LED Resistor Calculator

Estimate a series resistor for an LED and check its power dissipation. Enter your own values below to see series resistance and check the method behind it.

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Your inputs

Change the example values to match your situation.

Your result

Let’s work it out.

Fill in your inputs and select Calculate.
Your result and breakdown will appear here.

How this calculator works

R = (supply voltage − LED forward voltage) ÷ LED current. Choose a suitable standard resistance and power rating for the application.

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What is LED Resistor Calculator?

LED Resistor Calculator helps you estimate a series resistor for an LED and check its power dissipation.

The main result is series resistance, with a breakdown of resistor dissipation. The formula, example and assumptions below explain how to interpret it.

How to use LED Resistor Calculator

Use the example to get familiar with the form, then enter your own figures. Any rates in the example are illustrative. Check the field labels and units before calculating.

  1. Gather supply voltage (V), LED forward voltage (V) and LED current (mA). Use values from the same situation or reporting period.
  2. Replace the example supply voltage with your own value, then complete the other fields. Check the displayed units.
  3. Select Calculate and read series resistance with its result breakdown. Change one input and calculate again to compare a second scenario. Reset restores the original example.
  • Enter supply voltage in V, at least 0. Decimals are accepted.
  • Enter LED forward voltage in V, at least 0. Decimals are accepted.
  • Enter LED current in mA, at least 0.000001. Decimals are accepted.

LED Resistor Calculator: a worked example

With the inputs below, the result is 500 Ω (series resistance). Follow the example, then replace these illustrative values with your own.

Example inputs and output
Input or outputExample value
Supply voltage (V)12 V
LED forward voltage (V)2 V
LED current (mA)20 mA
Series resistance500 Ω
Resistor dissipation0.2 W

Understanding the LED Resistor result

Read series resistance alongside resistor dissipation. These describe different parts of the same calculation.

Displayed results use a readable number of decimal places. When checking a result by hand, keep extra precision until the last step. To compare scenarios, change one input at a time and keep the others fixed.

  • Example Resistor dissipation: 0.2 W

LED Resistor Calculator: assumptions and common mistakes

A useful estimate starts with the right inputs. Check the following assumptions and limits before applying the result to your situation.

  • DC, single-phase and three-phase calculations use different relationships. Equipment selection also depends on installation conditions and applicable standards.

For wider guidance, visit Bureau of Indian Standards. The formula and scope of this specific tool are stated on this page.

LED Resistor Calculator FAQs

What does LED Resistor Calculator calculate?

LED Resistor Calculator helps you estimate a series resistor for an LED and check its power dissipation. Its main output is series resistance in Ω.

What inputs do I need for LED Resistor Calculator?

You need supply voltage (V), LED forward voltage (V) and LED current (mA). Use the field labels and units to match your figures to the formula.

Which formula does LED Resistor Calculator use?

R = (supply voltage − LED forward voltage) ÷ LED current. Choose a suitable standard resistance and power rating for the application.

What is a worked example for LED Resistor Calculator?

Example inputs: Supply voltage: 12 V; LED forward voltage: 2 V; LED current: 20 mA. The result is 500 Ω (series resistance).

How should I enter supply voltage?

Enter supply voltage in V, at least 0. Decimals are accepted.

How do I choose LED forward voltage?

Enter LED forward voltage in V, at least 0. Decimals are accepted. The example uses 2 V; replace it if your circumstances differ.

How should I read series resistance?

Read series resistance alongside resistor dissipation. These describe different parts of the same calculation.

Why might my LED Resistor Calculator result differ from my records?

DC, single-phase and three-phase calculations use different relationships. Equipment selection also depends on installation conditions and applicable standards. Differences in entered values, units or rounding can also change the comparison.

Can I compare two scenarios in LED Resistor Calculator?

Yes. Note your first series resistance, change supply voltage or another input, and select Calculate again. Editing a field clears the old result so it is not mistaken for a new calculation.

Is LED Resistor Calculator free, and where are my inputs calculated?

This calculator is free to use without signing in. Its formula runs in your browser; calculator inputs are not sent to a calculation API. The inputs reset to the example values when you reset the form or start a fresh page.