LED Series Calculator
Design your multi-strand LED circuits with ease. Calculate the maximum LEDs per string, required resistors, and overall power draw for complex series-parallel LED arrays.
Series vs Parallel vs Series-Parallel
When wiring multiple LEDs, you have three main options depending on your supply voltage and total LED count:
- Series: Wiring LEDs end-to-end. The current is identical through all LEDs, but the voltage drops add up. This is highly efficient because you only waste a small amount of voltage on a single resistor.
- Parallel: Wiring each LED directly to the power source with its own resistor. The voltage is identical, but the current adds up. This wastes more power as heat in the resistors if the supply voltage is much higher than the LED forward voltage.
- Series-Parallel (Multi-Strand): The best of both worlds. You create strings of series LEDs to maximize efficiency, then wire those strings in parallel to reach your desired total number of LEDs.
How LEDs in series work
According to Kirchhoff's Voltage Law, the total voltage around a closed loop must equal zero. This means the supply voltage is entirely consumed by the components in the circuit. If you have 3 LEDs in series, their forward voltages (Vf) sum up, and the resistor must "drop" the remaining voltage.
Because there is only one path for the electrons to flow, the same current flows through all the LEDs in a series string. This guarantees they will all be exactly the same brightness (assuming matched LEDs), which is a major advantage of series wiring.
Maximum LEDs per series string formula
To ensure a stable circuit, you shouldn't use up 100% of your supply voltage on LEDs alone. You need to leave some "voltage headroom" for the resistor to regulate the current properly. We recommend at least a 0.5V to 1.5V margin depending on the supply.
Our calculator automatically determines the maximum number of LEDs you can place in a single string, and then calculates how many such strings you need to wire in parallel to reach your total LED count.
Maximum LEDs per String Reference
| Supply Voltage | Red LEDs (~2.0V) | Blue/White LEDs (~3.3V) |
|---|---|---|
| 5V USB | 2 LEDs (4.0V total) | 1 LED (3.3V total) |
| 12V Automotive | 5 LEDs (10.0V total) | 3 LEDs (9.9V total) |
| 24V Industrial | 11 LEDs (22.0V total) | 7 LEDs (23.1V total) |
| 48V Telecom | 23 LEDs (46.0V total) | 14 LEDs (46.2V total) |
Frequently Asked Questions
It depends on the forward voltage of the LEDs. For 2.0V red LEDs, you can safely put 5 in series (leaving 2V headroom). For 3.3V white LEDs, you can put 3 in series (leaving 2.1V headroom).
If the total forward voltage of the LEDs exceeds your supply voltage, the LEDs will not light up because there isn't enough voltage to forward-bias the string.
No, LEDs wired in series share the same current. You only need one resistor per series string to limit the current for all LEDs in that string.
In series, the voltage drops add up but the current stays the same. In parallel, the voltages stay the same but the total current drawn from the supply multiplies.
It is a combination where you wire several LEDs in series to form a string (with one resistor), and then wire multiple identical strings in parallel to increase the total number of LEDs without needing a higher supply voltage.
Methodology
This calculator determines the optimal array configuration using the following logic:
- Max LEDs per string: Evaluates how many LEDs can fit within the supply voltage while leaving a safe margin (min 0.5V headroom) for the current-limiting resistor.
- Number of strings: Divides your total desired LEDs by the max LEDs per string, rounding up, to find how many parallel strings are needed.
- Resistor sizing: Uses Ohm's law to find the exact resistance needed to drop the remaining voltage for a full string, then recommends the closest standard E24 resistor value.
- Total Power: Multiplies the supply voltage by the total current (Strings × Forward Current) to show the overall power requirement of the circuit.