Use the combined bitrate of the streams you intend to record. Variable-bitrate footage can depart from the chosen average.
Method and limits
Formula: Mb/s × 1,000,000 × seconds ÷ 8 = bytes. The extra allowance is then applied. GB uses 1,000,000,000 bytes; GiB uses 1,073,741,824 bytes. Neither number includes editing proxies, backups or operating-system files.
Container overhead and variable scene complexity can change the final file size. Use a short recording at your actual settings to get a better average, and keep a reserve for longer sessions. An encoder quality setting is not necessarily a fixed bitrate.
Drive capacity is only one requirement. Verify sustained write behavior, interface and application requirements separately. Use the storage capacity planner to combine the estimate with your existing files and free-space allowance.