LED Resistor Sizing Tool

Compute the series resistor and power rating for one or more LEDs on a DC supply. Designed for quick checks on indicator LEDs, panel lights, and small LED strings, not full lighting drivers.

LED series resistor calculator

Enter the supply voltage, LED forward voltage and current, and how many LEDs you’re putting in series. If you’re driving multiple identical strings, add the number of parallel strings to check total current.

Supply voltage (V)

LED forward voltage Vf (V)

LED current (mA) per string

LEDs in series (count)

Parallel strings (count, optional)

Extra voltage headroom (V, optional)

How to use this tool

Assumes a simple series resistor with one or more identical LEDs on a DC supply. For switching supplies, PWM dimming, or COB modules, this is a sanity check, not a full design.

Enter the supply voltage.
Enter the LED’s forward voltage Vf at your target current (from the datasheet or experience).
Enter the desired LED current in mA and the number of LEDs you intend to place in series.
If you’re duplicating that string several times, enter parallel strings so the tool can show total current.
Optionally add a little extra headroom voltage to keep the resistor from going to zero when supply and Vf are close.
Click Calculate. The tool shows:
- The required resistor value and a rounded “nearby” value
- Resistor power dissipation and minimum wattage
- Per-string and total current draws
- How much voltage actually drops across the resistor

Example – single 5 mm indicator LED on 12 V

• Supply = 12 V
• Vf = 2.0 V (red)
• I = 10 mA, series count = 1

The tool reports a resistor around 1 kΩ, ~0.1 W. You’d choose a standard 1 kΩ, ¼ W part and the LED would sit at a comfortable brightness.

What the LED resistor math is doing

For a string of N identical LEDs in series on a supply voltage Vs, with forward voltage Vf at the chosen current:

Voltage across resistor V_R = Vs − (N × Vf) − headroom
Resistor value R = V_R / I, with I in amps
Resistor power P_R = I² × R

If you have M parallel strings each with its own resistor, total current is simply M × I.

Rule of thumb. Whenever Vs is only slightly above N × Vf, build in some headroom. Real supplies and LEDs drift with temperature and tolerance. This tool will warn you when there isn’t enough voltage left across the resistor to reliably control current.

LED string checklist

Quick checks after you get a resistor value:

Power rating. Choose at least 2× the calculated resistor power, more in hot enclosures.
Supply tolerance. Consider minimum and maximum supply voltage and recalc extremes if necessary.
LED tolerance. Vf varies part-to-part. If color or brightness uniformity is critical, give yourself extra margin.
Parallel strings. Prefer one resistor per string, not a single resistor feeding several branches.
Thermal comfort. Warm resistors are fine; too hot to touch usually means you need a higher wattage or a different approach.

Examples from Celtic Engineering Solutions

Example A

Three white LEDs in series on 12 V

A small board needed a three-LED status bar from a 12 V rail.

• Vs = 12 V, Vf ≈ 3.2 V, N = 3, I = 15 mA
• V_R ≈ 12 − 3×3.2 = 2.4 V
• R ≈ 2.4 / 0.015 ≈ 160 Ω, P ≈ 0.036 W

A 160 Ω or 180 Ω, ¼ W resistor ran comfortably cool and gave a bright but not overdriven bar.

Example B

Panel indicator LEDs from 5 V logic rail

Multiple panel LEDs shared a 5 V logic rail, each with its own resistor.

• Vs = 5 V, Vf ≈ 2.0 V, I = 5 mA, N = 1
• R ≈ (5 − 2)/0.005 ≈ 600 Ω → standard 620 Ω

Current stayed modest, avoiding unnecessary rail load while remaining clearly visible on a brightly lit bench.

Unsure if a resistor is enough?

If your LEDs are part of a product with wide supply variation, very high brightness, or long expected life, a simple resistor may not be the right driver. Send us the numbers you see here and the LED part number, and we’ll let you know when we’d step up to a proper current driver.

Send one question about your LED string →