How to calculate ELF format .data section size manually - c

Question

I'd like to calculate the size of .data section in my program (compiled with gcc with no optimization) WITHOUT using readelf or any other tool, just by looking at the program. Is this possible? Here is the program:
ex1.h
#include <stdint.h>
#include <inttypes.h>
#define FORMAT_STRING "%s%" PRId64
#define MESSAGE "The Oddonaci Number is: "
int64_t FastOddonacci(int32_t n);
int64_t LinearOddonacciHelper(int32_t n, int64_t current, int64_t first, int64_t second, int64_t third);
ex1.c
#include "ex1.h"
#include <stdio.h>
int32_t n = 40;
int32_t a = 3;
int main(int argc, char* argv[]) {
printf(FORMAT_STRING, MESSAGE, FastOddonacci(n * a));
return 0;
}
int64_t FastOddonacci(int32_t n) {
if (n >= 1 && n <= 3) {
return 1;
}
return LinearOddonacciHelper(n, 3, 1, 1, 1);
}
int64_t LinearOddonacciHelper(int32_t n, int64_t current, int64_t first, int64_t second, int64_t third) {
if (n == current) {
return third;
}
current += 1;
return LinearOddonacciHelper(n, current, second, third, first+second+third);
}
From my understanding, .data section contains all initialized variables so here we should have two: n and a. The two in #define should go to .rodata instead. So we have 8 bytes in total.
However, readelf told me that the size is 0x18 = 24 bytes, why?
A second test: I changed both n and a to int64_t (and changed the signature of those functions) and readelf shew 32 bytes, which does make sense as the difference is exactly 8 bytes. Where is the missing 16 bytes? It doesn't look like alignment though.