LED Resistor Calculator

Use this led resistor calculator to determine the correct series resistor for your LED circuit. Enter supply voltage, forward voltage and desired current — and optionally number of LEDs in series — to get resistor value and power rating.

Supply Voltage (Vs) [V]

LED Forward Voltage (Vf) [V] Voltage drop of LED (from datasheet)

Desired LED Current (I) [mA] Typical values: 5-20 mA

Number of LEDs in Series (N) Multiple LEDs share resistor

LED Resistor Calculator – Find the Right Series Resistor for Your LED Circuit

When building circuits with LEDs (light-emitting diodes), selecting the correct series resistor is essential for ensuring safe current flow and long‐term reliability. Our led resistor calculator simplifies this process by applying Ohm’s Law: subtracting the LED forward voltage from the supply voltage and dividing by the desired current. You’ll get the ideal resistor value in ohms, along with the power dissipation and recommended wattage to use for safe operation.

Why You Need a Series Resistor for an LED

An LED cannot regulate current itself — its current rises rapidly once the forward voltage is reached. Without a proper series resistor or current driver, the LED may draw excessive current, overheat and fail. As Wikipedia explains, “the value for the series resistance may be obtained from Ohm’s Law, considering that the supply voltage is offset by the diode’s V_f.” :contentReference[oaicite:0]{index=0}

Formula and How the LED Resistor Calculator Works

R (Ω) = (V_s − N × V_f) ÷ I_LED
Where:
  V_s = supply voltage (volts)
  V_f = LED forward voltage drop (volts)
  N = number of LEDs in series
  I_LED = desired LED current (amps)

This equation underpins the LED series resistor calculator. After computing R, the tool also gives you the resistor power: P = (V_s − N×V_f) × I_LED. Then you choose a resistor with a wattage rating higher than this power (a common rule is ×2 or more) to avoid overheating.

Using the LED Resistor Calculator: Step by Step

Enter your supply voltage (for example: 5 V, 12 V, 24 V).
Enter the LED forward voltage (check your LED datasheet: common values ~1.8 V for red, ~2.1 V yellow/green, ~3.0-3.5 V for white/blue). :contentReference[oaicite:1]{index=1}
Enter your desired LED current in mA (e.g., 10 mA, 20 mA). For indicator LEDs lower current is fine.
Enter the number of LEDs connected in series (if multiple LEDs share one resistor).
Click “Calculate”. The led resistor calculator returns the resistor value (Ω), the power the resistor must dissipate (W), and suggests a suitable wattage rating.

Example Calculations

Example 1 – Single Indicator LED

Supply: 5 V, Forward voltage: 2.0 V, Desired current: 20 mA (0.02 A), N=1. R = (5 − 2.0) ÷ 0.02 = 150 Ω. Power in resistor: P = (5 − 2.0) × 0.02 = 0.06 W → choose a 0.125 W or 0.25 W resistor for safety.

Example 2 – Three LEDs in Series (12 V Supply)

Supply: 12 V, Vf: 3.2 V each (white LED), Desired current: 15 mA (0.015 A), N=3. Vdrop_total = 3 × 3.2 = 9.6 V. R = (12 − 9.6) ÷ 0.015 = 160 Ω. P = (12 − 9.6) × 0.015 = 0.036 W → a 0.25 W resistor is more than enough.

Selecting Resistor Wattage and Tolerance

Resistors heat up when dissipating power. Even if calculated P = 0.06 W, using a 0.125 W part leaves little margin. A typical recommendation: choose a resistor rated at ≥ 2× the calculated P (i.e., 0.125-0.25 W). For high-current LED strips or arrays, consult dedicated driver circuits since series resistors become inefficient.

When Multiple LEDs or Parallel Strings Are Used

If multiple identical LEDs are in series, the formula becomes R = (V_s − N×V_f) ÷ I. If you wire strings in parallel each with its resistor, calculate per string. The led resistor calculator simplifies this by accepting N (number in series). For long LED arrays or high brightness modules, use dedicated constant-current drivers instead of simple resistors.

Practical Tips for LED Resistor Design

Double-check your LED’s forward voltage (V_f) from its datasheet — value varies by color and manufacturer. :contentReference[oaicite:2]{index=2}
Account for supply voltage tolerance: if Vs may vary upward (e.g., 12.5 V instead of 12 V) design for worst case.
Use resistor with correct power rating – overheating resistor shortens life.
For many LEDs in series or high current, consider constant current drivers rather than resistors for efficiency and reliability.
Use series resistor only when supply voltage is comfortably higher than sum of LED drops; if Vs ≈ Vf_total, brightness will vary and design is unsafe. :contentReference[oaicite:3]{index=3}

Why It Matters: Stability, Safety & Efficiency

Without a proper resistor or current regulation, LED current can increase rapidly with small voltage changes, risking thermal runaway, reduced lifespan or catastrophic failure. A well-sized resistor ensures stable current flow and predictable brightness. Using the led resistor calculator helps you design correctly and avoid trial-and-error mistakes.

Related Tools for LED and Electronics Design

Summary

The led resistor calculator is an essential tool when designing LED circuits. It ensures you choose a resistor value that limits current correctly and a resistor wattage that safely handles the power dissipated. Follow good practices, check your datasheet values, and when in doubt opt for constant-current drivers for larger LED systems.

Sources: Wikipedia – LED circuit • DigiKey LED Series Resistor Calculator • RS Components LED Resistor Value Calculator

Disclaimer: This led resistor calculator is provided for educational purposes only and does not replace professional engineering advice. Always verify LED datasheet values, resistor ratings, and circuit conditions before prototyping or production.

Table of Contents