Why Is a 4TB Drive Only 3.63 TB? | The Space Isn’t Missing

A 4TB drive shows about 3.63TB in Windows because manufacturers advertise capacity in decimal units while operating systems display binary units.

You plug in a brand-new 4TB hard drive and Windows reports 3.63TB. The immediate reaction is that something is wrong, but nothing is defective. The gap comes down to how storage capacity gets measured, and it affects every drive sold. The “missing” space is a labeling difference, not a hardware fault.

Why Does Windows Show 3.63TB Instead of 4TB?

Seagate’s own capacity documentation explains the discrepancy. Manufacturers label drives using decimal units, where 1 kilobyte equals 1,000 bytes. A 4TB drive is marketed as 4,000,000,000,000 bytes on the label. Operating systems like Windows display storage using binary units, where 1 kilobyte equals 1,024 bytes.

When you divide 4,000,000,000,000 bytes by 1,024 three times, you land on approximately 3.63TB. The drive contains exactly the amount of storage promised; the number on the label and the number in the OS are just measured in different languages. Seagate’s support documentation lists this exact conversion in a table, showing a 4TB drive, labeled as 4000GB, displaying as 3.63TB in Windows.

Is Any Space Actually Lost to Formatting?

Yes, but formatting is a separate factor from the unit conversion. After the decimal-to-binary conversion, a small additional amount of space goes to the file system structure. Formatting creates metadata, file tables, and reserved areas that track where data lives. This overhead is necessary for the drive to function, but it makes the usable capacity slightly lower than the raw converted figure.

The amount varies by file system. NTFS, the default for Windows, uses a modest portion of the drive for its structures. The combined result is that the “missing” space comes from two sources: the unit mismatch that accounts for the bulk of the difference, and the file system overhead that accounts for a small additional slice.

Compatibility Caveat: When a Drive Caps at 2TB

If a 4TB drive shows roughly 2TB instead of 3.63TB, that is a separate issue entirely. This cap usually points to a partition style or platform limit, not a unit conversion. Older master boot record (MBR) partitions cannot address more than 2TB of space on a single volume. The GUID Partition Table (GPT) format removes this limit and is required for full capacity on larger drives.

On Windows, the disk management tools can convert an empty MBR disk to GPT. macOS also uses GPT by default for its boot drives. If your drive appears capped near 2TB, check the partition style—the fix is changing the partition table, not hunting for missing gigabytes.

How to Confirm Your Drive’s Real Capacity

If you want to verify that your drive is healthy and operating within expected parameters, the process is straightforward. On Windows, open disk management and check the raw capacity reported for the physical disk. On macOS, check the drive information in System Report or Disk Utility.

Windows: Right-click the Start button and select Disk Management. Find your 4TB drive and look at the unallocated space, which should read close to 3.63TB.

macOS: Click the Apple menu, select About This Mac, then System Report. Under Storage or USB, find your drive and read the raw capacity value.

A reading near 3.63TB means your drive matches the manufacturer’s specification. The drive is not faulty, and the space is not lost—it was never there in the binary measurement you were reading. When you’re ready to put your new drive to work, our roundup of the best 4TB NAS drive options covers tested models for network storage.

The same physical drive shows different numbers on the two platforms, which is the clearest proof that the capacity itself is unchanged. Nothing is missing; the measurement system just changed.