File Size Units Explained: Bytes, KB, MB, GB & TB Guide (2026)
Why does a "1 TB" drive show 931 GB? Why does the same photo look like 4 MB in one place and 4.2 in another? The answer is that there are two different ways to count data size. This guide explains both in plain English and shows you how to convert any value instantly with our free file size converter.
The building block: the byte
Everything digital is measured in bytes. One byte is 8 bits and can store a single character of text. From there, sizes scale up in big jumps: kilobytes, megabytes, gigabytes and terabytes. The confusion only starts when we ask how big each jump is.
Decimal vs binary: the two standards
The decimal (SI) standard scales by 1000: 1 KB = 1000 bytes, 1 MB = 1000 KB, and so on. This is what storage manufacturers print on the box. The binary (IEC) standard scales by 1024 and uses the names KiB, MiB, GiB and TiB: 1 KiB = 1024 bytes. This is what most operating systems use internally, even when they still display the label "MB" or "GB".
| Unit | Decimal (SI) | Binary (IEC) |
|---|---|---|
| 1 KB / KiB | 1,000 bytes | 1,024 bytes |
| 1 MB / MiB | 1,000,000 bytes | 1,048,576 bytes |
| 1 GB / GiB | 1,000,000,000 bytes | 1,073,741,824 bytes |
| 1 TB / TiB | 1,000,000,000,000 bytes | 1,099,511,627,776 bytes |
Why a 1 TB drive shows less space
A drive marketed as 1 TB holds 1,000,000,000,000 decimal bytes. Your operating system divides that by 1024 three times to display gigabytes, which gives roughly 931 GiB. Nothing is missing and nothing is broken — the manufacturer used the decimal unit and the OS used the binary unit. The converter shows both at once so you can reconcile the numbers in seconds.
How to convert a file size
- Open the File Size Converter.
- Type the value and choose the source unit (for example MB).
- Read the result in every other unit instantly — bytes, KB, MB, GB and TB.
- Compare decimal and binary results side by side to match your context.
Practical tips
- Quote storage and bandwidth in decimal units to match how vendors bill you.
- Quote RAM and OS-reported disk usage in binary units, since that is how they are measured.
- When in doubt, state the exact byte count — it removes all ambiguity.
- Shrinking files? Pair this with our Image Compressor.
Real-world examples
Putting the units to work makes them click. A typical high-resolution photo from a modern phone is about 4 MB, so a 1 GB album holds roughly 250 photos. A standard MP3 song runs around 5 MB, meaning a 1 GB drive stores about 200 tracks. A two-hour high-definition movie is often 4–8 GB, so a 1 TB drive comfortably holds well over a hundred of them. Knowing the conversions turns abstract numbers into decisions you can actually make about storage and uploads.
The units matter for cost, too. Cloud storage and bandwidth are almost always billed in decimal gigabytes and terabytes. If you estimate your usage in binary units by mistake, you can over- or under-budget by around 7% at the gigabyte level and nearly 10% at the terabyte level. For invoices and capacity planning, always convert to the same standard your provider uses before you do the math.
Where you meet these units every day
File size units show up far beyond drives. Email providers cap attachments in megabytes, so knowing that a 25 MB limit means roughly six high-resolution photos helps you decide what to compress before sending. Mobile data plans are sold in gigabytes, and streaming a single hour of high-definition video can burn through 1–3 GB, which is why an unmetered plan matters for heavy viewers. Web performance budgets are measured in kilobytes, where shaving a few hundred KB off a page can noticeably speed up load time on slow connections. Once the units are second nature, these everyday limits stop being mysterious and become simple, confident decisions.
A quick mental shortcut
You do not need to memorise the tables. Remember two anchors: each step up multiplies by about 1000 (decimal) or 1024 (binary), and the gap between the two standards grows with every step — small at kilobytes, noticeable at terabytes. When precision matters, skip the mental math entirely and let the converter show every unit at once.