Why Does My Hard Drive Show Less Space Than Advertised?

TB vs TiB: the decades-old byte counting confusion | Updated 2026

Your 16TB Seagate Exos showing 14.5TB in Windows isn't defective. Your 20TB WD Ultrastar showing 18.2TB isn't a scam. It's a decades-old clash between how drive manufacturers and operating systems count bytes.

The Short Answer

Drive manufacturers use decimal (base-10) units where 1 TB = 1,000,000,000,000 bytes.

Operating systems use binary (base-2) units where 1 TiB = 1,099,511,627,776 bytes.

Your drive has exactly as many bytes as advertised. Windows and macOS just display it using a different counting system.

TB vs TiB: The Full Explanation

This confusion exists because computers work in binary (powers of 2), but humans prefer decimal (powers of 10).

The "i" in KiB, MiB, GiB, and TiB stands for "binary" (kibibyte, mebibyte, gibibyte, tebibyte). These are the IEC standard names introduced in 1998 to clear up this confusion.

Unfortunately, Windows still labels binary values as "GB" and "TB" instead of "GiB" and "TiB", perpetuating the confusion.

How Much Space Will Your Drive Actually Show?

Here's what common drive sizes display in your operating system:

The "missing" percentage gets worse as drives get larger because the gap between 1000 and 1024 compounds at each unit level.

Can I Actually Store 6TB of Data on My 6TB Drive?

Think of it like measuring distance. A 100-kilometre drive is also 62.14 miles. You don't lose any road just because someone measured it differently.

Why the Mismatch?

Drive manufacturers use SI decimal units (1 TB = 1,000,000,000,000 bytes) because that's the international standard and matches how storage hardware is physically measured. Operating systems like Windows use binary units (1 TiB = 1,099,511,627,776 bytes) because memory and file systems are organized in powers of 2. macOS switched to decimal display in 2009, so a 1 TB drive shows as "1 TB" in Finder — but Windows still uses binary and mislabels it as "TB" instead of "TiB".

What About SSDs?

SSDs work exactly the same way. A 1TB SSD has 1,000,000,000,000 bytes and shows as 931 GB in Windows.

However, some SSD manufacturers slightly overstate capacity because SSDs reserve space for wear levelling and over-provisioning. A "1TB" SSD might have 1,024,000,000,000 bytes to account for this, but you'll only have access to 1TB after the reserved space is accounted for.

What About Formatted Capacity?

After formatting, you'll have slightly less space than even the binary calculation suggests. File systems need some space for:

• File system metadata — NTFS, ext4, and APFS all store organizational data
• Partition tables — GPT and MBR take a tiny amount
• Reserved blocks — ext4 reserves 5% by default (configurable)

This overhead is typically 0.1-1% on modern file systems, much smaller than the TB/TiB difference.