- Enter supply voltage — the voltage of your power source (e.g., 5V for USB, 9V for battery, 12V for automotive).
- Select LED color — choose from 9 presets (red, orange, yellow, green, blue, white, warm white, infrared, UV) or set custom values.
- Adjust forward voltage and current — these auto-fill from the preset but can be customized from the LED datasheet.
- Set number of LEDs — how many LEDs in the circuit (1-50).
- Choose connection type — series (LEDs in a chain sharing one resistor) or parallel (each LED branch gets a resistor).
- Click Calculate — view the required resistance, nearest E24 standard resistor, resistor color bands, power dissipation, and whether a higher-wattage resistor is needed.
LED Resistor Calculator — Design LED Circuits Correctly
Every LED circuit needs a current-limiting resistor. Without one, the LED will draw too much current and burn out. Our LED Resistor Calculator helps you find the exact resistor value, the nearest standard resistor from the E24 series, the resistor color bands, and the power rating you need — all instantly in your browser with complete privacy.
Why LEDs Need Resistors
LEDs (Light Emitting Diodes) are semiconductor devices with a very steep voltage-current curve. Once the forward voltage threshold is reached, even a tiny increase in voltage causes a massive increase in current. Without a resistor to limit this current, the LED overheats and fails within seconds. The resistor drops the excess voltage from the power supply, ensuring only the correct amount of current flows through the LED.
Key Features
- 9 LED Color Presets: Red (2.0V), Orange (2.1V), Yellow (2.1V), Green (2.2V), Blue (3.2V), White (3.3V), Warm White (3.2V), Infrared (1.5V), and UV (3.4V) — each with typical forward voltage and 20mA current.
- Custom Values: Enter your own forward voltage and current from the LED datasheet for precise calculations.
- Series and Parallel: Calculate for LEDs connected in series (one resistor for all) or parallel (one resistor per LED branch).
- E24 Standard Resistor: Automatically finds the nearest standard resistor value from the E24 series (the most commonly available).
- Resistor Color Code: Visual display of the 4-band resistor color code with band names.
- Power Dissipation: Calculates the power the resistor must handle and warns you if a higher wattage rating is needed.
- Visual LED Preview: Color-coded LED dot that changes to match your selected LED type.
- Formula Display: Shows the exact formula used and explains each variable.
The Resistor Formula
Series Connection
When LEDs are connected in series, their forward voltages add up:
R = (V_supply - V_forward × n) / I_forward
Where n is the number of LEDs. The supply voltage must be higher than the total forward voltage of all LEDs combined.
Parallel Connection
When LEDs are connected in parallel, each LED gets a separate resistor (or they share one with combined current):
R = (V_supply - V_forward) / (I_forward × n)
In practice, it is better to use a separate resistor for each parallel LED to ensure equal current distribution.
Common LED Forward Voltages
The forward voltage (Vf) depends on the semiconductor material used in the LED:
- Infrared: 1.2-1.6V (GaAs, GaAlAs)
- Red: 1.8-2.2V (GaAsP, AlGaInP)
- Orange: 2.0-2.2V (GaAsP, AlGaInP)
- Yellow: 2.0-2.2V (GaAsP, AlGaInP)
- Green: 2.0-3.5V (GaP, InGaN)
- Blue: 3.0-3.5V (InGaN, SiC)
- White: 3.0-3.5V (InGaN + phosphor)
- UV: 3.1-4.4V (InGaN)
Always check the specific LED datasheet for exact values, as they vary between manufacturers and LED sizes.
Understanding LED Current Ratings
Standard 5mm LEDs are typically rated at 20mA continuous forward current. However:
- 3mm LEDs: Usually 20mA, but some are rated at 10mA
- 5mm LEDs: Typically 20mA standard, some high-brightness at 30mA
- 10mm LEDs: Usually 20-40mA
- SMD LEDs (0805, 1206): Usually 20mA
- High-power LEDs (1W, 3W, 5W): 350mA to 1A+ — these require constant-current drivers, not simple resistors
E24 Standard Resistor Values
The E24 series provides 24 standard 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. These repeat for each decade (×10, ×100, ×1000, etc.). Using a standard value means you can easily purchase the resistor from any electronics supplier. Our calculator automatically rounds to the nearest E24 value.
Power Rating Selection
The resistor dissipates power as heat: P = V_resistor × I. Standard through-hole resistors come in these common power ratings:
- 1/8W (125mW): Very small, suitable for low-current indicator LEDs
- 1/4W (250mW): The most common through-hole resistor size
- 1/2W (500mW): Larger body, for moderate power dissipation
- 1W+: Large wirewound or metal film resistors for high-power applications
Always choose a resistor rated at least 2x the calculated power to provide a safety margin and prevent overheating.
Common LED Circuit Examples
- 5V USB + Red LED: R = (5V - 2.0V) / 20mA = 150Ω. Nearest E24: 150Ω. Power: 60mW — a 1/4W resistor works fine.
- 9V Battery + White LED: R = (9V - 3.3V) / 20mA = 285Ω. Nearest E24: 270Ω. Power: 114mW — a 1/4W resistor works.
- 12V + 3 Blue LEDs in Series: R = (12V - 3.2V × 3) / 20mA = 120Ω. Nearest E24: 120Ω. Power: 72mW.
Privacy and Security
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