How Big Is a Variable? (sizeof)

Big idea

Every type uses a fixed amount of memory, and sizeof tells you how much.

The idea

Memory is measured in bytes. sizeof(type) gives the number of bytes that type takes up.

Example

#include <stdio.h>

int main(void) {
    printf("char:  %zu\n", sizeof(char));
    printf("int:   %zu\n", sizeof(int));
    printf("float: %zu\n", sizeof(float));
    return 0;
}

output
char:  1
int:   4
float: 4

(The sizes are what most computers use today.)

%zu is the format code for printing a size.

Think of it like

Parking spaces. A motorbike (char) takes 1 space, a car (int) takes 4. sizeof counts the spaces.

🕷️ Spidey-sense: My spidey-sense tells me how big a threat is before I swing in. sizeof is your version: it tells you how many bytes something takes before you use it. Sizes can change between computers — so ask, don't guess.

Why it matters

A char is always exactly 1 byte. Other sizes can differ between computers — which is exactly why you ask sizeof instead of guessing. You'll need it later when you ask for memory yourself.

"With great power comes great responsibility." — Uncle Ben

This is C's deal with you. Most languages manage memory behind your back. C hands you the controls: you decide how much memory to take, and later you'll give it back yourself. That's the power. Knowing exactly how many bytes you're using — that's the responsibility. sizeof is where it starts.

Key terms

• Byte — The basic unit of memory.
• sizeof — Tells you how many bytes a type or variable uses.
• %zu — The format code for printing a sizeof result.