LED Resistor Calculator
Find the right resistor for your LED circuit
Typical: 10-20mA for standard LEDs, 5-10mA for high-brightness
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Multiple LEDs in Series
For LEDs in series, add up the forward voltages. For example, three red LEDs (2.0V each) need 6.0V total, so you'd need at least 7V supply. The formula becomes:
R = (Vs - (Vf₁ + Vf₂ + Vf₃)) / I
For LEDs in parallel, each LED needs its own resistor (same value as calculated above). Never connect LEDs directly in parallel without individual resistors—slight differences in forward voltage will cause uneven current distribution.
Why Do LEDs Need Resistors?
LEDs are current-controlled devices with very low internal resistance. Without a current-limiting resistor, the current would spike to dangerous levels, destroying the LED (and potentially your power supply) in milliseconds.
The resistor drops the excess voltage (Vs - Vf) and limits current to a safe level. Higher resistance = less current = dimmer LED. Lower resistance = more current = brighter LED (up to the LED's maximum rating).