Why 24 bytes per record overhead?

[chrislusf]

Low overhead: A database uses 2048 bytes, plus 24 bytes per record, plus the space for keys and data.

For my own calculation, for each record, it should be len(key), len(value), hash_value, entry_offset 4 numbers. Each is 4 bytes. So total 16 bytes overhead per record. Where the 24 bytes come from?