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Led Resistor Calculator

Calculate led resistor with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.

Reviewed by Manoj Kumar, Mathematics Educator

Reviewed by Manoj Kumar, Mathematics Educator

Formula

R = (V_supply - V_LED) / I_LED

The resistor drops the excess voltage. Use Ohm's law with the voltage difference across the resistor and desired LED current. Power dissipation P = IยฒR.

Worked Examples

Example 1: 5V with red LED

Problem:Vs=5V, Vled=2.0V, I=20mA

Solution:R = (5-2)/0.02 = 150ฮฉ, P = 0.02ยฒร—150 = 60mW

Result:150ฮฉ, 1/8W

Frequently Asked Questions

How do I read a resistor color code?

Each color band represents a digit or multiplier. The first two bands are digits (0-9), the third is the multiplier (number of zeros), and the fourth is tolerance. For example, brown-black-red-gold means 1-0-x100 = 1,000 ohms (1K) with 5% tolerance. Use the mnemonic: Bad Beer Rots Our Young Guts But Vodka Goes Well.

How do I calculate the current-limiting resistor for an LED?

Use R = (V_supply - V_forward) / I_desired. Typical LED forward voltages: red 1.8-2.2V, green 2.0-3.5V, blue/white 3.0-3.5V. Standard LED current is 20mA. For a red LED on 5V: R = (5 - 2) / 0.02 = 150 ohms. Always round up to the nearest standard resistor value.

What is a pull-up resistor and when do I need one?

A pull-up resistor connects a signal line to the supply voltage (typically 10K ohms to VCC) to ensure a defined high state when no other device is driving the line. Without it, floating inputs on microcontrollers read unpredictably. Pull-down resistors do the same to ground. Many microcontrollers have built-in pull-ups that can be enabled in software.

Reviewed by Manoj Kumar, Mathematics Educator ยท Editorial policy