Surveillance Storage Calculator: Cameras to Terabytes
Camera count, bitrate and retention in; raw capacity, usable capacity at your RAID level, annual write volume, and the workload tier the arithmetic actually requires out - with live picks for that tier, cheapest $/TB first.
Prices updated
The required tier is below even the standard-surveillance rating, but we do not recommend a desktop-class drive for surveillance regardless of workload headroom - it lacks RV sensors and surveillance firmware. See the standard tier's picks on /skyhawk or /wd-purple instead.
Reading the result
Raw capacity is retention days ร cameras ร bitrate, converted to terabytes. Usable capacity applies your chosen RAID level's overhead via the same calculation this site's RAID planner uses. Annual write volume is the same camera-count ร bitrate ร hours-per-day arithmetic used on the workload-ratings explainer, annualised over a full year regardless of your retention window - a drive's workload rating is a design target for continuous operation, not for one retention cycle.
The required tier follows directly from comparing that annual figure against the published ladder - it is not our judgement of what you need. A system that lands well under the standard-surveillance tier's 180TB/year rating does not need the AI tier regardless of camera count, and the tool will not recommend one.
Why raw capacity and annual write volume are different numbers
These two outputs answer different questions and deliberately do not share an input. Raw capacity answers "how big a volume do I need to hold my retention window" - it scales with retention days, and doubling your retention target roughly doubles this number. Annual write volume answers "what tier of drive is this workload rated for" - it is independent of retention entirely, because a workload rating describes how much data a drive is designed to have written to it in a year of continuous operation, regardless of how long you keep that data before it is overwritten.
A system with a short 7-day retention window and a system with a 90-day window, running the same cameras at the same bitrate, put the same annual write load on the drive - the drive rewrites its full capacity on a loop either way. Only the raw-capacity number changes with retention; the tier requirement does not. Conflating the two is the most common sizing mistake this tool is built to avoid.
Compression and the scene-activity adjustment
The scene-activity control is an adjustment on top of the base bitrate, not a separate vendor-sourced figure - modern cameras use variable bitrate encoding that rises with motion in frame and falls during quiet periods, so a camera's real average bitrate over a day is rarely its documented peak. We do not publish a specific multiplier as a vendor fact because no manufacturer datasheet we could render states one; the low/typical/high options here are a deliberately rough adjustment for you to calibrate against your own camera's actual observed bitrate if you have access to that figure from your NVR's own reporting.
Frequently asked questions
Where does the default bitrate come from?
WD's own published single-stream reference figure: 3.2Mbps for single stream, 1080p, h.265, 25fps (WD Purple / WD Purple Pro data sheets (footnote 3)). Real bitrates vary substantially by scene activity, resolution and encoder settings - change the input to match your own camera's documented or observed rate.
How is the required tier decided?
By comparing the computed annual write volume against the manufacturer-published workload ratings on the ladder (120/180/550TB/year) - see /surveillance/workload-ratings for the full ladder and citations. The tool never makes a suitability judgement of its own; it reports which published rating your number falls under.
What does 'usable capacity' mean here?
Capacity remaining after RAID overhead at your chosen level, computed via the same usableCapacity() function this site's RAID planner uses - RAID 1, 5, 6, 10, SHR or JBOD.
Is this an exact prediction of my storage needs?
No - it is arithmetic from the inputs you provide, not a measurement of your actual system. Real cameras use variable bitrate that rises with motion, and compression, codec and scene activity all change the true figure. Treat the output as an estimate to size against, not a guarantee.
Sources checked: WD Purple / WD Purple Pro data sheets (footnote 3), Seagate BarraCuda 3.5" HDD Data Sheet, DS2131.3-US2411 (2024), Seagate SkyHawk DS1902-6-1710US; WD Purple data sheet, November 2025, WD Purple data sheet, April 2019, pub. 2879-800012-A14, Seagate SkyHawk AI DS1960.15C-2310US; WD Purple Pro data sheet, July 2025.