Mbps vs MB/s: how fast will my download really be?
A 100 Mbps connection downloads at about 12.5 megabytes per second, not 100. Bits versus bytes, download time tables, and why real speeds come in a little lower.
Published
Tools for this job: Data Storage Converter · Time Converter
You pay for a 100 Mbps connection, start a download, and the browser shows 11 MB/s. Nothing is wrong. Internet speeds are measured in megabits per second (Mbps, small b) and file sizes in megabytes (MB, capital B). A byte is eight bits, so 100 megabits per second moves at most 12.5 megabytes per second.
The conversion
MB/s = Mbps ÷ 8
| Plan | Max MB/s | 5 GB download |
|---|---|---|
| 10 Mbps | 1.25 | about 67 min |
| 50 Mbps | 6.25 | about 13 min |
| 100 Mbps | 12.5 | about 6 min 40 s |
| 300 Mbps | 37.5 | about 2 min 13 s |
| 1 Gbps | 125 | about 40 s |
Times are the theoretical best for a 5 GB file (5,000 MB). Real downloads take a little longer.
Work out any download time
- Convert the file size to megabytes. 5 GB = 5,000 MB in decimal units.
- Divide your speed in Mbps by 8 to get MB/s.
- Divide the file size by MB/s to get seconds.
- Convert seconds to minutes or hours with the Time Converter.
The Data Storage Converter converts between bits, bytes, megabytes and gigabytes, with decimal units (MB, GB) and binary units (MiB, GiB) kept separate.
Why real speeds are lower
- Protocol overhead: some of every packet is addressing and error checking rather than your file, which typically costs a few percent.
- Wi-Fi: distance, walls and interference from neighbours often limit speed more than the broadband line. Test on a cable to see what the line itself delivers.
- The other end: a busy server may send more slowly than your connection can receive.
- Shared use: streaming and updates on other devices share the same connection.
What speed do you actually need?
Streaming one HD video uses a few megabits per second, and a 4K stream several times that. Video calls need less but are sensitive to upload speed and to delay. A household's need is roughly the sum of what everyone does at the busiest time of day. Beyond that, extra speed mainly shortens large downloads, such as games and system updates, which the table above lets you put in minutes.
Upload is often slower
Many home connections are asymmetric: 100 Mbps down but perhaps 10–20 Mbps up. Uploading large videos or backups takes much longer than downloading them. Check the upload figure in your plan or a speed test if you work with large files.
GB versus GiB
Operating systems sometimes report sizes in binary units, where a gigabyte means 1,024 × 1,024 × 1,024 bytes. A file shown as 4.66 GB in some programs is 5 GB in decimal units. For download estimates, the difference is about 7% at gigabyte scale. The full explanation is in why a 1 TB drive shows 931 GB.