wattstoamps
← Browse all tools

Simple DC LED string model

LED series resistor calculator

For one string of identical LEDs in series with one resistor, enter the DC supply voltage, a forward voltage per LED from its data sheet, the LED count and your chosen target current. The resistor takes the voltage left over.

Enter your values

Result

Results are based only on the inputs and assumptions shown here. Nothing you enter is sent or saved.

Formula: resistor voltage = supply V − LED count × forward V; resistance Ω = resistor V ÷ (target mA ÷ 1,000); resistor W = resistor V × target mA ÷ 1,000

Worked example

At 12 V, three identical 2 V-forward LEDs in series drop 6 V. A 20 mA target leaves 6 V across the resistor, giving an ideal 300 Ω and 0.12 W dissipated.

Use a data-sheet forward voltage

Forward voltage changes with current and temperature. A color alone is not a precise input. Use a forward-voltage value for the selected LED near your chosen current, and account for the supply and part tolerances before building a circuit.

One resistor serves one series string

All series LEDs carry the same current. This page does not model parallel strings, a regulated current driver or an LED connected directly to a supply. Adafruit’s LED guide applies Kirchhoff’s voltage law and Ohm’s law to a resistor and LED string.

Ideal resistance is not a component recommendation

The computed ohms and watts are ideal operating values. A real resistor’s standardized value, power rating, thermal environment and worst-case supply voltage must be selected separately. The page does not approve a design or guarantee the LED current.

Explore the related calculation

For a different starting point, try the resistor power calculator or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

Found a problem? Email contact@wattstoamps.app.