How big is a GB? A plain-English guide to file sizes

A gigabyte (GB) is 1,000 megabytes, or one billion bytes, in the decimal system used by storage manufacturers, Apple devices and internet providers. Windows still counts in powers of two, so it treats a gigabyte as 1,024 megabytes (1,073,741,824 bytes), which is why the same file or drive can show two different sizes depending on where you look.

That small difference explains a lot of everyday confusion: the 1 TB drive that shows 931 GB, the 100 Mbps connection that downloads at 12 MB per second, the 25 MB email limit that rejects a 20 MB file. This guide sorts it out, with conversion tables, real-world examples and the steps to check the size of any file on your computer or phone.

The units, from byte to terabyte

Everything starts with the byte, enough to store one character of plain English text. Each step up multiplies by 1,000 in the decimal (SI) system. The binary system multiplies by 1,024 and, strictly speaking, uses different names (kibibyte, mebibyte, gibibyte, tebibyte), approved by the International Electrotechnical Commission in 1998. As the US National Institute of Standards and Technology points out, the SI prefixes kilo, mega and giga strictly mean powers of ten. In practice almost nobody says “gibibyte”, which is where the mix-up comes from.

How many MB in a GB, and how many GB in a TB?

• 1 GB = 1,000 MB in decimal, the convention used on product boxes, by macOS and iOS, and by most online services.
• 1 GB = 1,024 MB in Windows File Explorer, which counts in binary but labels the result “GB”.
• 1 TB = 1,000 GB in decimal, or 1,024 GB in binary.
• 1 MB = 1,000 KB in decimal, or 1,024 KB in binary.

The gap grows with each step: about 2.4% at the kilobyte level, 4.9% for megabytes, 7.4% for gigabytes and almost 10% for terabytes. For a single photo it is irrelevant; for a 4 TB backup drive it is several hundred “missing” gigabytes.

Why does my 1 TB drive only show 931 GB?

Nothing is missing. The manufacturer sells 1 TB as 1,000,000,000,000 bytes. Windows divides that number by 1,024 three times and gets 931.3, which it labels “GB”. A Mac shows the same drive as 1 TB because Apple’s systems count in decimal; according to Apple, only OS X Leopard and earlier, and iOS 10 and earlier, reported storage in binary. Formatting then reserves a little space for the file system, and on phones and computers the operating system takes its share too, which is why a 128 GB phone never shows 128 GB free.

What fits in 1 GB?

It depends entirely on the type of file and how much it is compressed. The figures below are orders of magnitude, computed from typical file sizes and bitrates, to help you estimate quickly.

Video is what makes files big. A video’s size is simply its bitrate × duration, so the same 10-minute clip can weigh 300 MB or 6 GB depending on resolution, frame rate and codec. Our guide on sending video without losing quality explains how to deliver it intact.

Bits vs bytes: Mbps is not MB/s

Internet speeds are sold in megabits per second (Mbps, lowercase b), while files are measured in megabytes (MB, capital B). One byte is eight bits, so divide a connection speed by eight to know how many megabytes per second it can move.

These are best cases. When you send files, what matters is your upload speed, often much lower than the download speed in your contract. Our article on how long it takes to upload large files gives the full table, and why is my upload so slow? covers the usual culprits.

When is a file “too big” to send?

Size only matters relative to the channel you use. The same 30 MB PDF is too big for most mailboxes and tiny for a file transfer service. As a rule of thumb:

• Email: most providers stop at 20–25 MB per message, attachments included; see the email attachment size limits by provider.
• Messaging apps: limits vary from a few megabytes to a couple of gigabytes, and photos and videos are often recompressed; see our WhatsApp and Discord guides.
• Transfer services: free plans range from a few gigabytes to 100 GB per transfer on Transfr; for bigger sends, see how to send a 100 GB file.

Why a file can “grow” when you send it

Two things change the size you see along the way. First, email encoding: attachments are converted to text with Base64, which turns every 3 bytes into 4 characters, so a file grows by about a third inside the message. A 20 MB attachment becomes roughly 27 MB of email, which servers that count the encoded size, such as Microsoft’s, treat as over a 25 MB limit. Transfer links do not have this problem, because the file travels as-is.

Second, compression. Zipping a folder of documents can make it much smaller, but zipping photos, videos or MP3s barely changes anything, because those formats are already compressed. Our guide Should you zip files before sending? has the details, and the page on sending a folder without zipping shows the alternative.

How to check the size of a file

On Windows

1. 1

   Step 1

   Right-click the file or folder

   In File Explorer, right-click and choose Properties.

2. 2

   Step 2

   Read “Size”

   Size is the actual data. “Size on disk” is the space it occupies, rounded up to the drive’s allocation units, so it can be a little bigger.

On a Mac

1. 1

   Step 1

   Select the item in Finder

   Click the file or folder once.

2. 2

   Step 2

   Open Get Info

   Press Command-I (or right-click and choose Get Info). The size appears in decimal units, with the exact number of bytes in brackets.

On iPhone and Android

In the iPhone Files app, touch and hold a file and choose Get Info. In the Photos app, swipe up on a photo or video to see its details. On Android, open the file in Files by Google or your file manager and look at its details, or swipe up on a photo in Google Photos. If you need to send something heavy from your phone, our guide to sending large files from iPhone covers each option.

Quick reference

• 1 KB = 1,000 bytes · 1 MB = 1,000 KB · 1 GB = 1,000 MB · 1 TB = 1,000 GB (decimal).
• Windows shows binary values with decimal names: divide the advertised capacity by about 1.074 for GB, 1.1 for TB.
• Mbps ÷ 8 = MB/s. A 100 Mbps line moves about 12.5 MB per second at best.
• Video size = bitrate × duration. One minute at 8 Mbps is about 60 MB.