Led Resistor Calculator
Calculate led resistor with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed for accuracy by Manoj Kumar, Mathematics Educator
Led Resistor Calculator
Calculator
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Formula: R = (V_supply - V_LED) / I_LED
Worked example — 150Ω, 1/8W
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 = I²R.
Worked Examples
Example 1: 5V with red LED
Problem:Vs=5V, Vled=2.0V, I=20mA
Solution:R = (5-2)/0.02 = 150Ω, P = 0.02²×150 = 60mW
Result:150Ω, 1/8W
Frequently Asked Questions
What happens without a resistor?
Without a current-limiting resistor, excessive current flows through the LED, quickly destroying it. LEDs have very low resistance when forward-biased.
What are the forward voltages for different LED colors?
Forward voltage depends on the LED color: Red/Orange: 1.8–2.2V, Yellow/Green: 2.0–2.4V, Blue/White/UV: 3.0–3.6V, Infrared: 1.2–1.6V. Always check the datasheet for your specific LED. Using the wrong forward voltage will result in an incorrect resistor value and may overdrive or underdrive the LED.
Can I connect multiple LEDs in series with one resistor?
Yes. For series LEDs, add up all the forward voltages and subtract from the supply voltage before dividing by the current. For example, three red LEDs (2V each) in series on 12V: R = (12 - 6) / 0.02 = 300 ohms. Series wiring requires only one resistor but needs a higher supply voltage. Parallel wiring needs a separate resistor for each LED for reliability.
Why should I choose a standard resistor value instead of the exact calculated value?
Resistors come in standard E12 or E24 series values (e.g., 100, 120, 150, 180, 220 ohms). Choosing the nearest standard value above your calculated resistance slightly reduces current, which protects the LED and is totally acceptable. Exact calculated values are not available and would have to be custom-ordered, which is impractical.
How do resistors behave in series versus parallel?
In series, total resistance is the sum: R_total = R1 + R2 + R3. In parallel, reciprocals add: 1/R_total = 1/R1 + 1/R2 + 1/R3. For two parallel resistors, R_total = (R1 * R2) / (R1 + R2). Series increases total resistance while parallel decreases it.
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 for accuracy by Manoj Kumar, Mathematics Educator · Editorial policy
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