Electrical

LED Resistor Calculator

Resistor for LED circuits. Fast, accurate, and completely free.

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Supply Voltage (V)

LED Color Preset

LED Forward Voltage (V)

LED Forward Current (mA)

Number of LEDs (1–10)

Configuration

Results

Required Resistor

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📐 Circuit Details

Calculated Resistance

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Nearest E24 Standard

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Resistor Power Rating

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Total Current Draw

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🔌 Circuit Description

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⚠️ Design Notes

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Mathematical Formula

R = \frac{V_{supply} - (V_f \times n)}{I_f} \quad \text{(series)} \quad | \quad R = \frac{V_{supply} - V_f}{I_f} \quad \text{(parallel, per LED)}

R = Required resistance in Ohms (Ω)

V_supply = Power supply voltage (V)

V_f = LED forward voltage drop (V)

n = Number of LEDs in series

I_f = LED forward current (A)

How to Use this Calculator

Enter your power supply voltage (e.g., 5V USB, 9V battery, 12V adapter).
Select an LED color preset or enter a custom forward voltage (Vf).
Set the LED forward current (default 20mA is standard for indicator LEDs).
Choose the number of LEDs (1-10) and the configuration (Series or Parallel).
The calculator shows the exact resistance, nearest E24 standard resistor, power rating needed, and a circuit description.

LED Resistor Calculation Guide

LEDs (Light Emitting Diodes) are current-driven devices that require a resistor to limit the current flowing through them. Without a current-limiting resistor, an LED will draw excessive current, overheat, and burn out almost instantly. The resistor drops the excess voltage between the power supply and the LED's forward voltage, thereby controlling the current to a safe level.

How the Calculation Works

The basic principle is Ohm's Law applied to the resistor. The voltage across the resistor equals the supply voltage minus the LED forward voltage drop(s). For a series configuration, the total LED voltage is V_f × n (number of LEDs). For a parallel configuration, each LED string has its own resistor, so each resistor drops V_supply - V_f.

LED Forward Voltage by Color

The forward voltage (V_f) varies by LED color due to the semiconductor materials used. Typical values: Red = 1.8–2.2V, Yellow/Amber = 2.0–2.2V, Green = 2.0–3.5V, Blue = 2.8–3.5V, White = 3.0–3.5V. Always check the LED datasheet for exact values, as high-brightness and power LEDs may differ significantly.

Series vs. Parallel Configuration

In a series circuit, LEDs are connected end-to-end, sharing the same current. One resistor protects all LEDs. This is efficient but requires a supply voltage greater than the sum of all LED forward voltages. In a parallel circuit, each LED (or each LED string) has its own resistor. This uses more components but allows the circuit to work with a lower supply voltage and provides redundancy — if one LED fails, the others continue working.

E24 Standard Resistor Series

Resistors are manufactured in standard values. The E24 series includes 24 values per decade (1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1), each multiplied by powers of 10. When the exact calculated value isn't available, choose the next higher standard value to ensure the LED current stays within safe limits.

Power Rating

The resistor must be able to dissipate the heat generated. Power dissipated = I² × R or (V_drop)² / R. Common resistor ratings are 1/8W, 1/4W, 1/2W, and 1W. Always choose a resistor rated for at least twice the calculated power dissipation for reliable operation and long life.

Frequently Asked Questions (FAQ)

What happens if I don't use a resistor with an LED?

Without a resistor, the LED will draw maximum current from the supply. Since LEDs have very low internal resistance when forward-biased, the current surges far beyond the rated maximum, causing the LED to overheat and burn out within seconds. It can also damage the power supply.

Can I use one resistor for multiple LEDs in parallel?

While technically possible, it's strongly discouraged. LEDs have slight manufacturing variations in forward voltage. When paralleled with a single resistor, the LED with the lowest Vf draws more current, runs hotter, its Vf drops further, causing it to draw even more current — a thermal runaway loop. Use one resistor per LED in parallel configurations.

What if my supply voltage is lower than the LED forward voltage?

The LED simply won't light up. You need a supply voltage at least slightly higher than the LED forward voltage. For series configurations, the supply must exceed the total forward voltage of all LEDs combined. If your supply is too low, consider reducing the number of series LEDs or using a boost converter.

Why does the calculator suggest a higher resistor than I calculated?

The calculator rounds up to the nearest E24 standard resistor value. Using a slightly higher resistance is safer — it reduces the LED current slightly below the maximum rated value, increasing LED lifespan with minimal reduction in brightness.

How do I calculate for high-power LEDs (1W, 3W, 5W)?

The same formula applies, but high-power LEDs typically require 350mA (1W), 700mA (3W), or 1A+ (5W). At these currents, a resistor wastes significant power as heat. Instead, use a constant-current LED driver, which is far more efficient and provides better brightness control.

What is the E24 resistor series?

E24 is an international standard (IEC 60063) defining 24 preferred resistor values per decade: 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1 — each available in multiples of 10 (e.g., 47Ω, 470Ω, 4.7kΩ). This ensures you can find a resistor within ±5% of any needed value.

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