Best Allocation Unit Size For Your Drive: NTFS, exFAT (2026)
You are about to format a drive in Windows and the Format window asks you to pick an allocation unit size.
For most drives the default is the right answer, and picking a different size rarely changes real-world speed.
This guide explains what the setting does, what Windows picks for you, and the few cases where a different size is worth the effort.
What allocation unit size should you pick?
| Drive use | Pick this | Why |
|---|---|---|
| Windows system drive or general NTFS drive | Default (4096 bytes for NTFS) | Microsoft sources call it the best general-purpose size |
| External drive of mostly large media files | Default, or a larger size than the typical file only if you accept wasted space | Larger units mean fewer seeks on a mechanical drive |
| Drive shared between Windows and Mac | exFAT with the default size | NTFS is not natively supported on a Mac |
| Drive for SQL Server database files | 64 KB on NTFS | Matches a SQL Server extent |
| Drive that uses NTFS compression | 4 KB or smaller | Compression is not supported on larger clusters |
What is allocation unit size?
It is the cluster size, the smallest chunk of space the file system can hand to a file.
Microsoft’s cluster-size article says the average waste is about half a cluster per file.
A file that does not fill its last cluster still occupies the whole thing, which is why Size on disk is larger than Size in File Explorer.
As one Microsoft Q&A answer showed, with a 1 MB cluster a 1.1 MB file takes up 2 MB.
Why do larger and smaller units trade off?
- Larger units: fewer clusters to track, which can help big files, but more space wasted on small files.
- Smaller units: less wasted space, but more clusters to manage.
An archived Microsoft engineer’s blog advises matching the size to your average file size.
On a plain desktop, the difference is usually too small to notice, according to Help Desk Geek.
What does Windows choose by default?
| File system | Default cluster size | Limit |
|---|---|---|
| NTFS | 4 KB for volumes of 2,049 MB and up | Larger volumes may need 4 KB to 64 KB clusters |
| exFAT | 4 KB to 256 MB, 32 KB to 32 GB, 128 KB up to 256 TB | Per Microsoft KB 140365 |
| FAT32 | Up to 32 KB on 32 GB to 2 TB volumes | Volumes over 2 TB not supported |
These figures come from Microsoft’s default cluster size article and an archived Microsoft blog, so a newer Windows build may differ slightly.
How do you set the allocation unit size?
- Open File Explorer and go to This PC.
- Right-click the drive and choose Format.
- Pick the File system from the list.
- Open the Allocation unit size list and pick a size, or leave the default.
- Choose a Volume label and decide on Quick Format.
- Click Start, then OK at the warning.
Formatting erases everything on the drive, so copy your files off first.
Can you set it from the command line?
Yes, the Format command takes an /A switch, and Microsoft’s archived example formats drive D as NTFS with 64 KB units.
format D: /FS:NTFS /A:64K
Change the letter to the drive you mean, because this command wipes it.
The same source lists the manual choices as 512 bytes, 1 KB, 2 KB, 4 KB, 8 KB, 16 KB, 32 KB and 64 KB.
How do you check a drive’s current size?
Open an administrator command prompt and run fsutil fsinfo ntfsinfo C:.
Look at the Bytes Per Cluster line, where 4096 means 4 KB.
Can you change it without formatting?
No, an archived Microsoft blog says the only way to change the cluster size is to format the volume again.
Back up first, since the format removes the data.
Quick Format or full format?
Quick Format skips the sector scan and leaves old data on the disk until it is overwritten.
A full format scans for bad areas and wipes the data more thoroughly, so it suits a drive you plan to sell, but it takes much longer.
Does it matter for SSDs and games?
Help Desk Geek notes that fragmentation matters less on an SSD because there is no read-write head.
It also says a very small SSD, such as 16 GB or 32 GB, can save space with a smaller unit.
AWS’s SQL Server guide recommends 64 KB for database drives but admits performance on SSD storage may not depend on block size.
No official Microsoft recommendation for a bigger size on game drives turned up, so the default is the safe choice.
Why is FAT32 missing for a big USB drive?
A Microsoft Q&A answer reports that Windows does not format USB drives larger than 32 GB as FAT32.
On a 64 GB drive the Format window offered only exFAT and NTFS.
The poster’s attempt to force FAT32 failed with a Volume too Big error, and the advice was to use an alternative tool such as Rufus.
Our Rufus guide shows how to use that tool.
If a device does not need FAT32, exFAT with its default size is the simpler route.
Which file system should you choose first?
| Your drive | File system | Source note |
|---|---|---|
| Windows-only internal or external drive | NTFS | How-To Geek suggests NTFS for Windows-only use |
| Used on Windows and Mac | exFAT | Readable and writable on a Mac |
| USB drive over 32 GB, FAT32 wanted | Third-party tool needed | Windows format window does not offer it |
Pick the file system before the allocation unit size, since the size list changes with it.
When is a non-default size a mistake?
| Situation | Risk |
|---|---|
| 64 KB units on a drive full of small files | Lots of wasted space |
| Larger than 4 KB on NTFS | Compression not supported |
| NTFS for a Mac-shared drive | Mac cannot write natively |
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- How To Use An External Hard Drive With Xbox One
Sources: Microsoft Q&A on FAT32 over 32 GB, Microsoft KB 140365 (archive copy), Microsoft NTFS recommendations (archived blog), AWS SQL Server NTFS guidance.
What Allocation size should I for a 256GB Flash drive formatted in exFAT with average music files of 8MB’s I also have folders much larger but can omit them if it is going to be too large a compromise to allocate the drive to a larger allocation size I chose 64Kilobytes but am still unsure