File Size Calculator

How to use the file size calculator

1. Enter a size and unit.
2. Choose the binary or decimal standard.
3. Read every equivalent, and the transfer times.

About this tool

Both standards are shown because both are in use and the difference is real. A kilobyte is 1,024 bytes in the binary convention that operating systems use, and 1,000 bytes in the decimal convention that storage manufacturers use.

That is the whole explanation for the missing space on a new drive. A drive sold as 1 TB holds 1,000⁴ bytes; Windows divides by 1,024⁴ and still labels the result GB, so it reports about 931. Nothing is wrong and nothing is being withheld — two definitions of the same word are being applied to one number. The unambiguous binary names are KiB, MiB and GiB, though hardly anyone uses them.

Transfer times assume the full advertised speed with no protocol overhead, no congestion and no server limit, so treat them as a floor rather than a forecast. Note also that connection speeds are quoted in bits per second while file sizes are in bytes, a factor of eight that accounts for most surprised arithmetic.

Common uses

• Working out why a new drive shows less space than advertised.
• Converting an upload limit given in MB into bytes.
• Estimating how long a large file will take to send.

Frequently asked questions

Why does my 1 TB drive show 931 GB?

The manufacturer counts 1,000⁴ bytes as a terabyte; your operating system divides by 1,024⁴ and still writes GB. Same number of bytes, two definitions.

Is a kilobyte 1000 or 1024 bytes?

Both conventions are in use. Operating systems generally use 1,024; storage and network vendors use 1,000. KiB unambiguously means 1,024.

Why is my download slower than my connection speed?

Speeds are quoted in bits per second and file sizes in bytes — a factor of eight — before any protocol overhead or server limit is considered.