Tape length is usually measured in minutes of total playing time. The most popular varieties are C46 (23 minutes per side), C60 (30 minutes per side), C90, and C120. The C46 and C60 lengths are typically 15–16 µm thick, but C90s are 10–11 µm and C120s are just 9 µm thick, rendering them more susceptible to stretching or breakage. Some vendors are more generous than others, providing 132 meters (433 feet) or 135 meters (442 feet) rather than 129 meters (423 feet) of tape for a C90 cassette. C180 and even C240 tapes were available at one time, but these were extremely thin and fragile and suffered badly from effects such as print-through, which made them unsuitable for general use.
Although the TDK-D C180 was produced for two decades, it is very rare, because of several technical flaws. The tape had to be so thin that it was nearly transparent and therefore had fewer particles to magnetize, resulting in a poor sound quality and even worse durability. It required a strong motor to be driven, and had high wow and flutter. Finally, it took a relatively long time to rewind.
Other lengths are (or were) also available from some vendors, including C10 and C15 (useful for saving data from early home computers and in telephone answering machines), C30, C50, C54, C64, C70, C74, C80, C84, C100, C105, and C110.
Some companies included a complimentary blank cassette with their portable cassette recorders in the early 1980s. Panasonic's was a C14 and came with a song recorded on side one, and a blank side two. Except for C74 and C100, such non-standard lengths have always been hard to find, and tend to be more expensive than the more popular lengths. Home taping enthusiasts may have found certain lengths useful for fitting an album neatly on one or both sides of a tape. For instance, the initial maximum playback time of Compact Discs was 74 minutes, explaining the relative popularity of C74 cassettes.