Data Storage Converter
Convert between bits, bytes, kilobytes, megabytes, gigabytes and terabytes — and finally find out why your 1 TB drive shows as 931 GB.
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Quick reference table
Common file and drive sizes, converted between the two units you've selected.
Why your 1 TB drive shows as 931 GB
This is the single most asked question about data storage, and nobody is lying to you. Two different definitions of "gigabyte" are in use at the same time.
- Drive manufacturers use decimal. 1 GB = 1,000,000,000 bytes. So a 1 TB drive genuinely contains 1,000,000,000,000 bytes, exactly as advertised.
- Windows uses binary but calls it GB. It divides by 1,073,741,824 — that's 1024³ — and still writes "GB" on the screen.
Divide one trillion by 1,073,741,824 and you get 931. The drive is the size it says. Windows is measuring it with a slightly bigger ruler and using the same word.
The gap widens as capacity grows: about 2.4% at kilobytes, 4.9% at megabytes, 7.4% at gigabytes and 9.9% at terabytes. On a 4 TB drive that's nearly 400 GB of apparently missing space.
The two sets of units
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| Kilobyte (KB) | 1,000 | Kibibyte (KiB) | 1,024 |
| Megabyte (MB) | 1,000² | Mebibyte (MiB) | 1,024² |
| Gigabyte (GB) | 1,000³ | Gibibyte (GiB) | 1,024³ |
| Terabyte (TB) | 1,000⁴ | Tebibyte (TiB) | 1,024⁴ |
The binary names — kibibyte, mebibyte, gibibyte — were standardised by the IEC in 1998 specifically to end this confusion. macOS and Linux adopted the decimal definitions for display; Windows still reports binary values labelled with decimal names. That mismatch is the whole problem, and it is very unlikely to be fixed now.
Bits versus bytes: your internet speed
There are 8 bits in a byte, and the difference is why your "100 Mbps" connection downloads at about 12 MB/s rather than 100.
The capital B means bytes, the lowercase b means bits. Internet providers quote the bigger number, which is technically correct and conveniently eight times more impressive.
A sense of scale
- 1 KB — a short email, or about a page of plain text
- 5 MB — a high-quality photo from a phone
- 50 MB — a few minutes of 1080p video
- 5 GB — a two-hour film in HD
- 1 TB — roughly 200,000 phone photos, or 250 HD films
Frequently asked questions
Why does my 1 TB hard drive show up as 931 GB?
Because two different definitions of gigabyte are in play. The manufacturer uses the decimal one, where 1 TB is a trillion bytes — and the drive really does hold that. Windows divides by 1024³ instead but still writes "GB", which gives 931. No space is missing; it's being measured with a different ruler under the same name.
What's the difference between MB and MiB?
A megabyte (MB) is 1,000,000 bytes. A mebibyte (MiB) is 1,048,576 bytes — 1024². The MiB naming was introduced by the IEC in 1998 to end exactly this confusion. Both are in the dropdown above so you can see the gap for yourself.
Why is my 100 Mbps internet only downloading at 12 MB/s?
Because it's working perfectly. Internet speeds are quoted in megabits per second and file sizes are in megabytes, and there are 8 bits in a byte. 100 divided by 8 is 12.5 MB/s, which is exactly what you should expect. The lowercase b means bits; the capital B means bytes.
How many photos fit on 1 TB?
Roughly 200,000 phone photos at about 5 MB each, or around 250 films in HD. Those are rough figures — a RAW photo from a camera can be 25 MB or more, and 4K video eats storage several times faster than 1080p.
Should I use KB or KiB when I write about file sizes?
If precision matters, use the binary names (KiB, MiB, GiB) when you mean powers of 1024, and the decimal ones (KB, MB, GB) when you mean powers of 1000. Most everyday writing uses the decimal names loosely for both, which is exactly how the confusion spread in the first place.
Are my numbers sent anywhere?
No. Everything is worked out inside your own browser. Nothing you type is uploaded, stored or shared, and there's no sign-up.
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Please note: CalcuLane uses the internationally agreed definitions, which are exact for length, weight, temperature, volume, speed and data. Shoe sizing is a convention rather than a measurement and varies between brands, so treat those results as a starting point. Where precision genuinely matters, confirm against an authoritative source.