Arduino "Sketch too big": how to cut program size

Find what is large and remove it. Libraries and printed text take most of the flash.

What the message means

The flash holds your code and constant data. A big display or Wi-Fi library can fill a small chip on its own.

Fix it in this order

Read the program storage percentage in the build output.
Remove libraries you do not use and their include lines.
Cut long debug text, or move it behind a debug switch.
Use a lighter library for the same part.
Avoid heavy floating point and printf-style code where you can.
Choose a board with more flash if the sketch is still too big.

Why it happens

An Uno has about 32 KB of flash, a little of which the bootloader uses. Large libraries, many Serial.print() strings, and floating-point formatting use it quickly.

Remove unused libraries, cut debug printing, store lookup tables with PROGMEM, and move to a board with more flash such as a Mega or an ESP32 if you still need more space.

If you see this, do this

You see	Do this
Over 100 percent	Remove code or change board.
Close to the limit	Leave room for later changes.

low memory, library not found.

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

Why this happens

The flash on an ATmega328P Uno is 32768 bytes, and the bootloader takes 512 of those, so the IDE reports a limit of 32256 bytes. The compiler places program code, constant data and library code in it. A graphic LCD or OLED library carries a font table, a Wi-Fi stack is far bigger again, and every text string in Serial.print uses flash space.

Size problems are usually caused by a few large items. Finding them by trial is quicker than guessing: remove one feature, compile, and read the byte count. The difference is what that feature costs.

How to confirm the cause

Read the line ‘Sketch uses X bytes (Y%)’ after every compile. Note it before changes.
Comment out one large feature or library at a time and compile again. The drop in bytes is its cost.
Search for long text in Serial.print, lcd.print or web page strings. Each character is a byte.
Check which board is selected. An old Nano with the original bootloader has a limit near 30720 bytes, not 32256.

Worked example

Illustrative figures: if the report says 30,000 of 32,256 bytes, that is 30000 / 32256 = 93 percent. The switch below removes debug text from the final build.

#define DEBUG 0

#if DEBUG
  #define DBG(x) Serial.println(x)
#else
  #define DBG(x)
#endif

void loop() {
  DBG("sensor ok");     // compiled out when DEBUG is 0
}

Common mistakes

Believing F() or PROGMEM saves flash. They move data out of RAM; the bytes still occupy flash.
Including a whole library to use a single function.
Using floating-point formatting with printf-style code.
Leaving unused test code in the final sketch.

If this did not fix it

Check that the problem is flash and not RAM; the low-memory warning is a different limit.
Check for two copies of a library in different folders.
Consider a board with more flash, for example a Mega or an ESP32.

Arduino “Sketch too big”: how to cut program size