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LED Array Calculator

Electronics & PhysicsCircuits and Components

The regulated DC voltage feeding the array.

From the datasheet: about 2.0 V for red, 3.0–3.4 V for white or blue.

The current each string should carry.

Resistor per string (Ω) —
LEDs in series per string
—
Parallel strings
—
Last string
—
Power in each resistor (mW)
—
Total supply current (mA)
—
Total power drawn (W)
—
Share of power reaching the LEDs
—

An LED array calculator splits your LEDs into series strings that fit under the supply voltage, then sizes one current-limiting resistor per string. For nine 3.2 V white LEDs at 20 mA on 12 V, three LEDs go in each string, leaving 2.4 V, so each of the three strings needs a 120 Ω resistor drawing 60 mA in total.

About this tool

Hobbyists wiring signs, backlights or grow panels from discrete LEDs need two answers before soldering: how to group the LEDs, and what resistor goes with each group. This calculator places as many LEDs in series as the supply allows while still leaving voltage for a resistor, counts the parallel strings, and gives the resistor value, its power dissipation, the total supply current and how much of the input power ends up in the LEDs. If the count does not divide evenly, the shorter last string gets its own resistor value. The model assumes a fixed forward voltage; real LEDs vary by a few tenths of a volt between parts and with temperature, so check the current on a built string before scaling up.

How to use it

  1. Enter the supply voltage

    Use the regulated output voltage of your adapter or driver, in volts.

  2. Add the LED data

    Type the forward voltage and the current per LED from the datasheet.

  3. Set the LED count

    Enter how many LEDs the array holds in total.

  4. Read the layout

    The result gives the resistor per string, the string count, the resistor wattage and the total current.

Examples

9 white LEDs from a 12 V supply

Supply voltage
12
LED forward voltage
3.2
LED current
20
Number of LEDs
9

Result Resistor per string (Ω): 120
LEDs in series per string: 3
Parallel strings: 3
Last string: Same as the others
Power in each resistor (mW): 48
Total supply current (mA): 60
Total power drawn (W): 0.72
Share of power reaching the LEDs: 80%

  1. LEDs per string = floor(12 V ÷ 3.2 V), keeping some voltage for the resistor = 3
  2. Strings = ceil(9 ÷ 3) = 3
  3. Resistor = (12 − 3 × 3.2) V ÷ 0.02 A = 120 Ω
  4. Resistor power = 2.4 V × 0.02 A = 48 mW
  5. Supply current = 3 × 20 mA = 60 mA

Three LEDs fit per string with 2.4 V to spare, so the nine LEDs form three strings, each with a 120 Ω resistor.

5 red LEDs from USB 5 V

Supply voltage
5
LED forward voltage
2
LED current
20
Number of LEDs
5

Result Resistor per string (Ω): 50
LEDs in series per string: 2
Parallel strings: 3
Last string: 1 LED, 150 Ω
Power in each resistor (mW): 20
Total supply current (mA): 60
Total power drawn (W): 0.3
Share of power reaching the LEDs: 66.7%

  1. LEDs per string = floor(5 V ÷ 2 V), keeping some voltage for the resistor = 2
  2. Strings = ceil(5 ÷ 2) = 3
  3. Resistor = (5 − 2 × 2) V ÷ 0.02 A = 50 Ω
  4. Resistor power = 1 V × 0.02 A = 20 mW
  5. Supply current = 3 × 20 mA = 60 mA
  6. Last string: (5 − 1 × 2) V ÷ 0.02 A = 150 Ω

Two red LEDs fit per string, so five LEDs need three strings; the odd one out gets a larger 150 Ω resistor.

How it is calculated

n = floor(Vs ÷ Vf), reduced by 1 if n × Vf = Vs; strings = ceil(N ÷ n); R = (Vs − n × Vf) ÷ I; P = (Vs − n × Vf) × I

Vs
Supply voltage in volts
Vf
Forward voltage of one LED in volts
I
Current through each string in amperes
N
Total number of LEDs
n
LEDs in series per string

LEDs in series share one current, so their forward voltages add. The tool fits the largest whole number of LEDs whose combined forward voltage stays below the supply, because a resistor needs some voltage across it to regulate current. The leftover voltage divided by the target current is the resistor value by Ohm's law, and the leftover voltage times the current is the heat the resistor must handle. Strings run in parallel, so the supply current is the string current multiplied by the number of strings.

When not to use it

  • High-power LEDs above roughly 350 mA are better run from a constant-current driver than from resistors.
  • Addressable strips and modules with built-in resistors already set their own current.
  • An unregulated supply whose voltage sags or rises under load makes the resistor value unreliable.

Common mistakes

  • Wiring LEDs in parallel on one shared resistor, which lets the LED with the lowest forward voltage hog the current.
  • Using the typical forward voltage when the datasheet maximum is much higher, leaving the string dim or dark.
  • Choosing a 0.125 W resistor when the calculated dissipation is close to or above that rating.
  • Forgetting that the short last string needs a different resistor value.

Frequently asked questions

Why not put every LED in series?

The sum of forward voltages in a string must stay below the supply voltage. Nine 3.2 V LEDs need 28.8 V, so a 12 V supply can only light three of them per string. Splitting the array into parallel strings keeps each string under the supply.

Why does the tool leave some voltage unused even when LEDs would fit exactly?

If four 3 V LEDs sit on exactly 12 V, there is no voltage left for the resistor and nothing controls the current. A small change in supply or temperature would then cause a large current swing. The tool drops one LED from the string in that case so the resistor has room to work.

Which standard resistor value should I buy?

Round the calculated value up to the next E12 or E24 value, for example 120 Ω stays 120 Ω and 137 Ω becomes 150 Ω. Rounding up slightly lowers the current, which is the safe direction for LED life.

What does the share of power reaching the LEDs mean?

It is the LED power (count × forward voltage × current) divided by everything the supply delivers. The rest is heat in the resistors. Longer strings on a supply just above their total forward voltage waste less, but they also regulate current less tightly.

How do I find the forward voltage of my LEDs?

Check the datasheet for the forward voltage at your chosen current. Without one, typical figures are about 1.8 to 2.2 V for red and yellow, and 2.9 to 3.4 V for white, blue and true green. Measuring one LED with a bench supply and a test resistor gives a closer number.

Can I use one resistor for the whole array?

Not reliably. Parallel strings share voltage, not current, so small differences in forward voltage send more current into one string than another. One resistor per string balances them well enough for indicator and lighting use.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 18). LED Array Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/electronics-physics/circuits-and-components/led-array-calculator/

MLA

Tools Directory Editorial Team. “LED Array Calculator.” Tools Directory, 18 Oct. 2026, https://tools.milunir.com/electronics-physics/circuits-and-components/led-array-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “LED Array Calculator.” Tools Directory. Last modified October 18, 2026. Accessed October 27, 2026. https://tools.milunir.com/electronics-physics/circuits-and-components/led-array-calculator/.

Link<a href="https://tools.milunir.com/electronics-physics/circuits-and-components/led-array-calculator/">LED Array Calculator – Tools Directory</a>