Stsz

Stsz is a four-character code used in the MP4 and ISO base media file format to denote the Sample Size Box. It is part of the Sample Table Box (stbl) and plays a central role in describing the byte size of each media sample within a track. The stsz box is essential for determining where one sample ends and the next begins, which supports decoding, streaming, and seeking.

The box begins with a version field and flags, followed by a field called sample_size and a field called sample_count. If sample_size is nonzero, all samples in the track have the same size, and there are sample_count samples. If sample_size is zero, the box contains a table of sample_count entries, with each entry giving the size (in bytes) of the corresponding sample. This per-sample sizing allows for variable-sized samples, which is common in compressed video and audio streams.

In practical use, the stsz box works in concert with other metadata boxes in the MP4 container, such as stco or co64 (chunk offset boxes) and stsc (sample-to-chunk box), to map samples to their storage locations and decoding order. Playback applications and editing tools read stsz to efficiently locate and extract samples during decoding and seeking operations.

Etymology and scope: stsz stands for “Sample Size Box.” It is defined in the ISO/IEC 14496-12 specification and is one of several four-character box types used to organize sample data within MP4 files. While its primary and widely recognized use is in MP4/ISO BMFF, the term may appear in other contexts as a coded identifier or acronym in different software projects.