LED Resistor Calculator

An LED resistor calculator determines the series current-limiting resistor required between a DC supply and one or more LEDs connected in series. The resistor absorbs the voltage remaining after the LED string’s forward-voltage drop and limits current to the selected LED current.

The calculated resistor value is used when selecting the physical resistor installed in the LED branch circuit. It is not a conductor-sizing, ampacity, raceway-fill, voltage-drop, feeder, or branch-circuit load calculation. Those installation decisions require separate review of the actual equipment, supply, overcurrent protection, wiring method, terminal ratings, and applicable AHJ requirements.

Enter supply voltage, LED forward voltage, LED current, and LEDs in series to estimate the ideal current-limiting resistor and a rounded-up common resistor candidate.

Calculation Inputs

Input	Electrical purpose
Supply voltage	The available DC voltage applied to the LED and resistor series circuit
LED forward voltage	The estimated voltage drop across one LED at the intended operating current
LED current	The target current through the complete series LED string
LEDs in series	The number of LEDs connected in one series path

LED forward voltage is not a fixed value under all conditions. Actual forward voltage changes with LED type, selected current, junction temperature, production variation, and manufacturer specifications. Use a forward-voltage estimate appropriate for the LED and intended operating current.

LED current is the current through every component in the series string. Three LEDs in series do not use three times the entered current; the same current flows through each LED and through the current-limiting resistor.

Resistor Voltage and Value

The calculator first determines the total voltage used by the LED string:

\(\displaystyle \text{Total LED forward voltage} = \text{LED forward voltage} \times \text{LEDs in series}\)

It then calculates the voltage that must be dropped by the resistor:

\(\displaystyle \text{Resistor voltage} = \text{Supply voltage} - \text{Total LED forward voltage}\)

The ideal resistor value follows Ohm’s law:

\(\displaystyle \text{Ideal resistor value} = \frac{\text{Resistor voltage}}{\text{LED current in amperes}}\)

The result is expressed in ohms. Since LED current is entered in milliamperes, it must be converted to amperes for the formula:

\(\displaystyle 20\text{ mA} = 0.020\text{ A}\)

A valid result requires the supply voltage to exceed the total LED forward voltage. If the LED string requires the entire supply voltage—or more than the available supply—the resistor cannot establish the entered target current.

Resistor Power

The resistor converts its voltage drop into heat. The calculator determines resistor dissipation with:

\(\displaystyle \text{Resistor power} = \text{Resistor voltage} \times \text{LED current in amperes}\)

The output is the electrical power dissipated by the resistor under the entered conditions. Select a physical resistor with a power rating suitable for the expected operating environment and thermal conditions. The calculated wattage is dissipation, not the recommended resistor wattage marking.

A resistor installed in an enclosed luminaire, compact electronic enclosure, LED strip assembly, or other restricted-airflow location may operate hotter than the same resistor in open air. Component temperature rise and the resistor manufacturer’s rating conditions must be evaluated separately.

Calculation Example

For the entered values:

Item	Value
Supply voltage	12 V
LED forward voltage	2 V
LED current	20 mA
LEDs in series	3 LEDs

First, calculate the LED string voltage:

\(\displaystyle 2\text{ V} \times 3 = 6\text{ V}\)

Total LED forward voltage = 6 V

Next, calculate the resistor voltage:

\(\displaystyle 12\text{ V} - 6\text{ V} = 6\text{ V}\)

Resistor voltage = 6 V

Convert LED current:

\(\displaystyle 20\text{ mA} = 0.020\text{ A}\)

Calculate the ideal resistor value:

\(\displaystyle \frac{6\text{ V}}{0.020\text{ A}} = 300\text{ ohm}\)

Ideal resistor value = 300 ohm

Calculate resistor power:

\(\displaystyle 6\text{ V} \times 0.020\text{ A} = 0.12\text{ W}\)

Resistor power = 0.12 W

The calculator returns a rounded-up common resistor candidate of 330 ohm. Selecting a higher resistance than the ideal value reduces current below the exact target current, which avoids exceeding the entered LED-current value.

Field Verification

Verify the installed circuit against actual conditions before final component selection:

• Confirm the actual DC supply voltage, including expected variation under load.
• Use the LED manufacturer’s forward-voltage data at the intended current rather than a generic nominal value when current, brightness, color consistency, or LED life is critical.
• Confirm the actual LED configuration is a single series string. Parallel LED branches require separate current-control design.
• Check resistor power rating, temperature rise, enclosure conditions, and physical mounting.
• Verify polarity, source type, and whether the LED assembly already includes internal current regulation or a factory-installed resistor.
• Do not apply this arithmetic to select AWG or kcmil conductors, establish ampacity, apply adjustment or correction factors, determine current-carrying conductors, calculate voltage drop, or establish branch-circuit or feeder requirements.

Related calculations: Ohm's Law Calculator, Resistor Color Code Calculator.