How to use the led string count and resistor calculator
Find how many LEDs fit in series on a supply and the resistor needed for the string. Enter supply voltage, led forward voltage, led current in the fields above; the leds that fit in series, series resistor for the string are computed instantly and plotted on the chart so you can see how the answer changes.
Common searches this calculator answers include “LEDs in series”, “LED string resistor”, “how many LEDs can I chain”, “calculate led string count and resistor”, “find leds that fit in series”.
Formula
Variables
- Vs — Supply voltage (V)
- Vf — LED forward voltage (V)
- I — LED current (mA)
Worked example
With supply voltage = 12 V, led forward voltage = 3.2 V, led current = 20 mA, the calculator gives leds that fit in series ≈ 3 LEDs; series resistor for the string ≈ 120 Ω.
Frequently asked questions
What is the led string count and resistor calculator used for?
Find how many LEDs fit in series on a supply and the resistor needed for the string. It helps with questions such as “LEDs in series”, “LED string resistor”, “how many LEDs can I chain”.
What is the formula for the led string count and resistor calculation?
LEDs that fit in series (LEDs) = floor(Vs / Vf). Series resistor for the string (Ω) = (Vs-[LEDs that fit in series] × Vf) / (I / 1000). Here Vs is supply voltage (V), Vf is led forward voltage (V), I is led current (mA).
What inputs do I need for the led string count and resistor calculator?
You need supply voltage in V, led forward voltage in V, led current in mA. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use the led string count and resistor calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the led string count and resistor result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.