Storage calculator
Quick reality check: a drive labelled 256 GB does not give you 256 GB to fill. Enter your capacity, typical file size, and the reserve you want to keep free, and the calculator shows what actually fits after the overhead is subtracted.
Last updated: May 2026
Enter values to estimate how many files fit.
How much fits on your storage device?
The number on the box is not the number you work with. Camera card slots, NAS bays, phone settings, and backup planners all need the usable figure, not the marketed one. This calculator handles the gap: enter the labelled capacity, set a typical file size for your content type, and dial in a reserve percentage to hold back for system headroom and fragmentation buffer. The result is the realistic file count, not the theoretical maximum.
Video is heavier than most people expect: 4K footage at 100 Mbps generates about 750 MB per minute, so a 256 GB card holds around 5.7 hours at that bitrate. Switch to RAW stills (20-50 MB each) and the same card gives you somewhere between 5,000 and 12,800 frames, far fewer than the 25,600 a 10 MB JPEG estimate would suggest.
Common storage device capacities
| Capacity | Typical device | Approx. file count (photos at 5 MB) |
|---|---|---|
| 32 GB | Entry-level phone or USB stick | ~6,400 photos |
| 128 GB | Mid-range phone or SD card | ~25,000 photos |
| 256 GB | Laptop SSD or current-gen phone | ~50,000 photos |
| 512 GB | High-capacity phone or laptop | ~100,000 photos |
| 1 TB | Desktop SSD or external drive | ~200,000 photos |
| 4 TB | External HDD or NAS drive | ~800,000 photos |
Planning real usable capacity, not raw bytes
Buying or provisioning storage based on the marketed number leads to surprises at project time. These are the situations where this calculator earns its keep:
- Pre-shoot card planning: A wedding or sports photographer shooting RAW needs to know slots filled, not drive label. Run the numbers before the event, not after the card is full mid-session.
- NAS and server builds: RAID configurations hand back far less than the sum of the drives. A four-drive RAID 5 array built from 4 TB drives gives about 12 TB usable, not 16 TB. Add the filesystem overhead on top and the planning spreadsheet looks very different from the sales page.
- Phone and tablet purchasing decisions: The marketed capacity is the flash chip size. iOS and Android each consume several gigabytes for the OS and system partition, so a 128 GB phone typically offers 110-115 GB of user space before a single app is installed.
- Backup destination sizing: Compressed archives are smaller than source files, but incremental sets accumulate. Planners who size the backup target on a single snapshot consistently undersize it within six months.
Common mistakes
- Using the labelled GB as the working number: Drive manufacturers report in decimal gigabytes (1 GB = 1,000,000,000 bytes), but operating systems report in binary gibibytes (1 GiB = 1,073,741,824 bytes). A 1 TB drive shows as roughly 931 GiB in Windows Explorer. That 6.9% gap is not missing space, it is a reporting difference, but if you plan file counts using the label you will hit the ceiling sooner than expected.
- Skipping filesystem overhead: NTFS, ext4, APFS and FAT32 all reserve space for metadata structures. A freshly formatted drive is already a few percent smaller than the raw partition. This is before any files exist.
- Setting the reserve to zero: HDDs noticeably slow above 85-90% capacity because the controller has fewer contiguous free blocks to work with. SSDs lose write performance above 75-85% due to wear-levelling constraints. Running at 100% is how you get corrupted writes and sluggish transfers at the worst possible moment.
- Mixing up file-size units: Camera specs often list "average file size" in MB, but video specs often appear in Mbps (a rate, not a size). Convert bitrate to file size first: 1 minute of 100 Mbps footage = 100 megabits x 60 seconds / 8 bits per byte = 750 MB.
Frequently Asked Questions
I have a 512 GB phone but only see about 480 GB available. Where did the rest go?
Two things shrink that number. First, the OS partition (iOS or Android plus pre-installed apps) typically consumes 10-15 GB before you open a single third-party app. Second, the phone's storage chip is measured in decimal gigabytes by the manufacturer, but the OS may display in binary gibibytes, which are about 7% smaller. A 512 GB chip is 512 x 1,000,000,000 bytes, but the OS shows it as roughly 477 GiB. Both effects stack, so 480 GB available on a "512 GB" device is completely normal.
How do I calculate how much video fits when my camera spec shows Mbps, not MB?
Convert the bitrate to a per-minute file size first. Multiply the Mbps figure by 60 (seconds per minute), then divide by 8 (bits per byte), and the result is MB per minute. For example: 100 Mbps x 60 / 8 = 750 MB per minute of footage. Enter that as your average file size in MB, then set the capacity to your card size and include a reserve. The file count the calculator returns is your total number of shootable minutes.
Why does my NAS show far less than the sum of its drives?
RAID configurations trade raw capacity for redundancy. RAID 1 mirrors two drives, so four 4 TB drives in two mirrored pairs gives 8 TB usable. RAID 5 across four 4 TB drives yields about 12 TB usable (one drive's worth is used for parity). On top of that, the filesystem adds its own overhead layer. The calculator works on a single usable capacity figure, so enter the post-RAID usable number, not the sum of drive labels.
How much reserve should I plan for a backup drive?
At minimum 10-15%, but backup drives fill faster than primary drives because incremental sets accumulate silently. A practical rule for unattended backup targets is to size them at 1.5 to 2x the current data volume, then set the reserve to 20-25% in this calculator to model how long before you need another drive. HDDs show noticeable slowdowns above 85-90% full, which degrades long backup jobs at exactly the time your attention is elsewhere.