What this calculator works out
This calculator converts a data volume and a link speed into a transfer duration, applying an efficiency factor for the difference between the advertised speed and what a link actually sustains.
That factor is doing most of the work. A gigabit link almost never transfers at a gigabit, and planning a backup window on the nameplate figure produces a window that overruns.
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Before you plan a window
- Link speed is in megabits per second; data is in gigabytes. The calculation handles the conversion — one gigabyte is 8,000 megabits.
- 70% is a reasonable efficiency for a wired local network. Wi-Fi is frequently 40 to 50%; internet uploads are limited by the upstream speed, which is usually far lower than the download figure.
- The bottleneck is often the disk rather than the link. A mechanical drive reading many small files can be slower than the network carrying them.
- Incremental backups transfer far less than the full figure, which is the point of running them.
Domestic broadband is usually asymmetric — a 500 Mb/s download may come with 50 Mb/s upload or less. Cloud backup is an upload, so the advertised headline speed is the wrong number entirely. A first full backup of 500 GB at 50 Mb/s takes over a day of continuous transfer even at perfect efficiency.
How this calculator works
Bits over bits per second:
Seconds = data in GB × 8,000 ÷ (link speed in Mb/s × efficiency)
Hours = seconds ÷ 3,600The 8,000 converts gigabytes to megabits: eight bits to a byte, and a thousand megabytes to a gigabyte in the decimal convention networking uses. Storage capacity and network speed both use decimal units, which is one of the few places the two conventions agree.
Worked example: 500 GB over a gigabit link
Using the default figures — 500 GB at 1,000 Mb/s with 70% efficiency:
- Estimated duration: 1.59 hours
Comfortably inside an overnight window. Move the same backup to a 50 Mb/s upload for a cloud target and it becomes 31.7 hours — more than a day, and that is the initial seed rather than the routine run. This is why cloud backup providers offer physical seeding for large first backups, and why the incremental figure matters far more than the full one after week one.
Common mistakes
- Using the download speed for a cloud backup. It is an upload.
- Confusing megabits with megabytes. The factor is eight.
- Planning on the nameplate link speed. Real throughput is well below it.
- Ignoring the disk as a bottleneck. Many small files transfer far slower than one large one.
- Sizing the window on the full backup. After the first, incrementals are what run nightly.
Frequently asked questions
Why is my transfer slower than the link speed?
Protocol overhead, encryption, disk seek time, file count and contention all take a share. A wired gigabit link commonly sustains 700 to 900 Mb/s for large sequential files and far less for many small ones. Wi-Fi is worse again, since the advertised rate is shared and halves in practice for a single client.
How long should a backup window be?
Short enough to complete before the system is needed again, with margin. If a nightly backup routinely uses most of the window, any growth in data or a single failed run leaves no room to catch up. Sizing for about half the available window is a common rule.
Should I back up over Wi-Fi?
For a small incremental, it works. For an initial full backup of any size, a wired connection is worth the inconvenience — the difference is frequently a factor of two or three, and Wi-Fi is also more prone to interruptions that restart the transfer.
What is the 3-2-1 rule?
Three copies of the data, on at least two types of media, with one kept off-site. Each part defeats a different failure: drive failure, accidental deletion, and fire, theft or ransomware.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- National Cyber Security Centre — backup guidance, including offline copies and ransomware resilience
- Information Commissioner's Office — data protection obligations around retention and security of personal data
Sources are checked at publication and can change — how I choose and check references.