What do file sizes mean, and is a megabyte big?

Most people manage without knowing this until a moment when it matters: an email bounces, a drive fills, a phone plan is exhausted halfway through the month. A rough sense of scale settles all of those, and it does not require any technical knowledge — only knowing roughly what things weigh.

Step by step

1. Learn the ladder, roughly.A kilobyte is about a thousand bytes, a megabyte about a thousand kilobytes, a gigabyte about a thousand megabytes, and a terabyte about a thousand gigabytes. Each step is a big jump, which is why mixing them up produces such wrong answers.
2. Attach real things to each step.A page of plain text is a few kilobytes. A phone photo is two to five megabytes. A song is three to ten megabytes. An hour of streamed video is one to three gigabytes at ordinary quality and considerably more at high definition. A modern laptop drive holds a few hundred gigabytes to a terabyte or two.
3. Keep bits and bytes apart.Broadband speed is sold in megabits per second; file sizes are in megabytes. There are eight bits in a byte, so a hundred-megabit connection downloads at roughly twelve and a half megabytes a second at best. That factor of eight is behind most surprise about download times.
4. Expect a new drive to look smaller than advertised.Manufacturers count a gigabyte as a round thousand million bytes; operating systems often count in units of 1,024. The result is that a drive sold as one terabyte shows as roughly 931 gigabytes. Nothing is missing, and there is no fault to report.
5. Use the numbers for the decisions they actually settle.Will this attachment send by email — most services stop somewhere around twenty to twenty-five megabytes. Will the holiday photos fit on the stick. How much of a mobile data allowance an evening of video will consume. Those are the practical uses.
6. Know why the same content varies so much in size.Compression. A photo saved as a compressed image is a fraction of the raw version; the same video at higher quality or a higher frame rate can be several times larger. Two files of the same content and wildly different sizes are usually a quality difference.
7. Watch for the file types that surprise people.Video is far larger than anything else in a household, by an order of magnitude. Scanned documents saved as images are much larger than the same document as text. Presentations balloon because they contain full-resolution photographs nobody shrank.
8. Check where the space has gone before buying more.Both operating systems and phones have a storage breakdown showing what is using space by category. It almost always identifies one folder — usually video, downloads or a sync cache — rather than a general accumulation.

Tips

• If a file will not send by email, that is a size limit rather than a fault. A shared link, or compressing the file, solves it and both are quick.
• Photos taken in a high-efficiency format are roughly half the size of the older one for the same quality. Worth checking your phone's camera format if storage is tight.
• A mobile data allowance is spent almost entirely on video. Lowering the streaming quality setting on one or two apps usually does more than any other change.

Common mistakes

• Confusing megabits with megabytes — A factor of eight. It is why a fast connection appears to download slowly, and it is the single most common misunderstanding about broadband speed.
• Assuming a smaller reported drive size means a fault — It is a difference in how the units are counted, and every drive from every manufacturer does it. There is nothing to claim and nothing to fix.

If it doesn't work

Questions people ask