What this calculator does
This file transfer speed calculator works backwards from what you actually observed: a file of known size that took a known amount of time to move, whether that was a copy to a USB drive, an upload to a server, or a transfer across a local network. Dividing the size by the time gives the real, effective speed the transfer achieved, rather than the speed printed on a router box or a plan.
That effective speed is often well below any advertised figure. Overheads from the protocol being used, other traffic on the same connection, and the read or write speed of the drives at each end all eat into the theoretical maximum. Measuring the actual result from a size and a time, rather than trusting a rated speed, is the only way to know what a connection is really delivering.
The formula
Convert the file size to megabits (a gigabyte is treated as 1,000 megabytes, and each megabyte as 8 megabits) and the time to seconds, then divide size by time to get the speed in megabits per second. That figure is also shown converted to megabytes per second, since file sizes are usually quoted in bytes rather than bits.
| Term | Meaning |
|---|---|
| Mbps | Megabits per second, the standard unit for network and transfer speeds. |
| MB/s | Megabytes per second, the unit file sizes are usually shown in; equal to Mbps divided by 8. |
| Effective speed | The real transfer rate achieved, calculated from size and time rather than taken from an advertised rating. |
The inputs explained
| Field | What to enter |
|---|---|
| File size | The size of the file or files that were transferred. |
| Size unit | Choose from Megabytes (MB), Gigabytes (GB). |
| Time taken (min) | How long the transfer actually took to complete, measured from start to finish. |
| Time unit | Choose from Seconds, Minutes, Hours. |
When to use it
Checking a slow transfer
If a copy or upload feels slower than expected, working out the effective speed from the size and the time it took gives a concrete number to compare against the connection's rated speed.
Comparing two transfer methods
Timing the same file over two different connections or methods, such as Wi-Fi versus a cable, and calculating the effective speed for each gives a fair, like-for-like comparison.
Estimating time for a similar future transfer
Once the effective speed for a given connection is known, the related upload time or download time calculator can use that measured speed to predict how long a different-sized file will take.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Effective speed for different file sizes over the same 20-minute transfer
A fixed 20-minute transfer time, across a range of file sizes.
| File size | Effective transfer speed | In megabytes per second |
|---|---|---|
| 1 GB | 6.67 Mbps | 0.83 MB/s |
| 2 GB | 13.33 Mbps | 1.67 MB/s |
| 4 GB | 26.67 Mbps | 3.33 MB/s |
| 8 GB | 53.33 Mbps | 6.67 MB/s |
Effective speed for the same file at different transfer times
A fixed 2 GB file size, across a range of times it might have taken.
| Time taken (minutes) | Effective transfer speed | In megabytes per second |
|---|---|---|
| 2 min | 133.33 Mbps | 16.67 MB/s |
| 5 min | 53.33 Mbps | 6.67 MB/s |
| 10 min | 26.67 Mbps | 3.33 MB/s |
| 20 min | 13.33 Mbps | 1.67 MB/s |
Questions
Why is my measured speed lower than my internet plan's advertised speed?
Advertised speeds are a theoretical maximum for the connection alone. Protocol overhead, the speed of the storage at each end, other devices sharing the connection and the server being transferred to or from can all reduce the effective speed well below that rated figure.
How is this different from the upload or download time calculators?
Those calculators go the other way: they take a known speed and a file size and predict how long a transfer will take. This calculator solves for the speed itself, using a transfer that has already happened and a size and time you observed.
Why does the result show both Mbps and MB/s?
Network and connection speeds are almost always quoted in megabits per second (Mbps), while file sizes are shown in megabytes. Showing both lets you compare the result directly against either a plan's rated speed or a file size on screen.
Does this calculator account for overhead from the transfer protocol?
No, and it does not need to: because it works from the actual size and actual time taken, any overhead is already baked into the result. The effective speed shown is the true rate achieved, overhead and all.
To predict how long a future transfer will take once you know a connection's real speed, see the upload time calculator or the download time calculator.