Logical records may be of fixed or variable length. A logical record may be entirely contained in one physical record or may overflow into one or more physical records. Normally an event or trigger corresponds to one logical record but it may equally well consist of a sequence of related logical records.
Physical records may be of fixed or variable length.
A fixed record length is recommended (for ease of reading using standard system and particularly FORTRAN, input routines). A fixed record length is particularly important if the record has no header specifying the length.
A record length which is a multiple of 360 bytes is recommended (corresponding to an integral number of 16, 18, 24, 32, 36, 48, 60, 64-bits).
The following table indicates the increase in tape utilisation with record length up to the maximum of 32K bytes for IBM 370 [4].
| Usage() | 67 | 80 | 88 | 90 | 94 |
| Record length (bytes) | 3600 | 3600*2 | 3600*4 | 3600*5 | 3600*8 |
Table: Tape usage at 6250 bpi