What this tool helps you work out
A headline disk or network speed rarely matches the speed of a real Windows file copy. Protocol overhead, thousands of small files, antivirus scanning and the slower end of the transfer can all extend the job.
This calculator converts the selected data size and sustained throughput into an estimated duration. For a useful plan, enter a measured transfer rate rather than the maximum printed on a drive, cable or Wi-Fi box.
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Before you estimate the copy
Run a representative test if the deadline matters. Copying one 50 GB virtual-disk file can be much faster than copying 50 GB made up of a million tiny files. The same total size does not imply the same completion time.
Initial figures may be helped by cache, while later stages expose the sustained write speed of the destination. Treat the displayed transfer rate as a moving estimate, not a contractual finish time.
How the calculation works
Time in seconds = total bits ÷ bits per secondThe calculator converts bytes to bits and normalises the chosen speed unit. It then adds any overhead entered on the page.
For SMB copies, include time for network, storage and file-system overhead. The slowest component normally sets the sustained rate.
Worked example: copying 500 GB
At a sustained 85 MB/s:
- 500 GB is roughly 500,000 MB using decimal units
- 500,000 MB ÷ 85 MB/s = about 5,882 seconds
- That is about 98 minutes before extra overhead
Allowing 15% for scanning, metadata and speed variation produces a practical estimate of roughly 1 hour 53 minutes.
Common copy-time mistakes
- Entering a link rate in megabits per second as though it were megabytes per second.
- Using the advertised peak speed instead of measured sustained throughput.
- Ignoring the number of files and directory entries.
- Assuming the source and destination can both sustain the same speed.
- Skipping verification when the copy is a migration or the only backup.
Frequently asked questions
Why is Windows copying more slowly than the calculator?
The entered speed may be too optimistic, or the job may contain small files, permissions, antivirus scanning, retries or a slow destination.
Should I use File Explorer or Robocopy?
File Explorer is convenient for ordinary copies. Robocopy is often better for repeatable migrations because it supports restartable copies, retries, logging and many metadata options.
What does Robocopy /MT do?
It enables multithreaded copying. This can improve throughput for some collections of files, but too many threads can overload a slow disk, network link or remote server.
Does copying to the same drive take longer?
Usually. The same device may need to read and write at the same time, so throughput can be much lower than a copy between separate devices.
How much contingency should I add?
For a simple large-file copy on known hardware, 10–20% may be reasonable. Mixed files, Wi-Fi, cloud paths or active servers deserve more.
Related Windows and technology tools
References
These sources support the Windows behaviour or command guidance discussed on this page.
External documentation can change. Check the linked source before applying a setting to an important system.