NAS RAID Storage Planner

Why your NAS never gives you the full terabytes

You buy four 4 TB drives, slide them into your new NAS, and expect 16 TB of space. Then the dashboard shows 12 TB — or less — and you wonder where the rest went. The missing terabytes are not a mistake. Any RAID level that protects your data reserves part of the raw capacity for redundancy: parity blocks in RAID 5 and 6, full mirror copies in RAID 1 and 10. On top of that, drive makers count a terabyte as 1,000 gigabytes while your NAS reports in binary units, so a "4 TB" drive shows up as roughly 3.64 TB before RAID even enters the picture. This planner does the RAID math for you, so the number on the box stops being a surprise.

How RAID levels trade space for safety

Every RAID level is a deal between capacity, speed and fault tolerance. RAID 0 stripes data across all disks for maximum speed and zero wasted space — but one failed disk destroys everything. RAID 1 mirrors every byte, so you survive disk failures but pay for it with half your capacity. RAID 5 spreads one disk's worth of parity across the array, surviving a single failure at the cost of one disk's capacity, and needs at least three disks. RAID 6 keeps two parity blocks, surviving two simultaneous failures, at the cost of two disks' capacity, with a four-disk minimum. RAID 10 mirrors pairs and then stripes across them: fast and safe, but you lose half the raw space and need an even number of at least four disks. JBOD simply pools the disks with no redundancy at all. This calculator applies each level's formula to your disk count and size and shows usable space, overhead and how many disks can die.

Worked example: four 4 TB drives in RAID 5

Take four 4 TB drives. Raw capacity is 4 × 4 TB = 16 TB. RAID 5 reserves the equivalent of one disk for parity, so usable space is (4 − 1) × 4 TB = 12 TB, and the redundancy overhead is 4 TB — exactly 25% of the raw space. The array survives any single disk failure: pull one drive out and your files stay accessible while you order a replacement. Drop to three drives and usable space falls to 8 TB; switch the same four drives to RAID 1 and you get only 4 TB usable with three drives' worth of mirrors. Run the numbers above and the trade-offs become concrete instead of theoretical.

Practical tips before you buy disks

Buy identical disks — same size, same model line. A RAID array treats every disk as the size of the smallest one, so a lone 2 TB drive in a 4 TB array wastes 2 TB on every other disk. Budget for one cold spare: when a disk fails, rebuild time on multi-terabyte arrays can run for days, and a second failure during a rebuild is how RAID 5 arrays die. Remember that RAID is not a backup — it protects against disk failure, not accidental deletion, ransomware or fire, so keep an offline or off-site copy of anything irreplaceable. Finally, leave 10–15% of the usable space free; a NAS filled to the brim slows down and gives you no room to rebuild comfortably.