LED Series Resistor Calculator (DC Supply to Resistor Value)

This calculator finds the ideal series resistor for a DC LED string and estimates resistor power. Example: a 12 V supply, 2.0 V LED drop, and 20 mA target current need about 500 ohms.

The resistor absorbs the voltage remaining after the LED string’s estimated forward-voltage drop. Its resistance sets the approximate operating current, while its power result indicates the minimum dissipation level that must be considered when selecting the physical resistor.

This calculation applies to a basic DC series circuit. It does not select a regulated LED driver, branch-circuit conductor, AWG conductor size, overcurrent device, raceway fill, or voltage-drop conductor configuration.

LED String Voltage and Resistor Drop

The calculator uses these inputs:

Input	Electrical meaning
Supply voltage (V)	DC voltage applied to the complete LED-and-resistor series circuit
LED forward voltage (V)	Estimated voltage drop across one LED at the intended operating current
LED count	Number of identical LEDs connected in series
LED current (mA)	Target current through the entire series string

In a series string, the same current flows through every LED and through the resistor. LED forward voltages add together, so the calculator first determines the voltage used by the LEDs:

\(\displaystyle \text{Series LED voltage} = \text{LED count} \times \text{LED forward voltage}\)

The remaining supply voltage appears across the resistor:

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

A valid resistor-limited circuit requires a positive Resistor voltage. If the calculated series LED voltage equals or exceeds the supply voltage, the resistor cannot establish the entered target current using this arithmetic.

Ideal Resistance and Resistor Power

The ideal resistor value follows Ohm’s law. The calculator converts LED current (mA) to amperes for the calculation and reports Current used in amperes.

\(\displaystyle \text{Ideal resistance} = \frac{\text{Resistor voltage}}{\text{LED current}}\)

Resistor wattage is then calculated from the voltage across the resistor and the current through it:

\(\displaystyle \text{Resistor power} = \text{Resistor voltage} \times \text{Current used}\)

The Ideal resistance is the mathematical value needed for the entered voltage and current assumptions. The Recommended common resistance is the next higher common resistance value. Selecting the next higher value reduces current slightly rather than allowing a lower resistance to increase current beyond the ideal calculation.

That conservative direction is normally preferred for indicator LEDs and other resistor-limited LED circuits because LED current rises as resistance falls.

Calculation Example

For a 12 V supply feeding three identical LEDs with a 2 V forward voltage at 20 mA:

Calculator field or result	Value
Supply voltage	12 V
LED forward voltage	2 V
LED count	3
LED current	20 mA
Series LED voltage	6 V
Resistor voltage	6 V
Current used	0.02 A
Ideal resistance	300 ohm
Resistor power	0.12 W
Recommended common resistance	330 ohm

The calculation is:

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

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

\(\displaystyle \text{Ideal resistance} = \frac{6\text{ V}}{0.02\text{ A}} = 300\ \text{ohm}\)

\(\displaystyle \text{Resistor power} = 6\text{ V} \times 0.02\text{ A} = 0.12\text{ W}\)

A 330 ohm common resistor is higher than the 300 ohm ideal value, so it will operate the string below the 20 mA target assumed by the ideal calculation. The reported 0.12 W is the calculated resistor dissipation at the ideal 300 ohm value and entered 20 mA current.

Practical LED Resistor Selection

Use the calculation to establish the electrical starting point, then select components based on the actual circuit conditions.

• Confirm the LED forward-voltage value at the intended current from the LED manufacturer’s data.
• Verify the supply voltage under expected operating conditions, not only its nominal label value.
• Confirm that all LEDs in the series string are intended to operate at the same current.
• Review resistor voltage rating, power rating, ambient temperature, enclosure heat, mounting arrangement, and required reliability margin.
• Use a regulated constant-current LED driver rather than a simple series resistor where stable LED current is required over meaningful supply, temperature, or load variation.
• Treat each parallel LED path as a separate current-limiting design. A single resistor ahead of multiple parallel LEDs does not ensure equal current sharing.

A resistor-limited LED string is commonly suitable for low-power indicators, panel lights, control-circuit status lamps, and similar DC applications. It is less suitable where light output, thermal performance, or LED current must remain tightly controlled.

Field Verification

LED forward voltage is not a fixed value. It changes with LED type, current, junction temperature, production tolerance, and operating conditions. Supply voltage can also vary from its nominal value. Those changes alter both resistor voltage and LED current.

The calculator performs ideal series arithmetic only: it uses the entered Supply voltage, LED forward voltage, LED count, and LED current to calculate resistor value and dissipation. It does not evaluate LED regulation, temperature effects, component tolerances, pulse behavior, supply variation, thermal margin, manufacturer data, product listing, safety approval, or installation compliance.

Where the LED circuit is part of listed equipment, a control panel, a power-limited system, or building wiring, verify the final design against the equipment documentation, installation requirements, and the AHJ requirements applicable to the installation.