LED Series Resistor Calculator
Size series resistors for LED strings and arrays, including power rating.
About LED Series Resistor Calculator
Putting more LEDs in series reduces the voltage wasted in the resistor and raises efficiency. Individual resistors per string are needed because LED forward voltages differ enough that a shared resistor would leave current badly unbalanced. Maximising LEDs per string is the single most effective way to improve resistor-driven LED efficiency, which is why 24 V strips use six-LED groups.
Formula
LED Series Resistor
R per string = (V_supply − n V_f) ÷ I_string; efficiency = LED power ÷ total power
- string
- Series chain of LEDs sharing one resistor
- efficiency
- Fraction of power reaching the LEDs rather than the resistor
Worked example
With the default values below, LED Series Resistor Calculator returns Resistor per String of 240.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Supply voltageminimum 0 V
- 24 V
- Forward voltage per LEDminimum 0 V
- 3.2 V
- LEDs per series stringaccepts 1–30
- 6
- Parallel stringsaccepts 1–50
- 4
- Current per stringminimum 0.1 mA
- 20 mA
Results
- Resistor per String
- 240.00 ohm
- Total Current Drawn
- 80.00 mA
- Power per Resistor
- 0.09600 W
- Total Power Consumed
- 1.9200 W
- Share of Power Reaching the LEDs
- 80.00%
Step by step
- 1Values used
- 2LED Series Resistor
- 3Resistor per String= 240.00 ohm
- 4Total Current Drawn= 80.00 mA
- 5Power per Resistor= 0.09600 W
- 6Total Power Consumed= 1.9200 W
- 7Share of Power Reaching the LEDs= 80.00
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is LED Series Resistor calculated?
R per string = (V_supply − n V_f) ÷ I_string; efficiency = LED power ÷ total power Putting more LEDs in series reduces the voltage wasted in the resistor and raises efficiency. Individual resistors per string are needed because LED forward voltages differ enough that a shared resistor would leave current badly unbalanced.
Why does LED Series Resistor matter?
Maximising LEDs per string is the single most effective way to improve resistor-driven LED efficiency, which is why 24 V strips use six-LED groups.
What values do I need to enter?
This calculator takes 5 inputs: Supply voltage, Forward voltage per LED, LEDs per series string, Parallel strings, Current per string. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.