LED Series Calculator
This LED Series Calculator estimates how many LEDs fit within a supply voltage and calculates the current-limiting resistor for a selected series string.
Results include LED string voltage, voltage headroom, E24 resistor selection, resistor dissipation, recommended power rating, and circuit current for indicator and low-power lighting designs.
Use forward voltage at the intended current and temperature. Production designs should include supply tolerance, LED binning, thermal effects, resistor tolerance, and suitable current margin.
Engineering tool
LED Series Calculator
Calculate LED string capacity, voltage headroom, current-limiting resistance, E24 selection, and resistor power for one series branch.
Voltage available to the LED string and resistor.
Typical voltage drop of one LED at target current.
Desired operating current through the complete string.
Positive whole number of LEDs sharing one resistor.
Recommended E24 series resistor
100 Ω
R = (Vs - Vf × N) / I
Result console
- Maximum LEDs in series
- 6LEDs
- Total LED string voltage
- 10V
- Voltage headroom
- 2V
- Calculated series resistor
- 100Ω
- Nearest higher E24 resistor
- 100Ω
- Resistor power dissipation
- 40mW
- Recommended resistor power
- 0.25W
- Circuit current
- 20mA
LED Series String
Selected string: 5 LEDs in series with one current-limiting resistor.
Formula reference
LED Series String Formulas
The resistor must have positive voltage headroom. Maximum LED count is a voltage-ratio reference; practical resistor-driven strings may require fewer LEDs to preserve current-control margin.
LED string voltage: Vled_total = Vf × NMaximum count: Nmax = floor(Vs / Vf)Resistor voltage: Vr = Vs - Vled_totalSeries resistor: R = Vr / IResistor power: P = I²RVariable definitions
- Vs
- Supply voltage in volts (V)
- Vf
- Forward voltage of one LED in volts (V)
- N
- Number of LEDs in the selected string
- I
- Target string current in amperes (A)
- R
- Current-limiting resistance in ohms (Ω)
How to Use This Calculator
- Enter the minimum expected supply voltage.
- Enter one LED's forward voltage at the target operating current.
- Enter target string current and the desired whole-number LED count.
- Use the next higher E24 value and verify power rating, brightness, tolerance, and thermal margin.
Worked Example
Five 2 V LEDs from a 12 V Supply
Vs = 12 V, Vf = 2 V, N = 5, I = 20 mA = 0.02 A
Vled_total = 2 × 5 = 10 V
Vr = 12 - 10 = 2 V
R = 2 / 0.02 = 100 Ω
P = 0.02² × 100 = 0.04 W = 40 mW
Use 100 Ω E24 with a recommended rating of at least 0.25 W.
Engineering Notes
Forward voltage varies
LED forward voltage changes with color, temperature, manufacturing bin, current, and part number.
Leave regulation headroom
Always leave voltage across the resistor or driver at the lowest supply and highest expected LED string voltage.
Series strings improve efficiency
Long strings can waste less resistor power than many parallel branches when supply voltage provides adequate headroom.
Regulate each branch
Use one resistor or one current-regulator channel per parallel LED string to avoid uncontrolled current sharing.
Drivers suit high power
A constant-current driver is generally better than a simple resistor for high-power LEDs, wide input ranges, and precision brightness.
Common Mistakes
- Using maximum supply voltage and typical Vf without checking worst-case headroom.
- Selecting the mathematical maximum LED count even though no resistor voltage remains.
- Connecting parallel strings through one shared resistor.
- Ignoring resistor dissipation and temperature derating.
- Using a resistor for high-power LEDs that require active current regulation.
Support reference
FAQ
How many LEDs can I connect in series?
Divide supply voltage by the typical forward voltage of one LED and round down. For a resistor-driven string, select fewer LEDs when necessary to leave positive voltage headroom for current regulation.
How do you calculate the resistor for LEDs in series?
Subtract total LED forward voltage from supply voltage, then divide the remaining resistor voltage by target current: R = (Vs - Vf × N) / I.
Why do LEDs in series need a resistor?
LED current changes sharply with voltage. A resistor absorbs voltage variation and limits current so that forward-voltage tolerance and temperature changes do not cause uncontrolled current.
What happens if the supply voltage is too low?
The selected LED string will not have enough voltage to reach its intended current, and no positive resistor voltage remains. Reduce LED count or increase the supply voltage within component limits.
Should I use a resistor or a constant-current LED driver?
A resistor is suitable for many low-power indicators with a stable supply and adequate headroom. High-power LEDs, wide supply ranges, and precision brightness generally benefit from a constant-current driver.
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Browse the Diodes calculator category, the LEDs calculator category, or the Engineering Reference Center.
This calculator uses nominal LED and resistor values. Verify supply tolerance, LED forward-voltage distribution, current and temperature limits, resistor derating, thermal design, and measured string current before production use.