NAS Capacity and Drive Cost Calculator

The number that matters is cost per usable terabyte

A listing advertises cost per terabyte of raw capacity. That is not what your pool costs. Parity drives are bought at full price and store none of your data, so a six-drive RAIDZ2 array has already spent a third of its budget on redundancy before a single file lands. The calculator shows both figures side by side, because the gap between them is what decides whether a layout is worth it.

This is also why redundancy is not free advice. RAIDZ2 on six drives costs you two drives. On twelve drives it still costs you two. Wider arrays amortise parity better, which is the real argument for filling bays rather than half-filling them.

Why fewer large drives usually win

Most buyers assume small drives are the budget option. For a NAS the opposite is normally true. Capacity gets cheaper per terabyte as drive size goes up, so reaching a target with four large drives frequently costs less than reaching it with eight small ones, and it leaves bays empty for later expansion. Switch to the second mode above, set the capacity you need, and the table sorts the routes by total cost.

There is a real counter-argument and it is worth stating. Larger drives take longer to resilver, and a long resilver is precisely when a second failure hurts. That is why the calculator warns about RAIDZ1 on large drives: single parity plus a multi-day rebuild is a thin margin. Above about 8TB, double parity is the usual choice.

CMR only, and why that is not negotiable

Every drive in this calculator uses conventional magnetic recording. Shingled drives overlap their tracks to gain density, which makes sustained random writes collapse once the cache is exhausted. A resilver is exactly that workload, and shingled drives have turned rebuilds that should take hours into rebuilds that take days.

The trap to know: Western Digital shipped shingled drives inside the plain WD Red line.WD Red Plus is the conventional line, and plain WD Red should not go in a NAS. Seagate IronWolf, IronWolf Pro, WD Red Plus, WD Red Pro and Toshiba N300 are all conventional.

Workload ratings, in plain terms

Manufacturers rate drives for a sustained annual write volume. IronWolf and Red Plus are rated at 180TB a year, IronWolf Pro at 300TB, and Red Pro at 550TB. For a home media and backup array those are all far beyond what you will write, so the cheaper line is usually the right call and the Pro premium buys warranty length more than survivability. If the array backs up a busy network or runs constant scrubs and syncs, the higher rating starts to earn its money.

Terabytes versus tebibytes

Drives are sold in decimal terabytes and reported by your NAS in binary tebibytes, which makes a pool look about 9% smaller than the number on the box before you have written anything. That is not a fault and nothing is missing. The calculator shows both so the figure in the web interface is not a surprise, along with the practical capacity at the 80% fill guidance that keeps performance from degrading.

Next steps

Once you know the drive count, size the machine around it. TheTrueNAS hardware calculator works out the RAM those drives need, and thepower cost calculator covers what the array costs to leave running. For the boxes themselves see ourNAS mini PC roundup and theTrueNAS picks.

How these figures are sourced

Capacity and cost are arithmetic on your own inputs. Drive specifications, conventional recording, workload ratings and warranty terms come from the manufacturers' published specifications, and every Amazon listing in the catalogue was validated on August 25, 2026. Drives verified out of stock on that date are deliberately absent rather than listed and unbuyable. Prices move constantly, so every figure is approximate.