Usable Capacity Chart: Every Bay Count and Drive Size

This is the lookup table. Four drives give three drives of usable space under single parity and two under double. Six give five and four. Eight give seven and six. Multiply by your drive size and the answer falls out: four 8 TB drives is 24 TB or 16 TB, eight 16 TB drives is 112 TB or 96 TB. Every cell below is computed with the same functions behind the RAID capacity calculator, in decimal terabytes with the binary figure your NAS will print alongside.

Read the first table for capacity and the second for what that capacity costs, because the two together are the actual purchase decision. The third table works backwards from a capacity target to the cheapest set of drives that clears it.

How much usable capacity does each bay count and drive size give?

Usable capacity is the space the filesystem can address once redundancy is taken out, and for every layout here it is a fixed multiple of the drive size. Single parity costs one drive whatever the bay count, double parity costs two, and RAID 10 costs half. The only thing that changes with bay count is what percentage of your money that fixed cost represents.

All figures assume identical drives. Capacity is decimal TB, the unit on the drive label, and the last two columns are the binary TiB figures your NAS will print for the same bytes. A dash means the layout cannot be built at that bay count: RAID 5 needs three drives, RAID 6 and RAID 10 need four, and SHR-2 needs four. RAIDZ1 matches the RAID 5 column exactly and RAIDZ2 matches RAID 6.

Two observations worth taking away from that table. The first is that SHR and RAID 5 are identical in every row, and SHR-2 and RAID 6 are identical in every row. That is not a rounding coincidence. With matched drives, SHR collapses into a single parity group and yields exactly the RAID 5 formula. The columns exist so you can see it rather than take it on trust, and the SHR advantage appears only on mixed drive sizes.

The second is the two-bay block, where every layout except a mirror shows a dash. Two bays gives you one drive of usable space out of two, a 50 percent tax on everything you buy, and no arrangement of software changes that. It is the reason four bays is the volume tier of the whole category: three drives of usable capacity out of four is a 25 percent tax, and four bays is also the smallest count that can run double parity at all.

What does a usable terabyte actually cost?

Cost per usable terabyte is the drive spend divided by the capacity you can store data on, and it is the only price figure that compares fairly across drive sizes and layouts. Price per drive tells you nothing, because a cheap drive in an array that needs eight of them is not cheap. Price per raw terabyte is closer but still misleading, since it ignores the drive or two that parity consumes.

The last four columns are dollars per usable decimal terabyte, drives only, with no enclosure. Rows showing dashes are capacities where our product data holds no verified internal NAS drive, and a price has been left out rather than estimated. Prices change often, confirm on Amazon before ordering. Full drive-by-drive pricing lives on the cost per terabyte chart.

The pattern in those columns is the important part. Cost per usable terabyte drops sharply from the small capacities, sits on a broad flat floor through the middle of the range, and turns back up at the newest capacities. The cheapest cell in the four-bay RAID 6 column is the 8 TB class at $72.50 per usable terabyte, and the 16 TB class lands within a cent of it, so the two are a genuine tie on price and differ only on how many bays and how much power they consume to get there. The 4 TB and 6 TB rows are far worse, and the 20 TB and 24 TB rows carry a visible premium for being near the top of the range. Bay count pushes the whole curve down: the same drives in an eight-bay array under RAID 6 are cheaper per usable terabyte, because two parity drives are spread across six of usable capacity instead of two.

Notice also how heavily double parity taxes a four-bay array. RAID 6 on four drives gives up half of everything you bought, which is why the four-bay RAID 6 column runs at roughly one and a half times the four-bay RAID 5 column. At eight bays that penalty shrinks to about a sixth. If double parity looks unaffordable at four bays, the answer is usually a wider chassis rather than a riskier layout.

How many drives do I need for a capacity target?

Most people arrive from the other direction: they know roughly how much space they need and want to know what to buy. The table below runs every priced drive size against four, six and eight bays under RAID 6 and reports the cheapest combination that clears each target.

Drives only, no enclosure, searched across 4, 6 and 8 bays at every capacity with a verified price. Double parity throughout, because a chart that recommended single parity on large drives would be recommending a rebuild you should not want to run. Add roughly $400 to $1,400 for the chassis depending on bay count, and see best 4-bay NAS or best 8-bay NAS for that half of the bill.

Do not size for exactly what you need today. A pool that is 90 percent full behaves badly, particularly on copy-on-write filesystems where free space is what the allocator uses to avoid fragmentation. Plan to sit at 60 to 70 percent full on the day the array goes into service, which in practice means adding half again to whatever your current data measures. Then check the number against the backup sizing calculator, because the backup target has to hold all of it too.

What does the chart not show?

One thing, and it is the case a large minority of readers are actually in: mixed drive sizes. Every figure above assumes identical drives, because that is what a lookup table can honestly do. The moment the drives differ, classic RAID and every ZFS RAIDZ vdev truncate each member down to the smallest drive and strand the rest.

Take 8, 8, 12 and 16 TB. RAID 5 gives 24 TB usable with 12 TB stranded, because it truncates all four drives to 8 TB. SHR gives 28 TB usable at the same single-drive fault tolerance, a difference of 4 TB on drives you have already bought. That is the entire SHR pitch, and it is invisible on any calculator that only models standard RAID levels.

Our RAID capacity calculator takes an arbitrary list of drive sizes and shows SHR and SHR-2 next to RAIDZ1, RAIDZ2 and RAIDZ3 next to the standard levels, on the same physical drives, with stranded capacity called out. It is the only genuinely cross-vendor one, and mixed sizes are exactly the case it exists for. See also SHR against RAID 5.

Should I buy bigger drives or more bays?

Both roads reach the same capacity and they cost differently in three places: drives, chassis and rebuild exposure. Fewer larger drives win on chassis cost, power and noise. More smaller drives win on parity efficiency and on being able to lose a member without losing as large a fraction of the array.

• Chassis. A four-bay box starts around $365 and an eight-bay box around $675, with the popular units well over $1,000. That difference buys two or three large drives.
• Power. Each mechanical drive is roughly 4 to 6 W idle and 6 to 9 W active, so eight drives cost 30 to 40 W more than four, running continuously. Work it through on the power consumption chart.
• Rebuild. Larger drives take longer to rebuild and wider arrays read more members while doing it. Both push toward double parity. The rebuild time chart puts hours on it.
• Upgrade path. Empty bays are the cheapest capacity you will ever buy, because adding a drive to a set is far cheaper than replacing every drive in it. Buying a wider chassis than you need today is a defensible way to spend $300.

The practical default for most home builds is four bays with the largest drives the budget reaches, running double parity, in a chassis that can be replaced later. A Seagate IronWolf Pro 16TB NAS HDD (ST16000NT001) at $579.99 in a four-bay array under RAID 6 gives 32 TB usable, which is more than most households will fill in five years, and it leaves the enclosure decision cheap and reversible.

Why is my array smaller than the chart says?

Assuming the layout is what you think it is, there are four usual causes and they stack.

• Units. The largest single effect. Your NAS counts in binary tebibytes and often prints the letters TB anyway, so every figure looks 9.05 percent small. A 24 TB array reporting 21.8 is correct. See the TB against TiB chart.
• Filesystem overhead. Partition alignment, inode tables, journals and reserved blocks take a further 2 to 4 percent, and ZFS holds back roughly one part in 32 of the pool as slop space so it can always free blocks.
• A mismatched drive. One smaller member truncates every other drive in a classic RAID set. If one drive is 8 TB and the rest are 12 TB, the array is behaving like a set of 8 TB drives, exactly as designed.
• An unexpanded volume. After replacing drives one at a time with larger ones, the pool grows but the volume on top of it may not, until you expand it explicitly. This is the most common cause of an upgrade that appears to have done nothing.

Work through them in that order. Units explain most of it, filesystem overhead explains a little more, and if the number is still far off then the layout or the drive set is not what you believe it is.

Related reading

• RAID capacity calculator for mixed drive sizes and every layout at once
• RAID level comparison chart for the layout matrix behind these numbers
• TB against TiB chart for why the reported figure is 9.05 percent smaller
• Cost per terabyte chart for the full drive pricing table
• How to choose NAS drives for CMR, workload ratings and warranty
• How to migrate to a bigger NAS for the drive-by-drive upgrade path