C Data Types

C data types decide how much memory a variable uses and what kind of values it can hold. Choosing the right type makes your program correct and efficient.

Basic C Data Types

• char: one byte, a single character or small integer
• int: a whole number, usually 4 bytes (about -2.1 billion to 2.1 billion)
• float: a decimal number with about 6-7 digits of precision, 4 bytes
• double: a decimal number with about 15-16 digits of precision, 8 bytes
• short, long and long long: smaller or larger integers
• unsigned: an integer that holds only zero and positive values, doubling the positive range

Checking Sizes with sizeof

Sizes can differ between platforms, so the sizeof operator tells you how many bytes a type uses on your machine. Its result has type size_t, printed with %zu.

#include <stdio.h>

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

char:      1
int:       4
long long: 8
float:     4
double:    8

Float vs Double

Use double for most decimal math because it is more precise. Floating-point numbers are stored in binary, so some decimals can only be approximated.

#include <stdio.h>

int main(void) {
    float f = 1.0f / 3;
    double d = 1.0 / 3;
    printf("%.10f\n", f);
    printf("%.10f\n", d);
    return 0;
}

0.3333333433
0.3333333333

Booleans and Fixed-Width Types

C17 gets true and false from <stdbool.h>. For exact sizes, <stdint.h> gives types such as int32_t and uint8_t, which are popular in embedded code.

#include <stdio.h>
#include <stdbool.h>
#include <stdint.h>

int main(void) {
    bool isOpen = true;
    uint8_t level = 255;
    printf("%d %d\n", isOpen, level);
    return 0;
}

1 255

Frequently asked questions

What is the size of int in C?

The C standard only guarantees at least 16 bits, but on all modern desktop and mobile systems int is 4 bytes (32 bits). Use sizeof(int) to check.

Should I use float or double in C?

Use double by default because it is more precise and C math functions work with double. Use float only when memory is tight, such as large arrays or graphics.

Does C have a boolean type?

Yes. Include <stdbool.h> to use bool, true and false. In C23 they became built-in keywords.