LED Series Calculator
Find the maximum number of LEDs you can wire in series from a given supply, and calculate the current-limiting resistor needed for the string.
How to Use This Calculator
Enter your supply voltage (e.g. 12V), the forward voltage of each LED (shown on the datasheet, typically 1.8V–3.5V), and your desired current in mA (20 mA is standard for most through-hole LEDs). The calculator finds the maximum number of LEDs that fit in series and the resistor needed for the remainder of the voltage.
Formula
N = number of LEDs, V_supply = supply voltage, V_forward = LED forward voltage, I = current in amps, P = power in watts.
Example
N = floor(12 / 2.0) = 6 LEDs
V_resistor = 12 − (6 × 2.0) = 0 V → add a minimum resistor
Safer: 12V, Vf = 2.1V, 20 mA
N = floor(12 / 2.1) = 5 LEDs
V_res = 12 − (5 × 2.1) = 1.5V → R = 1.5 / 0.02 = 75 Ω
Frequently Asked Questions
Why must LEDs in series all have the same current rating?
In a series circuit the same current flows through every component. If LEDs have different forward voltages but are all rated for the same current, the current through all of them will be identical, which is fine. Problems arise when you mix LEDs with very different Vf values in parallel strings.
What happens if I exceed the maximum number of LEDs?
The supply voltage would be insufficient to forward-bias all the LEDs, meaning some or all would not illuminate. Always ensure N × Vf < V_supply, with enough headroom for the resistor.
Is a resistor always necessary?
Yes, unless your supply exactly equals the total LED voltage drop — which is unstable and not recommended. The resistor limits current and protects the LEDs from small supply variations.
Can I put multiple series strings in parallel?
Yes. Each string should have its own resistor to handle slight differences in Vf between LEDs. Never connect LED strings in parallel without individual series resistors.