---
id: mac-storage-basics.why-is-my-external-drive-smaller-than-advertised
url: https://storagestudio.us/faq/mac-storage-basics/why-is-my-external-drive-smaller-than-advertised
category: "Mac storage basics"
subcategory: "External storage and adding space"
question: "Why does my new external drive show less space than the box says?"
alt_questions:
  - "2tb drive shows 1.8tb"
  - "external drive capacity less mac"
  - "new ssd smaller than advertised"
  - "formatted capacity external drive"
  - "why 1tb shows 931gb"
short_answer: "Mac shows decimal units like the box, so a 1 TB drive shows close to 1 TB minus format overhead. Much less suggests binary units in another tool or an extra partition."
tags: ["capacity", "external drive", "disk utility"]
related: ["https://storagestudio.us/faq/mac-storage-basics/why-does-my-256gb-mac-show-less-than-256gb", "https://storagestudio.us/faq/mac-storage-basics/how-do-i-erase-or-format-a-drive-on-a-mac"]
applies_to: "all"
last_reviewed: 2026-10-01
---

# Why does my new external drive show less space than the box says?

On a Mac, a 1 TB drive should show close to 1 TB, because macOS and drive makers both count in decimal units. A little is used by the format's own structures and, on some drives, hidden partitions.

If you see a much smaller figure, such as 931 GB for a 1 TB drive, you're probably looking at a Windows tool or a utility that counts in binary units. A drive showing far less than either figure may have an extra partition — check **Disk Utility → View → Show All Devices**.