Free LED Resistor Calculator
Find the series resistor an LED needs from supply voltage, forward voltage and current.
What is the LED Resistor Calculator?
Find the series resistor an LED needs from supply voltage, forward voltage and current.
The resistor drops the supply voltage minus the total LED forward voltage, and its value is that voltage divided by the LED current. The result is rounded up to the next E12 standard value, with its actual current and power shown.
Quick, reliable engineering arithmetic saves time on site and in the workshop. These tools show their working so you can check the result.
How to use the LED Resistor Calculator
Using the led resistor calculator takes a minute or two. Your entries are saved in this browser, so you can come back later and carry on.
Step 1
Enter your values, choosing the units you are working in.
Step 2
Read the result and the supporting numbers shown beneath it.
Step 3
Check the result against a second method or a reference table before you rely on it.
Tips for better results
Choose a resistor power rating at least double the calculated power. Check the LED's datasheet for maximum current and forward voltage, which vary by colour.
Keep units consistent from start to finish.
These tools are for estimates. Safety critical designs need a qualified engineer.
Privacy and where your data goes
Everything you type into the led resistor calculator stays on your own device. Zapxa does not receive your numbers or documents. If you clear your browser data, your saved entries will be removed, so download a PDF of anything you want to keep.
Frequently asked questions
Why is the resistor rounded up?
Rounding up gives a slightly lower current than requested, which protects the LED. The calculator shows the actual current with that value.
What if the supply is lower than the LED voltage?
No resistor can work, and the calculator shows an error. Use a higher supply or fewer LEDs in series.
Is the led resistor calculator free?
Yes. It is free to use with no signup, no watermark and no usage limits.
Is my data private?
Yes. Your inputs are processed in your browser and saved only in its local storage. They are not uploaded to our servers.
Does it work on mobile?
Yes. Zapxa works on phones, tablets and desktops. Use the print button or download a PDF to keep a copy.
Related Engineering tools
- Ohm's Law Calculator — Solve for voltage, current, resistance and power from any two known values.
- Resistor Color Code Calculator — Convert a resistance value to 4-band colour code, or bands back to ohms.
- Voltage Drop Calculator — Calculate voltage drop and power loss along a copper or aluminium cable run.
- Electricity Cost Calculator — Estimate the daily, monthly and yearly electricity cost of running a device.
- Battery Runtime Calculator — Estimate how long a battery will power a load, allowing for depth of discharge and efficiency.
- Power Conversion Calculator — Convert power between watts, kilowatts, horsepower, BTU per hour and other units.