Before buying a drive, sizing a backup target, or provisioning a cloud bucket, you need a realistic total. This calculator turns a file count and an average file size — plus optional headroom — into a total shown in MB, GB, and TB.
How it works
The estimate is a single product:
total = number of files × average file size × (1 + headroom%)
The average size is converted from your chosen unit (MB, GB, or TB) before multiplying, and the result is shown across all three units using decimal sizing (1 GB = 1000 MB, 1 TB = 1000 GB) — the same convention drives are advertised under, so the figure maps straight onto what you would purchase.
The headroom percentage covers growth and the fact that filesystems reserve space for metadata and never fill to 100%.
Example
5000 photos averaging 4 MB each, with 20% headroom:
- Base = 5000 × 4 = 20,000 MB = 20 GB
- With headroom = 20,000 × 1.2 = 24,000 MB = 24 GB
| Quantity | Value |
|---|---|
| Files | 5000 |
| Average size | 4 MB |
| Base total | 20 GB |
| Headroom | 20% |
| Total needed | 24 GB |
Choosing the right headroom percentage
How much headroom to add depends on how predictable your growth is and whether the storage target is fixed or flexible:
- 10% headroom: Reasonable for a one-time archive you do not expect to grow — scanning a set of documents, for example. The archive is finished when you are done.
- 20–30% headroom: A good default for ongoing collections like photos, project backups, or media libraries that grow steadily over time.
- 50% or more: Sensible for fast-growing datasets — security camera footage, database backups that grow with user data, or collections where you regularly add large batches.
For cloud storage (S3, Backblaze B2, Google Drive), headroom matters less because you can expand on demand, but for physical drives or NAS you buy upfront, under-sizing means replacing hardware or adding another drive sooner than expected.
Decimal vs binary sizing
Storage manufacturers use decimal: 1 GB = 1,000 MB. Operating systems traditionally use binary: 1 GiB = 1,024 MiB. This is why a “1 TB” drive shows as about 931 GB in Windows or macOS. This calculator uses decimal (matching what you buy), so the output maps directly to drive sizes as advertised. If you need to account for the OS display gap, add roughly 7% extra headroom on top of your growth buffer.
Common file sizes for reference
| File type | Typical size |
|---|---|
| Smartphone photo (JPEG) | 3–8 MB |
| RAW photo (DSLR) | 20–50 MB |
| 1080p video per minute | 100–200 MB |
| 4K video per minute | 350–700 MB |
| Office document | 0.05–2 MB |
| PDF (text-heavy) | 0.5–5 MB |
These are approximate; actual sizes depend on quality settings, compression, and content. Measure a sample from your own files for the most accurate input.
Everything is calculated locally in your browser, with nothing sent anywhere.