AutocadHow-ToBeginner · 3 min read

How to Check Available Memory in Arduino: Simple Guide

To check available memory in Arduino, use a custom function that calculates free RAM by comparing the stack and heap pointers. This function returns the amount of free memory in bytes, helping you monitor memory usage with freeMemory().

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Syntax

The common way to check available memory in Arduino is by defining a function called freeMemory(). This function uses pointers to find the difference between the stack and heap, which tells you how much RAM is free.

Parts explained:

extern int __heap_start, *__brkval;: These are special variables that mark the start of the heap and the current break value (end of heap).
int v;: A local variable to get the current stack pointer.
return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);: Calculates free memory by subtracting heap end from stack pointer.

arduino

int freeMemory() {
  extern int __heap_start, *__brkval;
  int v;
  return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
}

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Example

This example shows how to use the freeMemory() function to print the available RAM to the Serial Monitor. It helps you see how much memory your Arduino has left while running.

arduino

#include <Arduino.h>

int freeMemory() {
  extern int __heap_start, *__brkval;
  int v;
  return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
}

void setup() {
  Serial.begin(9600);
  delay(1000); // Wait for Serial to start
}

void loop() {
  Serial.print("Free memory: ");
  Serial.print(freeMemory());
  Serial.println(" bytes");
  delay(2000); // Print every 2 seconds
}

Output

Free memory: 2048 bytes Free memory: 2048 bytes Free memory: 2048 bytes

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Common Pitfalls

Many beginners try to use sizeof() or RAMSIZE constants to check free memory, but these do not show current available RAM. Also, forgetting to declare extern int __heap_start, *__brkval; causes errors.

Another mistake is calling freeMemory() before Serial.begin(), which can cause confusing output if you print results immediately.

arduino

/* Wrong way: Using sizeof() to check free memory (incorrect) */
void loop() {
  Serial.print("Free memory: ");
  Serial.println(sizeof(int)); // This prints size of int, not free RAM
  delay(2000);
}

/* Right way: Use freeMemory() function as shown in the example above */

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Quick Reference

Use freeMemory() function to get current free RAM in bytes.
Call it after Serial.begin() for reliable output.
Monitor free memory regularly to avoid crashes from low RAM.
This method works on AVR-based Arduino boards like Uno and Mega.

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Key Takeaways

Use a custom freeMemory() function to check available RAM on Arduino.

Call freeMemory() after initializing Serial for accurate monitoring.

Avoid using sizeof() or constants to check free memory as they are incorrect.

Regularly checking free memory helps prevent unexpected crashes due to low RAM.

This method works best on AVR-based Arduino boards like Uno and Mega.