How the backup window calculator works
Incrementals are the whole strategy
Full backups of a 2 TB library are a weekend event; incrementals of a 20 GB day are a 30-minute chore. Sound backup design runs frequent incrementals (daily) with periodic fulls (weekly/monthly) — and the nightly window only ever needs to fit the incremental. The calculator prices both so the design is deliberate.
The 3-2-1 rule, made concrete
Three copies, two media, one offsite. The offsite copy is where this calculator matters: over a 110 MB/s pipe, your nightly 20 GB takes 30 minutes of internet. Over a 20 Mbps uplink, it takes 2.4 hours — the upload-time calculator and this one are the same problem wearing different units.
When the window is starving
Levers in order: dedup/compression (30–60% cuts, free), faster target storage (NAS 2.5GbE, SSD targets), longer windows (weekend fulls), or narrowing the dataset (exclude the movie archive from nightly jobs). Backups that chronically overrun the window get skipped by humans — the failure mode that only reveals itself the day restore is needed.