LED Series Parallel Calculator

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Calculate LED series strings, parallel branches, current-limiting resistor values, resistor power, total current, and supply power for practical LED circuit design.

Input Parameters

Supply Voltage

Available supply voltage for the LED strings.

LED Forward Voltage

Use the LED datasheet typical or worst-case forward voltage.

Total LED Count

pcs

Total number of LEDs to be connected.

LEDs Per Series String

pcs

Number of LEDs in each series branch.

Target Current per String

Desired current for each series LED string.

Resistor Power Derating

Use 50% to select a resistor rated at least twice the calculated dissipation.

Results

Series Resistor

Parallel Strings

Total Supply Current

Resistor Power / String

Estimated Supply Power

Design Note

Equations Used

Resistor Voltage: V_R = V_supply - N_series × V_F

Series Resistor: R = V_R / I_LED

Resistor Power: P_R = I_LED² × R

Parallel Strings: Strings = ceil(Total LEDs / LEDs per string)

Total Current: I_total = I_LED × Number of strings

Frequently Asked Questions (FAQ)

Q1: How do I calculate a resistor for LEDs in series?
Subtract the total LED forward voltage from the supply voltage, then divide the remaining voltage by the target LED current.

Q2: Why should LEDs use a current-limiting resistor?
Small voltage changes can cause large LED current changes. The resistor limits current and protects the LED from overcurrent.

Q3: Can I connect LEDs directly in parallel?
Direct parallel LEDs are not recommended because current may not share evenly. Use separate resistors for each string when possible.

Q4: Should I use typical or maximum LED forward voltage?
For robust design, check both. Maximum forward voltage affects headroom, while minimum forward voltage can increase current.

Q5: How should resistor power rating be selected?
Use a resistor with a power rating higher than calculated dissipation, commonly 2× or more for reliability.

Q6: Why does this calculator show a headroom warning?
If supply voltage is too close to the total LED forward voltage, the current becomes sensitive to LED tolerance and temperature.

Disclaimer: This calculator is for DC LED resistor estimation. LED forward voltage changes with current, temperature, color, binning, and manufacturer tolerance. Verify final designs with the LED datasheet and real measurements.

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