How many LEDs can go in one series string?

nmax = floor((Vsupply − headroom) / Vf). For example, with 12 V, a 2 V red LED and 2 V of resistor headroom: (12−2)/2 = 5 LEDs.

How do you calculate the resistor for an LED array?

For one series string, R = (Vsupply − n×Vf) / I. For example, 12 V, 5 red LEDs (10 V), 20 mA → (12−10)/0.02 = 100 Ω.

Can one resistor serve parallel LEDs?

No. Because their Vf values are not identical, the current does not split equally and one LED burns out. Give every parallel branch its own resistor.

How do you find the total current of an LED array?

Itotal = number of branches × branch current. For example, three parallel branches at 20 mA each draw 60 mA total.

Should I wire LEDs in series or parallel?

With a high supply voltage, series strings are more efficient (one current). For low voltage and many LEDs you need parallel branches, each with its own resistor.

How to Wire an LED Array (Series/Parallel) + Resistor Math

An LED array is built by wiring multiple LEDs in series and parallel. Correct wiring keeps every LED at a safe current and uses the supply voltage efficiently. This article covers how many LEDs fit in one series string, how to size the resistor per branch, and the total current. For a single LED, start with how to calculate an LED resistor.

Series wiring: voltages add up

In a series string the LEDs' forward voltages (V_F) add up: for n LEDs the total is V_total = n × V_F. The same current flows through the whole string, so a single series resistor sets the string's current:

R = (V_supply − n × V_F) / I

The supply must be high enough to cover the series LEDs' total voltage; leaving a few volts of headroom for the resistor keeps the current stable when the supply fluctuates.

How many LEDs fit in one string?

The maximum number of LEDs in a series string is the supply minus the resistor headroom, divided by V_F:

n_max = floor( (V_supply − headroom) / V_F )

For example, with a 12 V supply, a 2 V red LED and ~2 V of resistor headroom: (12 − 2) / 2 = 5 LEDs. The remaining 2 V drops across the resistor.

Parallel branches: a separate resistor per branch

To drive more LEDs you wire several series strings in parallel. The critical rule: give every parallel branch its own series resistor. Putting multiple LEDs (or branches) in parallel on a single resistor is dangerous; because their V_F values are not identical, the current does not split equally — one LED hogs current and burns out first, then the rest follow. A separate resistor fixes each branch's current independently.

The total current is the sum of the branches: I_total = number of branches × branch current.

Worked example

12 V supply, red LED (V_F = 2 V), branch current 20 mA:

Max LEDs per string: (12 − 2) / 2 = 5 LEDs (10 V total), 2 V left for the resistor.
Branch resistor: R = (12 − 5×2) / 0.02 = (12 − 10) / 0.02 = 100 Ω.
For 3 parallel branches: a separate 100 Ω per branch, total current = 3 × 20 mA = 60 mA.

Series or parallel? A comparison

Property	Series (one string)	Parallel branches
Voltage	V_Fs add up (needs high V_supply)	Each branch sees the same V_supply
Current	One current for the string (efficient)	Currents add up (high total current)
Resistor	One resistor per string	A separate resistor per branch
One LED fails	The whole string goes dark	Only that branch goes dark

Common mistakes

One resistor for parallel LEDs: the current does not split equally → LEDs fail one by one. Use a separate resistor per branch.
Insufficient supply voltage: if V_supply < n×V_F the string will not light; don't forget the resistor headroom either.
Mixing colors in series: the V_Fs differ; do the math with each LED's actual V_F.
Ignoring resistor power: for high-current branches, check the resistor's wattage (P = I²×R).

Use the calculator

To instantly work out the string layout and resistors from your supply voltage, LED count, V_F and current, use our LED Array Calculator.