Engineering

LED Resistor Calculator — Free 2026

Find the correct current-limiting resistor for your LED circuit with nearest standard value and power rating.

Source Voltage (V) Please enter a valid number.

LED Forward Voltage (V) Please enter a valid number.

LED Current (mA) Please enter a valid number.

LEDs in Series Please enter a valid number.

Results

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Calculated Resistance

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Nearest Standard (E24)

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Power Dissipation

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Actual Current (Standard R)

How It Works

Enter your circuit values
Review the calculated resistor
Choose your resistor

Understanding LED Resistors

Every LED circuit needs a current-limiting resistor to prevent the LED from drawing excessive current and burning out. LEDs are current-driven devices — once the applied voltage exceeds the LED's forward voltage, current rises rapidly without a resistor to limit it. The resistor value is calculated using Ohm's Law: R = (Vs - Vf × n) / I, where Vs is the supply voltage, Vf is the LED's forward voltage, n is the number of LEDs in series, and I is the desired operating current.

Choosing the Right Resistor

After calculating the exact resistance, you need to select a real resistor from standard values. The E24 series provides 24 values per decade (10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91) multiplied by powers of 10. Always round up to the next standard value — this slightly reduces the current, which is safer for the LED. Also verify that the resistor's power rating exceeds the calculated power dissipation. Most standard through-hole resistors are rated for 1/4 watt, which is sufficient for typical LED circuits. For more detailed resistance and power calculations, use our Ohm's law calculator.

Common LED Forward Voltages

Different LED colors have different forward voltages due to their semiconductor materials. Red LEDs typically drop 1.8-2.2V, orange and yellow LEDs drop 2.0-2.4V, green LEDs drop 2.0-3.5V (depending on type), blue and white LEDs drop 3.0-3.6V, and UV LEDs can drop 3.5-4.5V. Always check the LED datasheet for the exact forward voltage at your desired operating current. Using the wrong forward voltage in your calculation will result in an incorrect resistor value and potentially damage the LED or produce dim output.

Series vs Parallel LED Configurations

When connecting multiple LEDs, you can wire them in series or parallel. In series, the forward voltages add up, so the supply voltage must be higher than the total forward voltage drop. This calculator handles series configurations by multiplying Vf by the number of LEDs. For parallel configurations, each LED needs its own resistor to ensure equal current distribution. Never connect LEDs in parallel with a single shared resistor, as manufacturing variations will cause uneven current distribution and premature failure. For voltage-related calculations, try the voltage divider calculator.

Frequently Asked Questions

How do I calculate the resistor value for an LED?

Use the formula R = (Vs - Vf × n) / I, where Vs is the source voltage, Vf is the LED forward voltage, n is the number of LEDs in series, and I is the desired LED current (typically 20mA). The result is the minimum resistance in ohms needed to protect the LED.

What happens if I use no resistor with an LED?

Without a current-limiting resistor, excessive current will flow through the LED, likely destroying it almost instantly. LEDs have very low forward resistance once conducting, so even a small excess voltage above the forward voltage can cause dangerous current levels.

What is the typical forward voltage of an LED?

Forward voltage varies by LED color: red LEDs are typically 1.8-2.2V, yellow and green are 2.0-2.4V, blue and white are 3.0-3.6V, and UV LEDs can be 3.5-4.5V. Always check the LED's datasheet for the exact forward voltage.

What is the nearest standard resistor value?

Standard resistors come in preferred values from the E-series (E12, E24, E96). The calculator rounds your calculated value up to the nearest E24 standard value, which is what you will find at electronics stores. Using the next standard value up ensures the LED current stays at or below the target.