Use this LED Resistor Calculator to calculate the correct series resistor required to safely connect an LED to a power supply. Enter the supply voltage, LED forward voltage, and desired LED current to calculate the required resistance and resistor power rating.The calculator also helps you select a suitable standard resistor value and shows the actual current that will flow through the LED, making it useful for designing LED circuits with common supply voltages such as 3.3 V, 5 V, 9 V, and 12 V
Results
- Recommended resistor (calculated)
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- Nearest standard resistor
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- Next higher standard value
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- Actual current (standard resistor)
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- Resistor power
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- Suggested power rating
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How it works
An LED drops a roughly fixed forward voltage (Vf) when it conducts. The series resistor drops the remaining voltage and sets the current. Without a resistor (or another current limit) the current rises until the LED is destroyed.
LED Resistor Formula
R = (Vsupply − Vf) / I
P = I² × R
Example: 5 V supply, red LED (2 V), 20 mA: R = (5 − 2) / 0.02 = 150 Ω, P = 0.02² × 150 = 60 mW.
Safety information
- Always check the LED datasheet for its forward voltage and maximum continuous current.
- Never connect an LED directly to a voltage source without a current-limiting resistor or driver.
- Choose a resistor with a power rating at least twice the calculated dissipation.
- When LEDs are wired in parallel, give each LED its own resistor.
- High-power LEDs (350 mA and above) should be driven by a constant-current driver and need a heatsink.
- Do not build LED circuits powered directly from mains voltage unless you are qualified to do so.
