How to use it
- Choose Calculate: Time to transfer a file or Speed needed for a target time.
- Type the File size and choose the Size unit.
- For a time, type the Connection speed and choose the Speed unit. For a speed, type the Target time and choose the Target time unit.
- Type the Protocol overhead (%) if you know it, or leave 0, and read the Transfer time or the Speed needed (Mbit/s).
Megabits and megabytes
Connection speeds are given in bits per second and file sizes in bytes. A byte is 8 bits, so 1 MB/s is 8 Mbit/s, and a 100 Mbit/s connection carries 12.5 MB in a second. The Wikipedia article on data-rate units gives 1 Mbps as 0.125 MB/s, and uses decimal prefixes for data rates: 1 Mbit/s is 1,000,000 bit/s.
Sizes here are decimal too: 1 GB is 1,000,000,000 bytes (Wikipedia: Gigabyte). If your system shows binary sizes under a GB label, the figure is smaller than the decimal size. The same article gives an example: a 1 TB (1,000 GB) drive is reported by Windows as 931 GB. To turn a binary GiB figure into decimal GB, multiply it by 1.074.
Protocol overhead
The gross bit rate of a link includes the protocol overhead, such as headers, and the error-correction bits, as well as the file data. The net bit rate is the payload, the data that is actually carried. The Wikipedia article on bit rate describes this difference. The overhead field takes its share off the line speed: at 10 % overhead, 100 Mbit/s leaves 90 Mbit/s for the file, and the time grows by one ninth.
file data speed = line speed × (1 − overhead ÷ 100)
time = file bits ÷ file data speedA steady speed is assumed
The result assumes a steady speed for the whole transfer. Real transfers vary with the network, the server and other traffic, so the time is an estimate.
Example: a 4.7 GB film on a 100 Mbit/s line
4.7 GB is 4,700,000,000 bytes, which is 37,600,000,000 bits. A 100 Mbit/s line carries 100,000,000 bits each second, so with no overhead the transfer takes 376 seconds, or 6 min 16 s.
With 10 % protocol overhead only 90 Mbit/s carries file data, and the time grows to 418 seconds, or 6 min 58 s.
To finish in 30 minutes, the line needs 20.89 Mbit/s, which is 2.61 MB/s.
| Measure | Value |
|---|---|
| Time at 100 Mbit/s, no overhead | 6 min 16 s |
| Time with 10 % overhead | 6 min 58 s |
| Speed for 30 minutes | 20.89 Mbit/s |
| Speed for 30 minutes, in MB/s | 2.61 MB/s |
Frequently asked questions
Why is the real download slower than the calculator says?
The calculator assumes a steady speed and the overhead you enter. Real transfers vary with the network, the server and other traffic, so the time is an estimate.
How do I convert MB/s to Mbit/s?
Multiply by 8, because a byte is 8 bits: 12.5 MB/s is 100 Mbit/s. The Wikipedia article on data-rate units gives the same relation, 1 Mbps = 0.125 MB/s.
Why does my computer's file size differ from the figure here?
Some systems show binary sizes under a decimal label. The Wikipedia article on the gigabyte shows a 1 TB drive reported as 931 GB. Multiply a GiB figure by 1.074 to get decimal GB, and type that here.
What is protocol overhead?
The share of the transferred bits that is not file data, such as headers and error-correction bits. The Wikipedia article on bit rate says the gross bit rate includes protocol overhead, and the net bit rate is the payload.
Why is the time rounded to whole seconds?
A transfer changes speed as it runs, so a time to the hundredth of a second would suggest more accuracy than the calculation has. The result is rounded to the whole second.
Does the page test my connection?
No. It does not measure your connection. Only the numbers you type are used, and the sums run in your browser.
Sources
Figures and text last checked on Tuesday, November 10, 2026.