Why does a hard drive advertised as "1 TB" not show exactly 1 TB of available space in your operating system?
Why does a file-transfer tool use MB/s while a network plan uses Mbps?
The confusion comes from several related units being used for different purposes.
The most important distinction is this:
**MB and MiB are not the same unit.**
MB uses decimal prefixes
In the decimal system:
- 1 kB = 1,000 bytes
- 1 MB = 1,000,000 bytes
- 1 GB = 1,000,000,000 bytes
- 1 TB = 1,000,000,000,000 bytes
These are SI decimal prefixes.
Storage manufacturers commonly use decimal units when describing drive capacity.
MiB uses binary multiples
The IEC binary prefixes are different:
- 1 KiB = 1,024 bytes
- 1 MiB = 1,048,576 bytes
- 1 GiB = 1,073,741,824 bytes
- 1 TiB = 1,099,511,627,776 bytes
The binary units were introduced to make the distinction explicit.
So:
**1 MB = 1,000,000 bytes**
while:
**1 MiB = 1,048,576 bytes**
Those are close, but not identical.
Why did this become confusing?
Computers are deeply associated with binary arithmetic, so older software often used powers of 2 while labeling the result with decimal-style names such as "MB."
At the same time, storage and networking industries adopted decimal prefixes for practical reasons.
Eventually, people were using the same labels for different quantities.
That is why a modern file-size readout can look different from a manufacturer's storage specification even though nobody is necessarily doing the math incorrectly.
A simple example
Suppose a file is:
**10,000,000 bytes**
In decimal units:
**10,000,000 ÷ 1,000,000 = 10 MB**
In binary units:
**10,000,000 ÷ 1,048,576 ≈ 9.54 MiB**
Same file.
Different units.
Why a "1 TB" drive looks smaller
Suppose a manufacturer sells a drive with:
**1,000,000,000,000 bytes**
That is exactly:
**1 TB**
using decimal units.
But if software reports the capacity using tebibytes:
**1,000,000,000,000 ÷ 1,099,511,627,776 ≈ 0.91 TiB**
Nothing disappeared.
The number simply changed because the unit changed.
Formatting, partitioning, filesystem overhead, and reserved space can create additional differences in the amount of storage reported as usable.
MB/s vs Mbps
This is another very common source of confusion.
**MB/s** means megabytes per second.
**Mbps** means megabits per second.
There are eight bits in a byte, so:
**1 MB/s = 8 Mb/s**
ignoring the separate question of whether the units are decimal or binary.
Suppose your connection is advertised at:
**100 Mbps**
That is approximately:
**12.5 MB/s**
before accounting for protocol overhead and other real-world factors.
So a download speed of 100 Mbps does not mean the file-transfer meter should show 100 MB/s.
When should you use MB and when should you use MiB?
A practical rule:
Use **MB/GB/TB** when working with decimal quantities such as manufacturer storage capacities and many network specifications.
Use **MiB/GiB/TiB** when you want to be explicit about binary multiples.
The most important thing is consistency.
If you are comparing two measurements, make sure they use the same unit system first.
Why software sometimes still says "MB"
Many applications continue to display "MB" because users recognize the label.
That does not necessarily mean the underlying calculation follows the modern SI definition.
When precision matters, look for documentation describing the unit definition.
A professional data-processing workflow should not rely on the abbreviation alone if the exact quantity matters.
File size versus transfer size
A file can be 500 MB while a network connection might be advertised as 500 Mbps.
Those are different measurements.
The first describes the amount of data.
The second describes a rate of transmission.
If you download a 500 MB file over a 100 Mbps connection, the theoretical minimum transfer time based only on the headline rate is:
**500 MB × 8 = 4,000 megabits**
Then:
**4,000 ÷ 100 = 40 seconds**
Real transfers are usually slower because of protocol overhead, server speed, congestion, Wi-Fi conditions, and other factors.
The calculation is still useful as a rough upper-bound estimate.
Storage marketing versus operating-system reporting
This is the situation that causes many "where did my storage go?" questions.
A manufacturer may say:
**1 TB**
using decimal capacity.
The operating system may show a value in a different unit system and also reserve space for:
- Filesystem metadata
- System partitions
- Recovery partitions
- Formatting structures
- Reserved blocks or other platform-specific needs
So don't compare the advertised capacity and displayed free space until you know exactly what each number represents.
A useful conversion rule
When converting a raw byte count, always start with bytes.
For decimal:
MB = bytes ÷ 1,000,000
GB = bytes ÷ 1,000,000,000For binary:
MiB = bytes ÷ 1,048,576
GiB = bytes ÷ 1,073,741,824Starting from bytes avoids ambiguity.
Where PixelToFile fits
PixelToFile's File Size Unit Converter is useful when you need to translate between decimal and binary representations without doing the arithmetic by hand.
It is especially handy when:
- Comparing drive specifications
- Checking upload limits
- Estimating transfer sizes
- Understanding operating-system reports
- Documenting technical requirements
For software documentation, be explicit about the unit.
Instead of saying:
"The maximum file size is 50 MB."
say exactly what your application means by that limit if the distinction matters.
Final takeaway
The easiest way to avoid file-size confusion is to remember four facts:
**MB = 1,000,000 bytes**
**MiB = 1,048,576 bytes**
**MB/s is bytes per second**
**Mbps is bits per second**
Once you make the unit explicit, most of the apparent contradictions disappear.