Data Size Calculator
Convert bits, bytes, KB, MB, GB, and TB.
About the Data Size Calculator
Convert between bytes, kilobytes, megabytes, gigabytes and terabytes — and understand why your 1 TB drive shows as 931 GB.
How to use it
- Enter the size.
- Choose the unit you have.
- Read the equivalents.
The formula
Data units come in two families that use the same names and different numbers. This is the entire reason a 1 TB drive shows up as 931 GB.
Decimal (SI): 1 kB = 1,000 B · 1 MB = 1,000² · 1 GB = 1,000³ · 1 TB = 1,000⁴
Binary (IEC): 1 KiB = 1,024 B · 1 MiB = 1,024² · 1 GiB = 1,024³ · 1 TiB = 1,024⁴
Drive manufacturers use decimal, because computers count bytes and marketing counts in thousands. Operating systems — Windows especially — report in binary while labelling it 'GB'. Neither is lying; they are using different definitions of the same abbreviation.
The gap widens with size. It is 2.4% at kilobytes and 9.95% at terabytes, which is why nobody noticed in the floppy disk era and everybody notices now.
Worked examples
| Advertised | Actual bytes | Windows shows | Shortfall |
|---|---|---|---|
| 500 GB | 500,000,000,000 | 465.66 GiB | 6.9% |
| 1 TB | 1,000,000,000,000 | 931.32 GiB | 6.9% |
| 2 TB | 2,000,000,000,000 | 1.819 TiB | 9.1% |
| 8 GB RAM | 8,589,934,592 | 8.00 GiB | none — RAM is sold in binary |
The last row is the exception that explains the rule. Memory is genuinely manufactured in powers of two because of how addressing works, so 8 GB of RAM really is 8 GiB and reports as exactly 8. Storage has no such constraint, so it is sold in round decimal numbers. Same abbreviation, two industries, two meanings — and one confused customer per drive.
Common mistakes
- Assuming the drive is faulty or misadvertised. A 1 TB drive really does contain a trillion bytes. Your operating system is dividing by 1,024 three times and still writing 'GB'.
- Confusing bits with bytes. Eight bits make a byte. Network speeds are quoted in bits per second and file sizes in bytes, so a 100 Mbps connection transfers about 12.5 MB per second.
- Expecting the full capacity to be usable. Formatting, the file system, and recovery partitions all consume space beyond the decimal conversion. A few percent more disappears before you store anything.
- Using KB and KiB interchangeably in technical writing. They differ by 2.4%, and the gap compounds at every step up. In storage and networking documentation the distinction is worth keeping.
Terms explained
- Bit
- A single binary digit, 0 or 1. The smallest unit there is.
- Byte
- Eight bits. Historically the space for one character of text.
- Decimal prefix
- kB, MB, GB, TB — powers of 1,000. Used by drive manufacturers and by the SI standard.
- Binary prefix
- KiB, MiB, GiB, TiB — powers of 1,024. Defined by the IEC in 1998 to end exactly this ambiguity.
- Formatted capacity
- What remains after the file system takes its share. Always less than raw capacity.
- Mbps
- Megabits per second, a network speed. Divide by eight for megabytes per second.
Common questions
- Why does my 1 TB drive show as 931 GB?
- Because it holds a trillion bytes, and your operating system divides by 1,024 three times to reach 931.32 — then labels the result GB. The drive is the size advertised; the two are counting differently.
- What is the difference between GB and GiB?
- A GB is 1,000³ bytes and a GiB is 1,024³. The GiB is about 7.4% larger, which is exactly the discrepancy you see on a new drive.
- How many bits are in a byte?
- Eight. This is why a 100 Mbps connection delivers about 12.5 MB per second at best — and less in practice, after protocol overhead.
- Does macOS report differently from Windows?
- Yes. Since 2009 macOS reports storage in decimal units, so a 1 TB drive shows as roughly 1 TB. Windows still uses binary maths with decimal labels.
- Am I being cheated by drive manufacturers?
- No. They use the standard SI meaning of the prefixes, where a terabyte is a trillion bytes. The ambiguity comes from operating systems reusing the same abbreviations for binary units.
- Why do computers use powers of 1,024 at all?
- Because memory addressing is binary, and 1,024 is 2¹⁰ — conveniently close to 1,000 and exactly aligned with how hardware works.
- How much space does formatting take?
- Typically 1% to 3%, depending on the file system and cluster size. That is on top of the decimal-to-binary difference, not instead of it.
- What comes after terabytes?
- Petabytes (1,000 TB), then exabytes, zettabytes and yottabytes, each a thousand times the last — with PiB, EiB and so on as the binary equivalents.