Mbps vs MB/s: Why Your Downloads Are Slower Than Your Plan

Bits vs bytes: the 8× gap

Network equipment counts bits because it sends them one at a time down a cable or radio link. Files and storage are counted in bytes (8 bits each). Internet providers, routers and speed tests use bits per second (Mbps, with a small b); browsers, Steam, Windows and macOS show bytes per second (MB/s, capital B).

The realistic columns assume 10% overhead, explained next. Swap any numbers into the calculator's “Download time” tab.

Why even the converted number is too optimistic

Your line rate isn't all payload. Part of every packet is addressing and error-checking (TCP/IP headers), encrypted connections add a little more, and lost packets get sent twice. On a good wired connection about 5–10% disappears this way; on busy or weak Wi-Fi it can be far more.

Then there are limits that have nothing to do with your plan:

• The server. A download can't go faster than the other end will send. Busy game launch days and small hosting plans are common bottlenecks.
• Wi-Fi. Distance, walls and neighbouring networks cut speed sharply. A cable to the router is the quickest test.
• Sharing. Streams, cloud backups and updates on other devices all take a slice.
• The disk. Writing to an old hard drive or USB stick can be slower than a fast fibre line.

The second trap: MB vs MiB

There are two definitions of a megabyte. Drive makers, network gear and the SI standard use powers of 1,000 (1 MB = 1,000,000 bytes). Windows — and many apps — use powers of 1,024 but still write “MB”; the strict name for that unit is the mebibyte (MiB, 1,048,576 bytes).

The gap grows with each prefix: 2.4% at kilo, 4.9% at mega, 7.4% at giga and 10% at tera. That's why a file Windows calls 4.7 GB is really 5.05 billion bytes, and why a “1 TB” drive appears as about 931 GB. When a download speed and a file size disagree slightly, mixed units are usually the reason.

How to estimate any transfer time

1. 1
   Convert the file size to megabits: GB × 8,000 (or MB × 8).
2. 2
   Take off overhead: multiply your speed in Mbps by 0.9.
3. 3
   Divide the first number by the second to get seconds.

Example: a 4.7 GB file over a 50 Mbps connection is 37,600 megabits ÷ 45 Mbps ≈ 836 seconds, about 14 minutes. A 100 GB backup over gigabit Ethernet takes about 15 minutes at the same 10% allowance.

Don't forget upload speed

Many cable and DSL plans upload 5–20 times slower than they download. A 500 Mbps / 50 Mbps plan downloads a 10 GB video in about 3 minutes but needs around 30 minutes to upload it. If you back up to the cloud, stream, or send large files, check the upload figure before you choose a plan — and plan big uploads overnight. Shrinking files first helps too: the Image Compressor and other file tools can cut upload sizes dramatically.

Turning a bitrate into data usage

The same conversion tells you how much data something uses. A continuous 25 Mbps stream moves 25 × 3,600 ÷ 8 = 11,250 MB, about 11 GB an hour. Pick “GB/hour” or “TB/month” in the converter to compare a bitrate with a data cap.

Units for data sizes on their own — KB, MB, GB, KiB and so on — are handled by the Data Storage Converter.