Computers & Laptops

Usable Storage Capacity Calculator

Why a 1 TB drive shows as 931 GB, and roughly what is left after formatting and a recovery partition.

4 inputs Free, no sign-up

Your figures

A recovery partition, a preinstalled operating system, or anything else already occupied. In decimal GB. Leave blank if there is none.

Optional. A rough percentage for what the filesystem itself takes. Leave blank to skip it - the honest figure depends on the filesystem and how it is configured.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

Nothing is missing from your new drive. It holds exactly the bytes on the box; your operating system is counting them in binary units and labelling them with decimal names.

A 1 TB drive is 1,000,000,000,000 bytes, which is 931.32 GiB — and Windows calls that "931 GB". Anything set aside for recovery or lost to the filesystem comes off on top.

How to use this calculator

  1. Enter the capacity printed on the drive.
  2. Add anything already occupied — a recovery partition, a preinstalled system.
  3. Add a rough filesystem overhead percentage if you want one. Leave it blank if you do not, since the honest figure depends on the filesystem.

The formula

Advertised bytes = Capacity x 1,000,000,000 (decimal)
Reported figure  = Advertised bytes / 1,073,741,824 (binary GiB)
Usable           = Advertised - reserved - overhead

The first two lines are exact arithmetic. The third depends on figures you supply.

SymbolMeaningUnit
Advertised The figure on the box TB or GB
Reserved Already occupied GB
Overhead Filesystem share, roughly %

Worked example

A 1 TB drive with 20 GB of recovery partition and 2% filesystem overhead:

Advertised    = 1,000 GB = 931.32 GiB
After recovery = 980 GB
After overhead = 960.4 GB = 894.44 GiB

Notes and limits

  • The decimal-to-binary difference is arithmetic and exact. The filesystem overhead is not: it depends on whether you are using NTFS, exFAT, ext4 or APFS, on the allocation unit size, and on how many files you store.
  • macOS has reported decimal gigabytes since 10.6, so the same drive shows about 1,000 GB there and 931 GB on Windows. Neither is wrong.
  • SSDs also hold back some capacity for over-provisioning, which never appears to the operating system at all and is part of why the drive lasts.
  • Leave 10–20% free on an SSD. Filesystems slow down when nearly full and wear levelling needs free blocks.

What this calculation assumes

  • The decimal-to-binary conversion is exact arithmetic.
  • The reserved space and overhead are figures you entered; no filesystem is assumed.
  • The usable figure is an estimate and the page says so.

Frequently asked questions

Why is my new drive smaller than advertised?

It is not. The drive holds the bytes it says; your operating system divides by 1,024 rather than 1,000 and keeps the decimal name. A 1 TB drive is 931.32 GiB, which Windows displays as 931 GB.

How much space does formatting take?

It depends on the filesystem and its settings, which is why this page asks rather than assuming. On a large modern drive the filesystem structures themselves are a small percentage; the bigger effects are small-file waste and any recovery partition.

Is this the same on Windows and Mac?

No. macOS reports decimal gigabytes and Windows reports binary ones under decimal names, so the same drive reads about 7-10% larger on a Mac.

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