KB, MB, GB and GiB Explained: Why Your Drive Looks Smaller

A "1 TB" drive shows up as about 931 GB on Windows because two different systems of units share similar names. Drive makers use decimal units, where 1 GB = 1,000,000,000 bytes. Windows counts in binary units, where 1 GiB = 1,073,741,824 bytes, but labels them "GB". No space is missing; it is the same number of bytes divided by a bigger unit. You can check any conversion with the Storage category of ToolMany's Unit Converter.

The two systems side by side

The gap grows with each step: about 2.4% at the kilo level, 4.9% at mega, 7.4% at giga and 10% at tera. That is why the difference is barely noticeable for a photo but very noticeable for a hard drive.

Who uses which?

So the same file can show as 4.9 MB on a Mac and 4.7 MB on Windows. Neither is wrong.

Worked examples

• "1 TB" drive on Windows: 1,000,000,000,000 bytes ÷ 1,073,741,824 = 931.3 GiB, shown as "931 GB".
• "500 GB" drive: 500,000,000,000 ÷ 1,073,741,824 = 465.7 GiB.
• "64 GB" phone storage: 64 × 10⁹ bytes = 59.6 GiB before the operating system takes its share.
• A 1 GiB file in decimal megabytes: 1073.741824 MB. Our tests check this exact value in the Unit Converter.

Why it matters for upload limits

Upload forms say things like "maximum 2 MB". Which megabyte do they mean? Usually you cannot tell, so:

• assume the smaller one: 2 MB = 2,000,000 bytes, and stay a little under it;
• a file that Windows reports as "1.95 MB" (binary) is about 2.04 million bytes and could be rejected by a form that counts decimal megabytes;
• aim for about 5% under the limit to be safe.

If a photo or PDF is just over a limit, the Image Compressor and our guide on getting a PDF under an upload limit help you get it down.

Bits vs bytes: the other trap

Internet speeds are measured in bits per second (Mbps), while file sizes are in bytes (MB). One byte is 8 bits.

• A 100 Mbps connection moves at most about 12.5 MB per second.
• A 1 GB download on that connection takes at least about 80 seconds, before overheads.

Watch the capitalisation: Mb is megabits, MB is megabytes.

Converting with ToolMany

1. Open the Unit Converter and choose Storage.
2. Pick the units, such as GB to GiB.
3. Type the number. The result appears immediately, to 10 significant digits.

The converter labels the units carefully: KB, MB, GB and TB are 1000-based; KiB, MiB and GiB are 1024-based.

Where the rest of "missing" space goes

Even after the unit difference, a new device shows less free space than its capacity because:

• the operating system and preinstalled apps take space;
• formatting creates file-system structures;
• some space is reserved for recovery partitions or system updates.

On a phone, these can take 10 GB or more.

How big are everyday files?

Rough sizes help when you are deciding what fits in an email, an upload form or a storage plan. These ranges vary a lot with content and settings:

Two practical consequences follow. First, photos and video, not documents, fill phones and cloud storage. Second, a document PDF that is several megabytes almost always contains images; see how to get a PDF under an upload limit.

Email attachment limits use the same units

Most email providers cap attachments at around 20–25 MB per message, and the attachment is encoded in a way that makes it roughly a third larger in transit. A file that is 19 MB on your disk can therefore fail against a 25 MB limit. When in doubt, share large files through a cloud storage link instead.

Choosing a storage plan or a USB stick

• Phones: count the operating system. A 128 GB phone usually leaves somewhat more than 100 GB for your own files.
• Cloud storage: plans are quoted in decimal GB or TB, like drives.
• USB sticks and memory cards: expect about 7% less in Windows than the label for GB-sized cards, and about 9% less for TB-sized drives.
• Backups: buy at least twice the space you currently use. Files only grow.