Video storage estimation is simpler than it looks once you break it into its parts -- here's the math, and how to use the calculator built into this site.
Both camera databases include a bandwidth & storage calculator (scroll to the bottom of the Synology site or the QNAP site) that estimates network throughput and storage footprint from a camera count, resolution tier, codec, frame rate, and retention period. This guide explains what it's actually computing and, just as importantly, what it deliberately doesn't claim to know.
Storage planning for video comes down to four numbers multiplied together: bitrate (how much data one camera generates per second, driven mainly by resolution and codec), camera count, retention period in days, and duty cycle (what fraction of the time it's actually recording). Bitrate times camera count gives total network throughput; that times seconds-per-day times retention days gives total storage for continuous recording; multiplying by duty cycle gives a lower estimate for motion-triggered recording instead.
The calculator uses common rule-of-thumb bitrate figures for H.264 at 30fps "good quality" continuous recording, scaled by resolution tier -- these are industry-standard approximations, not a specific manufacturer's published per-camera bitrate table, since that field isn't mined from camera datasheets on this site (or, generally, published by manufacturers in a consistent way at all). H.265 is applied as roughly half the H.264 figure, matching the widely-cited real-world compression improvement of that codec. Treat the calculator's output as a reasonable planning estimate, not a guarantee of your actual real-world bitrate, which depends on scene complexity, lighting, and camera-specific encoder tuning too.
Continuous (24/7) recording is the simplest to reason about and the safest if you might need footage from any arbitrary moment, but consumes storage linearly with retention period regardless of whether anything happened. Motion-triggered recording only writes footage when the camera (or Surveillance Station/QVR Pro's own motion detection) flags activity, which the calculator estimates at roughly a 20% duty cycle as a rough rule of thumb -- real-world duty cycle varies enormously by location (a front door sees far more triggered activity than a fenced backyard) and by how motion sensitivity is tuned, so treat that 20% figure as a starting point to adjust once you have real usage data from your own install, not a fixed constant.
The estimate is raw video storage only. It doesn't account for RAID/redundancy overhead (a mirrored or parity RAID configuration reduces your usable capacity relative to raw drive size), filesystem/snapshot overhead, or any non-surveillance data you're also storing on the same volumes. Budget meaningful headroom above the calculator's raw number -- and revisit the estimate after your NAS's camera license count and channel ceiling are settled too, since that's a separate constraint from raw storage capacity.