Data Size Converter

What this converter does

From a photo measured in megabytes to a drive measured in terabytes, data has its own ladder of units, and famously two rival versions of it. This converter handles both: the decimal units that storage makers print on the box, and the binary units your computer counts in.

How to use it

• Type your data value into the value box.
• Set From to the unit you have, and To to the unit you want.
• Press Calculate.

The result reads back in plain words, like "1 GB is equal to 1000 MB." Use Swap to flip the two units, and Reset to clear.

How it works

Every unit is built up from the bit. A byte is 8 bits, and each larger unit has its own factor, so the tool converts to bits and then out to the unit you want. The one thing to watch is which family you pick, decimal or binary, which the next section explains.

Common data size conversions

The ones that come up most:

Decimal or binary: why a 1 TB drive shows 931 GB

This is the big one, and it explains a mystery almost every computer owner has hit. There are two ways to count data, and they disagree.

The decimal way uses powers of 1,000, like the rest of the metric system: a kilobyte is 1,000 bytes, a megabyte is a million, a gigabyte is a billion. Storage and network makers use this.

The binary way uses powers of 1,024, because computers count in base 2 and 2 to the tenth power happens to be 1,024, close to 1,000. To keep the two apart, the binary units have their own names: a kibibyte (KiB) is 1,024 bytes, a mebibyte (MiB) is 1,024 kibibytes, a gibibyte (GiB) is 1,024 mebibytes. This tool gives you both families side by side.

So why does a new 1 TB drive show up as only 931 GB in Windows? Because the maker sells it as a decimal terabyte, a clean trillion bytes, while Windows measures in binary but still writes the label "GB". A trillion bytes divided by 1,073,741,824 (one binary gigabyte) comes to about 931. The drive is not faulty, the two sides are simply counting differently. The gap grows with size: it is under 2.4% at the kilobyte level but about 10% by the terabyte. macOS sidesteps the whole thing by reporting in decimal, which is why the same drive reads as 1 TB on a Mac.

Data size or data rate: which one you need

Data size and data rate sound alike but answer different questions. Size is how much data there is: the weight of a file, the capacity of a drive, measured in bytes and their multiples. Rate is how fast data moves: a download speed or a connection's bandwidth, measured per second.

A 4 GB film is a size. Downloading it over a 100 Mbps line is a rate. So if your value has a "per second" in it, you want the Data Rate Converter. If it is a fixed amount of stored data, you are in the right place.

Questions people ask

References

1. National Institute of Standards and Technology (NIST). Definitions of the SI units: the binary prefixes (kibi, mebi, gibi). https://physics.nist.gov/cuu/Units/binary.html