to get the resistor that absorbs the difference. The core relationship is R = (V_supply − n·V_f) / I_f, shown above the calculator so you can see exactly how your inputs turn into the result.

To use it well: (1) Check the LED's datasheet for forward voltage (V_f) and forward current (I_f) — common defaults are 2V and 20mA for a standard red LED. (2) Enter your supply voltage and how many LEDs are wired in series (not parallel). (3) Use the nearest standard resistor value at or above the calculated result, and check the suggested wattage before soldering it in. Keep your units consistent as you go, and re-run a case you already know the answer to — it's the fastest way to catch a typo before it throws off a result you're relying on.

Worked example: entering Supply = 9V, LED V_f = 2V, I_f = 20mA, 1 LED returns R ≈ 350 Ω → nearest standard value 390 Ω. Try swapping in your own numbers next, especially a case you're unsure about, before you use this for something that matters.

Quick retention checklist

  • Speak the formula aloud (or annotate it) so the relationships stick.
  • Write each step in your own words and compare with the numbered list above.
  • Swap in new numbers for the Example to make sure the calculator (and your logic) handles edge cases.
  • Check at least one related calculator below — it's the fastest way to confirm your numbers still line up from a different angle.

FAQ & notes

Why round up to a bigger resistor instead of down?

Rounding up drops the current slightly below the LED's rated maximum, which is safe. Rounding down pushes more current than rated through the LED and shortens its life or burns it out.

What if I'm wiring LEDs in parallel instead of series?

Each parallel branch needs its own resistor sized off that branch's LED, because LEDs don't share current evenly — a single shared resistor across parallel LEDs will run the lowest-voltage LED hotter than the rest.

My supply voltage is lower than the total LED forward voltage — what happened?

That string of LEDs physically can't turn on from that supply; reduce the number of LEDs in series or use a higher supply voltage.

What formula does the LED Series Resistor Calculator use?

Subtract the combined forward voltage of every LED in the series string from the supply voltage, then divide by the LED's rated forward current to get the resistor that absorbs the difference.