Backup Time & Window Calculator

How we calculate

• Data moved. Full = all the data you protect. Incremental = the logical data changed per day. Differential = changed data per day × days since the last full backup, capped at the whole dataset. Dividing by your data reduction ratio (1 = none, the default) gives the data sent.
• Speed. The parts of a backup run at the same time, so the part that takes the longest sets the transfer time. Reading the drive and processing the backup (encrypting, compressing, deduplicating) handle all of the data, before any reduction. Only the network or internet upload and the drive it writes to handle the smaller amount sent. So a reduction ratio shortens the network and destination time, never the time to read the data. Each part's time is its data divided by its speed. Encryption and compression count only as the processing ceiling, never as a percentage taken off afterwards, so they cannot slow a job the network already limits.
• Files. Each file adds a small handling time on top of its bytes, so the same data as many small files takes longer. The number of files comes from the data before reduction. The slowest part is whichever takes the most time, the bytes or the file handling.
• Internet upload. With a plan speed only, the range runs from NTK's planning assumption of 75% of plan upload speed (the slow end) to the full line rate (the fast end). A measured upload is used as it is.
• Measured backup speed. If you enter one, it replaces the whole estimate. It is read as data backed up per second before any reduction, so it applies to all of the data and your reduction ratio does not shorten it a second time. Any fixed overhead you enter is added to it, so leave the overhead blank if your measured time already includes scanning and verification.
• Window. A window can cross midnight (10 PM to 6 AM is 8 hours), and the same start and finish hour is a full 24 hours. "Fits" means even the slow end finishes in time, "only at the fast end" means the fast end does and the slow end does not.
• Transfer only. Unless you enter a fixed overhead, the time is the transfer alone. Scanning for changes, snapshots, verification and retention clean-up add to a real job.
• Units. Decimal: 1 TB = 1,000 GB = 1,000,000 MB. Network speeds are megabits per second (Mbps), drive speeds megabytes per second (MB/s), and 8 bits make a byte.

The drive, network and per-file figures are NTK planning assumptions, labelled as such in the table below, not measurements of your hardware. Use the result to plan, then compare it with your first real backup and enter the measured speed.

Worked examples

A local full backup. 1 TB of large files from a NAS with 4 x 7,200 RPM drives in RAID 5 (reads 306 to 459 MB/s) over 1 GbE (100 to 117.5 MB/s) to one 7,200 RPM drive (writes 120 to 180 MB/s). The network is the slowest part, so the transfer takes 2 h 22 min to 2 h 47 min. Faster drives would not change that; a faster link would, until the destination drive becomes the limit.

Incremental and differential. 2 TB with 1% changed a day is 20 GB of logical changed data. With a 1.25 reduction ratio that sends 16 GB, about 2 min to 3 min over the same path. Six days after the last full backup, a differential holds 120 GB, sends 96 GB and takes 14 min to 16 min.

Reduction does not speed up the read. 1 TB of large files from one 7,200 RPM drive (reads 120 to 180 MB/s) to another over 1 GbE, with a 10 reduction ratio. Only 100 GB crosses the network, but all 1 TB is still read from the drive, so the drive is the slowest part and the transfer takes 1 h 33 min to 2 h 19 min.

When processing is the limit. 1 TB from a SATA SSD to a SATA SSD over 2.5 GbE, where the backup software encrypts at 100 MB/s: 2 h 47 min. Over 10 GbE it is still 2 h 47 min, because the network was never the slowest part.

An offsite backup over nbn. 4 TB to a NAS at another site over a 20 Mbps upload (nbn Home Standard on FTTP, FTTC or HFC): 18.5 to 24.7 days for the first full backup. A 20 GB daily incremental then takes 2 h 13 min to 2 h 58 min, which fits an 8-hour overnight window.

From a measured speed. If your backup software reported 45 MB/s on a previous job, 1 TB takes 6 h 10 min, with no other assumption applied.

The assumptions behind the estimate

nbn upload speeds for offsite and cloud backups

A backup to another site or the cloud uses your upload, which on nbn home tiers is much lower than the download. These are nbn's wholesale upload speeds by connection type, the most nbn sells to providers; your provider's plan and your line can be slower. Where nbn gives a range, the maximum can fall anywhere in it.

Source: nbn Co speed plans, checked 2026-10-07. For a cloud upload in more detail, use the Cloud Backup Time Estimator.

Offsite backups, CGNAT and seeding

A cloud backup connects outward from your NAS, so carrier-grade NAT (CGNAT) usually does not block it. Backing up to your own NAS at another site is different: that site has to accept incoming connections, so behind CGNAT you need an overlay VPN such as Tailscale or ZeroTier, or a relay service. Synology says a relayed QuickConnect connection may be slow because of longer network latency, and recommends port forwarding for better performance. This help page gives no speed figure or cap. (Synology DSM Help, checked 2026-10-07) Whether your provider uses CGNAT, and whether it offers a public or static IP address, varies by provider: compare plans in the NBN Plan Finder.

If the first full backup over the internet would take weeks, back up the most important data first, or check whether your backup or cloud provider can seed the backup from a physical drive. Availability, shipping and cost vary by provider and region.

Facts this page uses

Each fact below is taken from the publisher's own page and was re-found at the source on the date shown.

Frequently Asked Questions

Method last reviewed: 2026-10-14. Facts last checked at source: 2026-10-07.