File Size Converter

A file size means nothing until you compare it with something: the limit on an email attachment, the space left on a USB stick, the size of the drive you are about to buy. Our file size converter does that comparison properly. Type a size in any unit — bits, bytes, KB, MB, GB, TB or PB — and you see it in every other unit at once, in both counting systems: decimal (KB, MB, GB) and binary (KiB, MiB, GiB). That second column is the one most converters hide, and it is the reason your numbers never quite seemed to match.

File sizes and internet speeds are close cousins, and confusing the two is common. If you have not measured your connection yet, the free speed test on the Speed Test UK homepage gives you the Mbps figure you can later set against the file sizes you convert here.

How to Use This File Size Converter

• Step 1: Type your size. Enter the number you have — the one on a download page, in an email warning, or in your file manager.
• Step 2: Pick its unit. Choose the unit that came with the number. If a website says 25 MB, choose MB; if Windows says 25 MB, it is really measuring MiB, so choose MiB for an exact answer.
• Step 3: Read both tables. The blue table is decimal (powers of 1,000), the grey table is binary (powers of 1,024). The same bytes, counted two ways.
• Step 4: Copy it if you need it. The copy button puts the whole conversion into your clipboard, handy for quoting a size in an email, a listing or a support ticket.

Why There Are Two Ways to Count: 1,000 vs 1,024

Here is the oddity at the heart of every file size argument. For most measurements, kilo means 1,000: a kilometre is 1,000 metres. Computers, though, work in powers of two, and 2¹⁰ is 1,024 — close to 1,000, but not the same. For decades the industry used “kilobyte” for both, and at small sizes the 2.4% gap barely mattered.

It matters now, because the gap grows with the unit. At megabyte size it is about 4.9%, at gigabyte size about 7.4%, and at terabyte size about 10%. To end the confusion, the International Electrotechnical Commission standardised separate binary names — kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB) — in the late 1990s; the US National Institute of Standards and Technology (NIST) publishes the full prefix table if you want the authoritative version. The naming fix worked on paper. In everyday life, the two systems still run side by side:

• Hard drives, SSDs and USB sticks are sold in decimal: a 1 TB drive really does hold 1,000,000,000,000 bytes.
• Windows counts in binary but labels the results KB, MB and GB — so that same drive appears as about 931 “GB”.
• macOS and iOS count in decimal, so Apple devices show the full number on the box.
• RAM is genuinely binary: 16 GB of memory is 16 GiB of chips.
• Downloads and cloud storage are usually decimal, though individual services vary — which is why this converter shows you both.

Neither system is wrong. They are two rulers measuring the same thing, and this page exists so you never have to guess which ruler a number was measured with.

The Units, From Smallest to Largest

A bit is a single 0 or 1 — the smallest possible piece of data. Eight bits make one byte, enough to store one character of text. File sizes are almost always quoted in bytes and above, but bits sneak in via internet speeds, which is why the converter includes them. From there, the ladder climbs:

The decimal prefixes themselves — kilo, mega, giga, tera, peta — are the standard SI prefixes maintained by the International Bureau of Weights and Measures (BIPM), the same body behind the metre and the kilogram. When a drive maker says GB, they are using the prefix exactly as the BIPM defines it.

Worked Examples You Will Actually Meet

“My 1 TB drive only shows 931 GB”

The most common conversion puzzle in Britain. Your drive holds 1,000,000,000,000 bytes — a true decimal terabyte. Windows divides by 1,024 three times instead of 1,000, and reports 931.3 GiB while calling it GB. Convert 1 TB above and you will see both figures: nothing is missing, and no, the shop did not short-change you. A small amount of space also goes on formatting, but the counting system explains nearly all of the “loss”.

“This email attachment is too large”

Gmail and Outlook.com both cap attachments at 25 MB. A folder of phone photos at roughly 4 MB each means about six photos per email — and if your mail app counts in MiB, your real headroom is a touch less than the label suggests. Convert your file above, and if it will not fit, the practical fix is a sharing link from cloud storage rather than splitting the photos across three emails.

“Will this game fit on my console?”

A modern game at 100 GB (decimal, as the store lists it) needs 100,000,000,000 bytes free. Your console reporting free space in binary will want to show at least 93.1 GiB free. Run the conversion before you start a download the size of a small hard drive, and you will not meet the “not enough space” message at 98%.

“How many photos fit on this memory card?”

A 64 GB card holds 64,000,000,000 bytes. At an average 4 MB per photo, that is roughly 16,000 photos in decimal terms — but your camera may report capacity and file sizes in binary, shaving the real count to nearer 14,900. The honest answer is “somewhere between the two”, and now you know why estimates from different devices never agree exactly.

Quick Reference: Everyday File Sizes

The converter above includes a reference table, but these are the sizes worth remembering. A single page of plain text is about 2 KB. A smartphone photo is around 4 MB, an MP3 song around 7 MB. An HD film download is roughly 4.5 GB, a 4K film around 20 GB, and a big modern game 80–120 GB. If a size on your screen looks wildly different from these anchors — a “photo” claiming to be 2 GB — something has gone wrong, and the number is worth a second look before you send or store it.

Why the Same Photo Can Be 2 MB or 25 MB

Conversion tells you what a size means; it does not tell you why the size is what it is. The biggest factor is format. A camera saving a JPEG compresses the image heavily, throwing away detail your eye is unlikely to miss, and produces a file of perhaps 4 MB. The same shot saved as a RAW file keeps everything the sensor captured — 25 MB or more — which is why photographers fill cards so quickly and everyone else never sees a RAW file at all. A PNG screenshot sits between the two: lossless, but often larger than a JPEG of the same picture.

Video is steeper still, because size follows bitrate multiplied by time. A minute of phone video at a typical bitrate lands around 100–200 MB; an hour of streamed HD video uses roughly 3 GB; 4K pushes that toward 7 GB an hour. This is where size conversions stop being trivia: they are how you work out whether a download fits your remaining storage, whether a file will squeeze through an attachment limit, and how much of a monthly data allowance a single film night actually eats.

Mistakes People Make When Converting File Sizes

• Mixing up MB and Mb. Capital B is bytes, small b is bits. “100 MB” and “100 Mb” differ eightfold, and the small-b version usually appears in speeds, not sizes.
• Assuming 1 GB is always 1,024 MB. It is 1,000 MB in decimal and 1,024 MiB in binary. Quoting one against a limit set in the other is how files get rejected by upload forms.
• Comparing numbers from two devices directly. A file that is 500 MB on a Mac can show as 477 “MB” on a Windows PC. It has not shrunk — Windows divided by 1,024 twice and kept the MB label. Convert both to bytes to settle any argument.
• Forgetting formatting and system files. Even in the same counting system, a new drive shows slightly less than its raw capacity because a slice is reserved for the filesystem itself. That is normal and usually only a fraction of a percent.
• Rounding too early. Converting GB to MB to KB in steps and rounding at each stage drifts the answer. This converter works from bytes every time, so the figures stay exact.

Converting Speeds Is a Different Job

This page converts sizes: how much data something is. The moment a size gains a “/s”, it becomes a speed, and that is a different conversion with its own trap (bits versus bytes, a factor of eight). For speeds, use our internet speed converter, which handles Kbps, Mbps, Gbps and MB/s together. If you only ever convert the one pair that causes most confusion, our Mbps to MB/s converter keeps it deliberately simple. And when you want to know how long a file of a given size will take on your connection, the download time calculator does the whole sum in one go.

Frequently Asked Questions

How many MB are in a GB?

It depends which GB. In decimal, 1 GB is exactly 1,000 MB. In binary, 1 GiB is 1,024 MiB. Most of the time people mean the decimal version, but Windows measures in the binary one while using the same labels — type 1 into the converter above, choose GB, and read both answers side by side.

Why does my hard drive show less space than the box says?

Because the box counts in decimal and your computer probably counts in binary. A 1 TB drive holds a trillion bytes; Windows divides that by 1,024 three times and displays 931 “GB”. The space is all there. The UK consumer group Which? has covered the related question of how much storage people genuinely need, and the counting gap is part of why advertised and usable figures differ on every device.

Is a KB 1,000 or 1,024 bytes?

Both, depending on who is speaking — which is precisely the problem the IEC names were created to solve. Strictly, KB means 1,000 bytes and KiB means 1,024. In casual use, especially in Windows, “KB” can mean either. When the difference matters (comparing against a strict upload limit, for instance), convert using the binary column to be safe.

How many bytes are in a bit?

It runs the other way: there are 8 bits in a byte, so one bit is 0.125 bytes. Bits matter here mainly because internet speeds are quoted in bits per second, while files are measured in bytes — the factor of eight between them is why a 100 Mbps connection downloads at 12.5 MB/s.

What is the difference between MB and MiB?

An MB is 1,000,000 bytes; a MiB is 1,048,576 bytes — about 4.9% more. The gap grows with size: GB vs GiB is about 7.4%, TB vs TiB about 10%. That growing gap is why the discrepancy is invisible for a text file but very visible on a terabyte drive.

How big is a petabyte?

One petabyte is 1,000 terabytes — a quadrillion bytes. You will not meet one at home: a PB is data-centre territory, the scale of large cloud backups or a streaming service’s catalogue. This converter includes PB and PiB so figures quoted from those contexts can be brought down to earth in GB or TB.

Convert the Size, Then Put It to Work

Now any file size you meet — on a download page, in an error message, on the side of a box — can be translated into units you can compare. Convert the number above, and if the file is one you plan to download, check it against your real connection speed on the Speed Test UK homepage so you know roughly how long you will be waiting.