File sizes, explained

This is a simplified, practical explanation of data file sizes in computing & communications, for anyone who doesn't understand them, but doesn't care for scientific detail.

All examples given are from actual uses, focusing on communications such as broadband internet and mobile phones.

Data "size"

Table of File size units
Unit	How much?	Description
bit	1 binary digit	This is the smallest possible unit; it has value 0 or 1. You will never, ever see a file size listed in bits.
byte	8 bits	eg: 01110001. That's enough bits to count from 0 - 256; and is enough to store an alphabetical letter. ("01110001" is 'q'). When typing, every key-press you make generates one byte.
kilobyte (KB)	1024 bytes about '1000' bytes.	1KB is enough for about 2 paragraphs of plain text. Most file sizes you'll see online are expressed in KB. Most JPG photos are several hundred KB.
megabyte (MB)	1024 kilobytes about 1 million bytes	Enough for a 150 page Word document, or one smallish, uncompressed ('bitmap') image; or more realistically, about 3 medium-sized JPG photos (i.e. compressed images).
gigabyte (GB)	1024 megabytes about 1 billion bytes.	Few singular files are this large. It's enough to store about 1 hour 40 minutes of compressed DVD-quality video.

Transfer speeds

Please note!
Transfer speeds are always given in kilobits / megabits, NOT kilobytes / megabytes.
This means they are 8 times slower than you might assume.

Table of network transfer speeds/caption>
Unit	Description/examples
kilobits per second (kbps)	Maximum dial-up modem speed is 56.6kbps. In practice, you only get 48kbps. So you can download 48 kilobits each second. Dividing by 8 gives the more useful number: 6KB (kilobytes) per second. GPRS has a maximum theoretical speed of 115 kbps. In practice, you only get 30-40kbps. 3G networks vary, but the maximum speed is up to ~2mbps. One 3G standard, called WCDMA provides speeds from 144 kbps to 2 mbps. Modem speed: ~48kbps; (~6KB (kilobytes) per sec.) Low-end aDSL ("broadband"): 512 kbps; 64 KB (kilobytes) per second.) GPRS Phone: ~35kbps (~4.4 KB (kilobytes) per second)
megabits per second (mbps)	This speed range applies to mid-high range DSL connections ("broadband") and Local Area Networks. As before, to get megabytes/second it's necessary to divide by 8, since a megabit is 1/8 of a megabyte. 1Mbit broadband = 1 mbps = 0.125 Megabytes per second (1/8 MB). 2Mbit broadband = 2 mbps = 0.25 Megabytes (1/4 MB). Local Area Network (LAN) speed: usually 100 Mbps = 12.5 MB per sec Wireless LAN (802.11b) has up to 11mbps speed - that's 1.375 MB per second. 'Fast' Wireless LAN (802.11g) can go up to 54mbps - i.e. 6.75MB each second.