Sample rate, stored bits, channels, and duration fully determine an uncompressed PCM payload. Compression introduces another process. Lossy encoders may target constant, average, or variable bitrate; lossless encoders exploit patterns in the actual samples. The same PCM format and duration can therefore produce different compressed sizes.
PCM has a fixed payload rate
Every complete PCM frame stores a known number of bytes, and a known number of frames occur each second. This makes exact payload planning possible before recording. Headers and metadata are separate, but the sample bytes themselves do not depend on whether the content is silence, speech, music, or noise.
Lossy size follows the encoded stream
For a true constant-bitrate stream, bitrate multiplied by duration gives a useful payload estimate. Variable and average bitrate modes allocate different amounts to different passages, while tags, frames, padding, and containers add overhead. AudioSize does not invent a codec preset or imply that equal bitrate means equal quality across codecs.
Lossless compression depends on sample content
FLAC and similar codecs preserve decoded samples but remove predictable redundancy. Compression ratio changes with signal complexity, noise, bit depth, channel correlation, encoder settings, and metadata. A fixed percentage of WAV may be a planning range, never an exact result derived only from PCM settings.
Use the correct evidence for each question
Use AudioSize when planning the uncompressed master payload or establishing an upper reference before compression. For a compressed deliverable, use the encoder's configured stream bitrate when its contract is fixed, or inspect a representative encoded output when it is variable or content-dependent. Keep operational safety capacity separate from both numbers.