Mbps vs MB/s and GB vs GiB: Why Your Internet Speed and Storage Never Match the Box

You pay for a 500 Mbps internet plan, start a big download, and your browser says 55 MB/s. Then you plug in a new 1 TB drive and Windows tells you it holds 931 GB. It is natural to wonder whether you were shortchanged twice in one afternoon.

You were not. Both surprises come from the same two habits of the computing world: speeds and sizes are measured in different units that look almost identical, and the same abbreviation can mean two slightly different amounts. Once you know the two rules, the mismatch disappears, and you can work out in seconds how long any download will take and how much space you really have.

All tables calculated from the formulas shown; real-world figures checked against published protocol math and common test results

The 30-Second Answer

• Internet speeds are in bits per second, file sizes are in bytes. There are 8 bits in a byte, so divide Mbps by 8 to get MB/s. A 500 Mbps plan is at most 62.5 MB/s, and a 100 Mbps plan is at most 12.5 MB/s.
• Drive makers count in thousands, Windows counts in 1,024s. A 1 TB drive holds exactly 1,000,000,000,000 bytes. Windows divides that by 1,073,741,824 and shows 931 GB. The space is not missing. It is being measured with a bigger ruler.
• Real speeds land below the plan number because of protocol overhead, Wi-Fi, and the slowest device in the chain.

Figure 1 Plans Count Bits. Files Count Bytes. — Eight bits make one byte, so divide Mbps by 8 to get MB/s

Rule 1: Bits Versus Bytes (Mbps vs MB/s)

A bit is the smallest piece of digital information, a single 1 or 0. A byte is a group of 8 bits, and it is the unit that file sizes are counted in. Network speeds have traditionally been measured in bits per second, the amount of data moving down the wire each second, while the files you download and store are measured in bytes. Bits per second also gives bigger-sounding numbers, which is one reason internet plans are sold that way.

The letter case matters. A lowercase b means bit and an uppercase B means byte. So Mbps (or Mb/s) is megabits per second, and MB/s is megabytes per second. They differ by a factor of exactly 8.

The conversion is exact: MB/s = Mbps ÷ 8 and Mbps = MB/s × 8. Network speeds use decimal prefixes, so 1 Gbps is exactly 1,000 Mbps, not 1,024. A speed-test app will report your result in Mbps, while the download bar in your browser or game launcher will often show MB/s, which is why the same connection seems to have two different speeds.

How Long Will a Download Take?

The formula is straightforward. For a file size in GB and a speed in Mbps:

Time in seconds = file size in GB × 8,000 ÷ speed in Mbps

The 8,000 comes from 1 GB being 8,000 megabits. The table below applies it. These are the best-case times for a connection running at its full plan speed, using decimal units, with no overhead.

Another handy way to think about it: a 100 Mbps connection moves 45 GB per hour at best, 300 Mbps moves 135 GB, 500 Mbps moves 225 GB, and a full gigabit moves 450 GB.

Why Real Speeds Fall Below the Plan Number

The table above is a ceiling, not a promise. Several things take a bite out of it, and it helps to know which ones you can fix.

Protocol Overhead: The Part Nobody Can Avoid

Data does not travel as a clean stream of file content. It is split into packets, and every packet carries headers for Ethernet, IP, and TCP that are needed to deliver it but are not part of your file. Wikipedia's explanation of goodput works the numbers out: on gigabit Ethernet carrying TCP over IPv4, the best an application can achieve is about 949 Mbit/s, or 119 MB/s. On a 100 Mbps link the equivalent figure is 94.9 Mbit/s, or 11.9 MB/s. In real tests, a wired gigabit connection typically measures around 940 Mbps, about 117 MB/s, and everyday file copies across a home network often settle at 110 to 115 MB/s.

So a gigabit result of about 940 Mbps on a cable is a perfect result, not a shortfall. As a rule of thumb, expect anywhere from about 5 to 20 percent less than the plan number once everything is included.

The Other Things in the Chain

• Wi-Fi. The same connection over Wi-Fi is usually slower than over a cable, and it varies with the router, the distance, walls, and interference. If you want to know what your plan can really do, test with a cable.
• The slowest device wins. A fast connection cannot write to a slow disk faster than the disk allows, and a server that limits each download caps you regardless of your plan.
• Sharing. Other devices streaming, backing up, or updating share the same connection.
• "Up to" speeds. Plans are sold as maximums, and many are much faster downloading than uploading.

Figure 2 Where your gigabit goes — on the box 1,000 Mbps (125 MB/s), wired after overhead about 940 Mbps (117 MB/s), typical file copy about 110 to 115 MB/s

Rule 2: Decimal Versus Binary (GB vs GiB)

The second mix-up is about what "kilo," "mega," and "giga" mean. In everyday language and in the metric system they mean 1,000, 1,000,000, and 1,000,000,000. Early computers, though, worked naturally in powers of two, and 1,024 (2 to the power of 10) is close to 1,000. For years, people used "kilobyte" for 1,024 bytes, and the habit stuck, which meant the same word meant two slightly different things.

To settle it, the International Electrotechnical Commission introduced separate binary prefixes in the late 1990s: kibi, mebi, gibi, and tebi, abbreviated KiB, MiB, GiB, and TiB. The decimal and binary versions are now different, named units, and the gap grows as the numbers get bigger.

Who Uses Which

• Drive and storage makers use decimal. A 1 TB drive is 1,000,000,000,000 bytes.
• macOS (since 10.6) and iOS report sizes in decimal, so a Mac shows a 1 TB drive as about 1 TB.
• Windows calculates in binary but labels the result KB, MB, or GB instead of KiB, MiB, or GiB. That is the whole source of the 931 GB result.
• Linux depends on the tool you use, and many tools let you choose.
• Memory (RAM) is built in powers of two, so a "16 GB" module genuinely holds 16 GiB. That is why RAM never seems to go "missing" the way drive space does.
• Network speeds use decimal throughout, so 1 Gbps is exactly 1,000 Mbps.

Why Your Drive Shows Less Space

Take a 1 TB drive. It holds 1,000,000,000,000 bytes. Windows divides by 1,073,741,824 (one binary "gigabyte") and reports 931.3, then prints "GB" after it. Nothing was removed from the drive. The same quantity was measured with a longer ruler, so the number shrinks by about 6.9 percent.

A freshly formatted drive then loses a little more to file-system structures, and some computers set aside a recovery partition, so the usable figure is slightly lower again. To see the exact number for yourself, right-click the drive in Windows and choose Properties, which shows the capacity in bytes alongside the rounded figure.

Figure 3 Same bytes. Different ruler. Nothing is missing. — A 1 TB drive holds 1,000,000,000,000 bytes; Windows labels that as 931.3 GB

Three Worked Examples

Example 1: A 60 GB Game on a 500 Mbps Plan

Time = 60 × 8,000 ÷ 500 = 960 seconds, which is 16 minutes at the full plan speed. Allowing roughly 10 to 20 percent for overhead and everything else, plan on about 17 to 19 minutes. If your browser is showing around 55 MB/s, that is a normal result for this plan, since the ceiling is 62.5 MB/s.

Example 2: Will 900 GB From a Mac Fit on a 1 TB Drive?

The Mac reports in decimal, so 900 GB means 900,000,000,000 bytes. A 1 TB drive holds 1,000,000,000,000 bytes, so it fits with room to spare, minus a little for formatting. Windows would display that same 900 GB as roughly 838 GB, which is again the same bytes under a different ruler. The safe way to compare any two sizes is to convert both to bytes first, so you are always using one ruler.

Example 3: Will a 2 TB Backup Finish Overnight on a 50 Mbps Upload?

2 TB is 2,000,000,000,000 bytes, or 16,000,000,000,000 bits. Dividing by 50,000,000 bits per second gives 320,000 seconds, which is about 89 hours, or nearly 3.7 days of continuous uploading. Many internet plans have a much slower upload than download, and upload speed, not the headline download number, decides how long a big backup takes.

Common Mistakes

Use the Free Converters

You do not need to do this arithmetic by hand. Two free ToolsCrow converters cover everything in this guide.

The Data Transfer Rate Converter converts between units such as Mbps, Gbps, and MB/s, which is the quickest way to see what a plan speed means in the megabytes per second your download bar uses. The Data Storage Converter handles bits and bytes in both decimal units (KB, MB, GB, TB) and binary units (KiB, MiB, GiB, TiB), so you can see the same quantity both ways and settle any "why does it say 931" question immediately. If you want a result in hours and minutes, the Time Converter turns seconds into larger units. All of them are in the Unit Converters collection.

If large video files are the reason you are watching your storage and your speeds, the video compressor guide shows how to shrink a file to a target size before you upload it, and the duplicate file finder comparison helps you win back space you did not know you were wasting.

Frequently Asked Questions

What is the difference between Mbps and MB/s?

Mbps is megabits per second and MB/s is megabytes per second. A byte is 8 bits, so MB/s is Mbps divided by 8.

How do I convert Mbps to MB/s?

Divide by 8. For example, 100 Mbps is 12.5 MB/s, 500 Mbps is 62.5 MB/s, and 1,000 Mbps is 125 MB/s.

Why is my download slower than my internet plan?

Usually because the plan number is a ceiling measured in bits, while your download is shown in bytes, and because protocol overhead, Wi-Fi, other devices, and the server or disk at the other end all reduce the real figure. A wired gigabit connection commonly measures about 940 Mbps, or about 117 MB/s.

Is 1 Gbps equal to 1,000 or 1,024 Mbps?

1,000. Network speeds use decimal prefixes.

Why does my 1 TB drive show 931 GB?

The drive holds exactly 1,000,000,000,000 bytes. Windows divides by 1,073,741,824 and labels the result GB, so it shows about 931.3. A Mac, which uses decimal, shows it as 1 TB.

Am I being cheated on drive capacity?

No. Drive makers use the decimal meaning of the prefixes consistently, which is legitimate, and the capacity you receive in bytes matches the label. The confusion comes from Windows labeling binary amounts with decimal-looking names.

Is a GiB bigger than a GB?

Yes. A GiB is 1,073,741,824 bytes and a GB is 1,000,000,000 bytes, so a GiB is about 7.4 percent bigger.

Why does RAM show the full size when drives do not?

Memory is built in powers of two, so a "16 GB" module really holds 16 GiB, and Windows reports it that way. Drives are sold in decimal sizes, which creates the gap.

How long will it take to download a file?

Time in seconds equals the file size in GB multiplied by 8,000, divided by your speed in Mbps. Add roughly 10 to 20 percent for real-world overhead.

Why do macOS and Windows show different sizes for the same file?

macOS reports decimal and Windows reports binary under the same labels. The file is identical, so compare the byte counts to confirm.

The Bottom Line

Almost every "my speed and storage do not match the box" complaint comes down to two rules. Speeds are counted in bits and file sizes in bytes, so divide Mbps by 8. And drive makers count in thousands while Windows counts in 1,024s but keeps the decimal-looking names, so a 1 TB drive shows as 931 GB.

When you convert any size or speed to bytes first, you are always comparing like with like.

Figure 4 Bits, Bytes and Boxes cheat sheet — save this for quick reference: 8 bits = 1 byte, MB/s = Mbps ÷ 8, download time = GB × 8,000 ÷ Mbps, and 1 TB drive = 931.3 GB in Windows

Convert it in a click, free:
Data Transfer Rate Converter: Mbps, Gbps, MB/s and more
Data Storage Converter: bits, bytes, KB, MB, GB, TB and KiB, MiB, GiB, TiB
All Unit Converters