Backup Window Calculator
Work out how long a backup or data transfer takes from the data size, throughput, transfer efficiency and any compression — and whether it fits your window.
How it works
effective rate = rate × efficiency% · data transferred = data ÷ compression ratio · time = data transferred ÷ effective rate
A backup finishes only as fast as the slowest link in the chain sustains, which is always below the peak line rate — disks seek, protocols acknowledge, and other traffic competes, so applying an efficiency factor keeps the estimate honest. Compression reduces how many bytes actually cross the wire, shortening the window in rough proportion to its ratio (though CPU-bound compression can itself become the bottleneck). Comparing the result against the available window tells you whether the job completes before the business day resumes, or whether you need faster storage, incremental backups, or a bigger pipe.
Worked example
500 GB at 100 MB/s peak but 80% efficient is 80 MB/s effective. With 1.5× compression, only 500 ÷ 1.5 ≈ 333 GB (341,333 MB) crosses the wire: 341,333 ÷ 80 ≈ 4,267 s ≈ 1.19 hours — comfortably inside an 8-hour window.
Frequently asked questions
Why not just use the rated transfer speed?
Rated speeds are peak, best-case numbers. Real backups run slower because of disk seeks, protocol overhead, encryption and contention, so an efficiency factor of 70–90% gives a realistic window.
How does compression change the estimate?
Compression shrinks the number of bytes sent, so a 2× ratio roughly halves the transfer time — provided the CPU can compress fast enough to keep the link full. Already-compressed data (video, archives) barely benefits.
What if the backup does not fit the window?
Switch to incremental or differential backups so only changed data moves, increase throughput with faster disks or a wider network link, or stagger jobs so they do not all run at once.
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This calculator is for educational purposes. Double-check important results before acting on them.