The calculation
Uncompressed PCM storage equals sample rate × bit depth × channels × seconds, divided by eight to convert bits to bytes. Doubling sample rate or channel count doubles the result. Moving from 16-bit to 24-bit increases sample data by 50%.
What the estimate does not include
A DAW session also creates project data, waveform caches, fades, alternate takes, rendered effects, exports, and backups. Compressed formats such as FLAC reduce storage without discarding decoded audio data; MP3 and AAC use lossy compression and should not replace production masters.
Plan the archive, not only the recording drive
The “2× backup” result means three total copies: the working data and two additional copies. Keep at least one copy on a separate device, and verify that important sessions can actually be restored before reusing recording media.