Quick answer- Use the default allocation unit size. How-To Geek notes you probably will not notice a difference in speed with a modern drive, and the default is tuned to the drive size.
- Pick the file system by where you use the drive: exFAT for Windows and Mac, NTFS for Windows only, APFS for Mac only.
- Formatting erases everything on the drive. Back up first.
You may have seen advice to set an exact allocation unit size, such as 4KB for small files or 32KB for media, to speed up an external SSD. In practice, the file system you choose matters much more than the cluster size, and for most people the default is right. This guide explains what allocation unit size is, when changing it makes sense, and how to format an external SSD in Windows 11 or on a Mac.
1. What Allocation Unit Size Means
Allocation unit size, also called cluster size, is the smallest chunk of space the file system uses to store a file. A bigger unit wastes more space on small files, but needs fewer lookups for very large ones.
How-To Geek explains that, on average, each file wastes about half an allocation unit, so 100,000 files with a 4KB unit waste around 200 MB. On a modern SSD that is negligible.
My take (20 years in IT): Decide where the drive will be used before you format it. Pick exFAT if it moves between Windows and Mac, NTFS for Windows only, or APFS for Mac only.
2. Choose the File System First
| File system | Works on | Good for |
|---|
| exFAT | Windows and Mac | Sharing one drive between both systems; large files |
| NTFS | Windows (Mac can usually read only) | Windows-only use and Windows features |
| APFS | Mac | Mac-only drives, especially SSDs |
| FAT32 | Almost everything | Small drives and older devices; 4 GB file limit |
3. Default Allocation Unit Sizes
- NTFS: 4KB for volumes from 7 MB to 16 TB, and 8KB from 16 TB to 32 TB.
- exFAT: the default grows with drive size, so a large external SSD gets a large default unit.
- How-To Geek’s advice: use the default that Windows recommends when you format.
- If you store mostly very large files, a larger unit can improve storage efficiency. If you store many small files, keep the default.
Narrowing it down? Prices and availability change often, so check the current listing.
NVMe SSD enclosures → 4. How to Format in Windows 11
- Back up your data. Formatting erases the drive.
- Open File Explorer, right-click the drive, and choose Format.
- Pick the File system, leave Allocation unit size on Default, and keep Quick Format checked.
- Select Start, then confirm the warning.
5. How to Format on a Mac
- Open Disk Utility and choose View, Show All Devices.
- Select the disk and click Erase.
- For a drive used with Windows and Mac, Apple says choose exFAT if the disk is over 32 GB, or MS-DOS (FAT) if it is 32 GB or less.
- Choose APFS if the drive is only for Mac. Name the volume and click Erase.
Ready to compare options? Prices and availability change often, so check the current listing.
10 Gbps USB-C cables → My take (20 years in IT): Copy anything you need off the drive first, because formatting erases everything. Then leave the allocation unit size at the default, as the post advises.
Frequently Asked Questions
What allocation unit size should I use for an external SSD?
Use the default. It is chosen for the drive size, and the speed difference from changing it is small on modern drives.
Is exFAT or NTFS better?
Use exFAT if you move the drive between Windows and Mac. Use NTFS if it only connects to Windows.
Does quick format wipe the data?
Quick format erases the file index so the drive looks empty. The data can sometimes be recovered, so use a full format or secure erase if you are giving the drive away.
Will a bigger unit make my SSD faster?
Not noticeably for most uses. A bigger unit mostly changes how much space small files waste.
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Last updated: October 22, 2026.
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