Backup & RAID Calculator
Calculate RAID usable storage, estimate backup capacity needs, compare cloud storage costs across providers and size a NAS for your home or office.
Results
Setting up a NAS, choosing a RAID level or estimating cloud storage costs requires math that gets surprisingly complex. How much usable storage does RAID 5 with 4 drives give you? How much backup space does a 500 GB dataset need with 4 weeks of retention? Is Backblaze B2 actually cheaper than AWS S3 for your archive? These calculators handle the formulas for RAID redundancy overhead, incremental backup sizing, cloud provider pricing and multi-year NAS capacity planning.
Frequently asked questions
RAID 5 with 4 drives gives you 3 drives worth of usable storage - 75% of the raw total. With four 4 TB drives (16 TB raw), you get 12 TB usable. One drive's capacity is dedicated to parity data, allowing the array to survive a single drive failure without data loss.
RAID 5 uses one parity drive (N-1 usable) and can survive one drive failure. RAID 6 uses two parity drives (N-2 usable) and can survive two simultaneous failures. For arrays with 6 or more drives, RAID 6 is recommended because the risk of a second failure during a rebuild increases with drive count.
With full-only weekly backups and 4 weeks of retention, you need 2 TB (4 copies of 500 GB). With incremental backups at 5% daily change rate you need about 1.2 TB - one full backup plus 28 days of incremental changes. Incremental is far more space-efficient.
Backblaze B2 at $0.006/GB/month is cheapest for all tiers. For archive specifically, Google Cloud Archive at $0.0012/GB/month beats AWS Glacier at $0.004/GB/month. However, retrieval fees vary significantly - factor those in if you need frequent access.
With 500 GB per person (2.5 TB total) and 20% annual growth over 3 years, you need about 4.3 TB of usable storage. In RAID 5 with 4 drives, buy four 2 TB drives (6 TB usable) for comfortable headroom. For more safety margin, four 4 TB drives in RAID 6 gives 8 TB usable.
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