All data items are held in the native byte order of the AVR processor. The plain character type is signed by default. The floating-point types float and double are implemented as 32-bit and 64-bit IEEE floating-point.
| Data Type | Size in bytes | Alignment in bytes |
| char, signed char, and unsigned char | 1 | 1 |
| int and unsigned int | 2 | 1 |
| short and unsigned short | 2 | 1 |
| long and unsigned long | 4 | 1 |
| long long and unsigned long long | 8 | 1 |
| float and double (compiled with -msd) | 4 | 1 |
| double and long double | 8 | 1 |
| type * (pointer) | 2 | 1 |
| enum (enumeration) | 2 | 1 |