LED Parallel Calculator

LED Parallel Calculator – Parallel Strings Resistor, Total Current & Efficiency
Parallel LED Strings — Resistor, Total LEDs & Efficiency
Vs Supply Voltage
V
Vf LED Forward Voltage (per LED)
V
I LED Current
ns LEDs Per String
LEDs
np Number of Parallel Strings
strings
Parallel LED Strings Analysis
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Resistor Per String (E24)
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Exact Value
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Current Per String
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Resistors Needed
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Total Current
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Total LEDs
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Power Per Resistor
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heat wasted
Total Resistor Power
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Efficiency
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LED power / total
Total Power
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Parallel LED Strings

parallel LED strings each with their own resistor share the same current. Each string works like the series calculator. Adding parallel strings increases total current and LED count, with enough left over for the current-limiting resistor. Each string is independent — if one LED fails, only its string goes dark.

Vs R I Vf Vf Vf Vs = VR + n × Vf R = (Vs − n × Vf) / I Same current flows through all LEDs and the resistor
Vs — 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.
Vf — 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 Parallel Calculator

When you need more LEDs than one series string can hold, you run multiple strings in parallel. Each string has its own current-limiting resistor and operates independently. If one string fails, the others keep working. This calculator finds the resistor value per string, total current draw, total power, efficiency, and the number of resistors needed.

How Parallel LED Strings Work

Each parallel string is a self-contained series circuit — LEDs in series with a resistor, all connected across the same supply voltage. The strings share the supply but carry independent currents. This is exactly how LED strip lights work: segments of 3 LEDs + 1 resistor repeated along the strip, all connected in parallel across 12V or 24V. For single-string calculations, use the LED Series Calculator.

Why Each String Needs Its Own Resistor

You cannot connect LEDs directly in parallel without individual resistors. LEDs have a steep V-I curve — a tiny difference in forward voltage between two LEDs causes a large difference in current. The LED with the lowest Vf hogs most of the current, overheats, its Vf drops further, it takes even more current, and it burns out. Then the next-lowest takes over and the cascade continues. One resistor per string forces equal current distribution regardless of Vf variations.

The Formulas

Per string (same as series):
R = (Vs − ns × Vf) / I

Parallel totals:
Itotal = Istring × np
Ptotal = Vs × Itotal
LEDs total = ns × np
Resistors needed = np (one per string)

12V / 3s × 4p — 12 Red LEDs

12V supply, 3 red LEDs (Vf = 2.0V) per string, 4 parallel strings, 20 mA each.

Per string: R = (12 − 6.0) / 0.020 = 300 Ω
4 strings × 20 mA = 80 mA total
Ptotal = 12 × 0.080 = 960 mW
Presistors = 4 × 6.0 × 0.020 = 480 mW
Efficiency = 50%

Need: 4 × 300 Ω resistors + 12 LEDs

480 mW wasted in resistors. For indicator LEDs at 20 mA this is fine. For high-power lighting, the waste adds up — 50 parallel strings would waste 6 W in resistors alone. For the energy cost of running LED arrays, see the LED Power Calculator.

5V / 1s × 10p — 10 White LEDs

5V supply, 1 white LED (Vf = 3.3V) per string, 10 parallel strings.

Per string: R = (5 − 3.3) / 0.020 = 85 Ω → 91 Ω (E24)
10 strings × 18.7 mA = 187 mA total
Ptotal = 5 × 0.187 = 935 mW
Efficiency = 66%

Need: 10 × 91 Ω resistors + 10 LEDs

Each string has only 1 LED because 2 × 3.3V = 6.6V exceeds 5V. This is the worst case for efficiency — 34% wasted. A 12V supply with 3 per string would be 83% efficient for the same 10 LEDs (4 strings of 3, with one extra). For high-efficiency solutions, consider an LED driver IC — the LED Driver Calculator sizes boost and buck-boost topologies.

24V / 6s × 3p — 18 Blue LEDs

24V supply, 6 blue LEDs (Vf = 3.2V) per string, 3 parallel strings.

Per string: R = (24 − 19.2) / 0.020 = 240 Ω
3 strings × 20 mA = 60 mA total
Ptotal = 24 × 0.060 = 1.44 W
Efficiency = 80%

Need: 3 × 240 Ω resistors + 18 LEDs

80% efficient. 24V systems allow long strings that use most of the supply voltage, leaving little for the resistor. This is the standard approach for architectural and signage LED lighting.

Series vs Parallel — When to Use Which

Use series only when all LEDs fit in one string (supply voltage ≥ n × Vf + 1V). Simplest, fewest components, most efficient. But one LED failure kills the whole string.

Use parallel strings when you need more LEDs than one string can hold, or when you need fault tolerance (one failure only kills one string). Each string needs its own resistor. Total current = strings × current per string.

LED strip lights use series-parallel: segments of 3 LEDs + 1 resistor (series), repeated in parallel along the strip. Cut marks are between parallel segments.

Power Supply Sizing

Total current = current per string × number of strings. Size the supply with at least 20% margin. For 10 strings at 20 mA each: 200 mA minimum, choose a 250 mA supply. Total power = Vs × Itotal. For the underlying V = IR relationship, see Ohm’s Law.

Frequently Asked Questions

How many parallel strings can I run?
As many as your power supply can handle. Total current equals current per string times number of strings. A 1A supply at 20 mA per string supports 50 parallel strings (1000 / 20 = 50).
Why does each string need its own resistor?
Without individual resistors, the LED with the lowest forward voltage hogs all the current and burns out. Separate resistors force equal current distribution regardless of Vf variations between LEDs.
Can I mix different LED colours in parallel strings?
Yes, as long as each string has its own resistor calculated for that string’s forward voltage. A red string (R = 300 Ω) and a blue string (R = 240 Ω) can run in parallel from the same supply with different resistor values.
What happens if one string fails?
Only that string goes dark. The other parallel strings continue working because each has its own independent current path and resistor. This is the main reliability advantage over a single long series string.
Is parallel less efficient than series?
No — each parallel string has the same efficiency as a single series string with the same number of LEDs. The total power waste increases because there are more resistors, but the percentage efficiency is the same.
How do LED strip lights work?
LED strips use series-parallel topology. Each segment contains 3 LEDs and 1 resistor in series (designed for 12V or 24V). Segments are connected in parallel along the strip. The cut marks indicate where you can safely cut between parallel segments.

Last updated: March 2026