Why do LEDs in series need a resistor?

LEDs are current devices not voltage devices. Without a resistor to limit the current, the LED draws excessive current and burns out. The resistor drops the excess voltage and sets a safe current.

Do all LEDs in series get the same brightness?

Yes. Series LEDs share the same current so they all produce the same brightness. This is the main advantage of series connection over parallel.

What happens if one LED in a series string fails?

If it fails open the entire string goes dark because the current path is broken. If it fails short the remaining LEDs get slightly more current which may increase brightness or cause a cascade failure.

Is series or parallel better for LEDs?

Series is simpler and ensures equal brightness but one failure kills the whole string. Parallel allows independent strings but needs a resistor per string. Series is more efficient because it wastes less power in resistors.

Why is efficiency higher with more LEDs in series?

More LEDs use more of the supply voltage leaving less for the resistor. The resistor power is wasted heat so less resistor voltage means less waste. Three red LEDs on 12V waste 6V in the resistor while five waste only 2V.

LED Series Calculator – Series String Resistor, Max LEDs & Efficiency

Series LED String — Resistor, Max LEDs & Efficiency

V_s Supply Voltage

V_f LED Forward Voltage (per LED)

I LED Current

n Number of LEDs in Series

LEDs

Series LED String Analysis

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Series Resistor (E24)
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Exact Resistor

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Actual Current

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Resistor Voltage

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Total LED Voltage

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Max LEDs

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Resistor Power

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heat wasted

Power Per LED

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Efficiency
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LED power / total

Total Power

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LEDs in Series

LEDs connected in series share the same current. The supply voltage must exceed the sum of all LED forward voltages, with enough left over for the current-limiting resistor. More LEDs in series = higher efficiency (less voltage wasted in the resistor).

V_s — Supply voltage. Must exceed the total LED voltage plus ~1 V minimum for the resistor.
R — Current-limiting resistor. Drops the excess voltage and sets the LED current.
V_f — Forward voltage per LED. Red ~2.0 V, Green ~2.2 V, Blue/White ~3.0–3.5 V.
I — Current through the entire series string. Same for all LEDs. Typically 20 mA for standard LEDs.

LED Series Calculator

LEDs connected in series share the same current — one resistor sets the brightness for the entire string. This is the simplest and most efficient way to drive multiple LEDs from a fixed voltage supply. The calculator finds the resistor value, checks whether the supply voltage is high enough, shows the maximum number of LEDs the supply can handle, and computes the efficiency (how much power goes to light vs heat in the resistor).

How Series LEDs Work

In a series string, the same current flows through every LED and the resistor. Each LED drops its forward voltage (V_f), and the resistor drops whatever voltage is left over. The resistor’s job is to absorb the excess voltage and limit the current to a safe value. Because all LEDs see the same current, they all produce the same brightness — no matching required. For the single-LED version of this calculation, see the LED Resistor Calculator.

The Formula

V_string = n × V_f — total LED voltage
V_R = V_s − V_string — voltage across resistor
R = V_R / I — resistor value
P_R = V_R × I — power wasted in resistor
Efficiency = (n × V_f × I) / (V_s × I) × 100%

The efficiency formula simplifies to V_string / V_s — the fraction of the supply voltage used by the LEDs. More LEDs means more of the supply voltage goes to useful light and less is wasted as heat in the resistor.

LED Forward Voltage Reference

LED Colour	Typical V_f	Range
Infrared	1.2 V	1.0–1.5 V
Red	2.0 V	1.8–2.2 V
Orange / Yellow	2.1 V	1.9–2.4 V
Green	2.2 V	2.0–3.5 V
Blue	3.2 V	2.8–3.5 V
White	3.3 V	3.0–3.6 V
UV	3.5 V	3.2–4.0 V

12V / 3 Red LEDs (V_f = 2.0 V, 20 mA)

V_string = 3 × 2.0 = 6.0 V
V_R = 12 − 6.0 = 6.0 V
R = 6.0 / 0.020 = 300 Ω → 300 Ω (E24)
P_R = 6.0 × 0.020 = 120 mW
Efficiency = 6.0 / 12 = 50%

Half the power goes to LEDs, half to the resistor. Acceptable for indicator LEDs but wasteful for lighting. Adding more LEDs improves efficiency — 5 red LEDs on 12V gives 83% efficiency (only 2V across the resistor). To check whether a 1/4 W resistor can handle the 120 mW, use the Resistor Power Dissipation Calculator.

24V / 6 Blue LEDs (V_f = 3.2 V, 20 mA)

V_string = 6 × 3.2 = 19.2 V
V_R = 24 − 19.2 = 4.8 V
R = 4.8 / 0.020 = 240 Ω → 240 Ω (E24)
P_R = 4.8 × 0.020 = 96 mW
Efficiency = 19.2 / 24 = 80%

80% efficiency — good. 24V supplies allow long series strings. You could fit 7 blue LEDs (22.4 V) with only 1.6 V for the resistor (93% efficient), but the margin is tight — V_f variations between LEDs could push the total over 24V and the string would stop working.

5V / 1 White LED (V_f = 3.3 V, 20 mA)

V_string = 1 × 3.3 = 3.3 V
V_R = 5 − 3.3 = 1.7 V
R = 1.7 / 0.020 = 85 Ω → 91 Ω (E24)
Efficiency = 3.3 / 5 = 66%

Only one LED fits because 2 × 3.3 = 6.6V exceeds the 5V supply. The 34% wasted in the resistor is inherent to the voltage mismatch. For multiple white LEDs on 5V, you need parallel strings (one LED per string, each with its own resistor) or an LED Driver Calculator to design a boost converter that raises the voltage.

Maximum LEDs Per String

Rule: n_max = floor(V_s / V_f) − 1

Subtract 1 to leave at least ~1V for the resistor to regulate current. Without enough headroom, the resistor voltage is too small to absorb V_f variations, and current regulation is poor.

12V supply: 5 red (2.0V), 3 white (3.3V), 5 green (2.2V)
24V supply: 11 red, 6 white, 10 green
5V supply: 1 white, 2 red, 1 blue

Efficiency and Power

Efficiency = V_string / V_s. The only way to improve it is to use more of the supply voltage for LEDs. Three strategies: add more LEDs (if the supply voltage allows), reduce the supply voltage (if the system permits), or switch to an LED driver IC (constant-current, no resistor, 85–95% efficiency regardless of voltage). For total power and cost analysis across your LED installation, see the LED Power Calculator.

Frequently Asked Questions

How many LEDs can I put in series?

Divide the supply voltage by the LED forward voltage, then subtract 1 for resistor headroom. For 12V with 2.0V red LEDs: 12/2 − 1 = 5 LEDs. The calculator shows the exact maximum for your inputs.

Why do series LEDs need a resistor?

LEDs are current devices, not voltage devices. Without a resistor to absorb the excess voltage and limit current, the LED draws too much current and burns out instantly. The resistor is not optional — it is the current regulator.

Do all LEDs in a series string have the same brightness?

Yes. The same current flows through every LED in the string, so they all produce the same light output. This is the main advantage of series over parallel — no current-sharing problems.

What happens if one LED fails?

If it fails open (breaks), the entire string goes dark because the current path is broken. If it fails short (shorts out), the remaining LEDs get slightly more current because the resistor voltage increases. For fault-tolerant designs, use multiple parallel strings so one failure only affects one string.

Is series more efficient than parallel?

Yes, for the same number of LEDs. Series uses one resistor dropping a small voltage. Parallel uses one resistor per LED (or per string), each dropping the full difference between supply and V_f. More resistors = more wasted power.

Can I mix different LED colours in one series string?

Yes, but use the sum of the individual forward voltages for the string voltage. A red (2.0V) + green (2.2V) + blue (3.2V) string needs 7.4V. All three share the same current, so brightness depends on each LED’s efficiency at that current.

Last updated: March 2026