A light emitting diode is not like an ordinary lamp: it has a fairly fixed forward voltage drop, and once that voltage is reached, its current rises very steeply with any further increase. Connected directly to a supply, an LED will draw far more current than it can survive and burn out in moments. A series resistor solves this by dropping the excess voltage and limiting the current to a safe value, typically around twenty milliamps for an indicator LED. This calculator finds that resistor from three numbers you can look up: the supply voltage, the LED’s forward voltage, and the desired current. It also reports the power the resistor dissipates, so you can choose a part rated comfortably above it.
Quick Answer: Enter the supply voltage, the LED forward voltage and the desired current in milliamps, then click Calculate Resistor.
Free Online Tool
LED Series Resistor Calculator
The right series resistor, from supply, forward voltage and current.
How to Use the LED Series Resistor Calculator
- Find your supply voltage, such as five volts from USB or nine volts from a battery.
- Find the LED’s forward voltage from its datasheet, typically around two volts for red and three volts for white LEDs.
- Choose the desired current, usually twenty milliamps for a standard indicator LED.
- Click Calculate Resistor to see the required resistance and the power it will dissipate.
- Pick the nearest standard resistor value at or above the calculated resistance, with a power rating above the dissipated power.
Key Features
- Series resistor calculation from the voltage difference divided by the desired current.
- Power dissipation reporting, so the resistor’s wattage rating can be chosen safely.
- Milliamp input for current, matching how LED datasheets specify operating current.
- Nearest standard value guidance in the tips, since exact calculated values are rarely manufactured.
- Instant local calculation for bench and hobby electronics work.
Best Practices and Tips
- When in doubt, round the resistor up to the next standard value. A slightly dimmer LED is far better than a burnt out one.
- Use a higher power rating than the calculated dissipation, ideally double, for a comfortable safety margin.
- Check the forward voltage for your LED colour, since red, green, blue and white LEDs differ by a volt or more.
- For several LEDs, calculate per LED rather than sharing one resistor, unless the LEDs are identical and in series.
Common Mistakes to Avoid
- Connecting an LED with no resistor at all, which destroys the LED almost immediately on most supplies.
- Using the supply voltage alone instead of the difference between supply and forward voltage, which overstates the resistor.
- Entering current in amps when the field expects milliamps, or the reverse, which changes the result by a factor of one thousand.
Frequently Asked Questions
Why does an LED need a resistor?
An LED’s current rises extremely steeply once its forward voltage is reached, so a small voltage excess produces a huge current. The series resistor drops the remaining supply voltage and sets the current to a safe, predictable value, which is what keeps the LED alive and at a stable brightness.
What is forward voltage?
Forward voltage is the voltage drop across the LED while it is conducting, and it depends mainly on the LED’s colour and semiconductor material. Red LEDs are typically near two volts, while blue and white LEDs are typically near three volts. The datasheet for your exact LED gives the precise figure.
What current should I choose?
Twenty milliamps is the standard operating current for most small indicator LEDs and a safe default. High brightness LEDs may specify more, and some applications deliberately run lower, such as five or ten milliamps, for a dimmer indicator or longer battery life.
What if the calculated resistor is not a standard value?
Choose the next standard value above the calculated resistance. The LED will run very slightly dimmer, which is invisible in practice, and the current will be slightly lower, which is always the safe direction.