NAS Sizing Wizard: How Many Bays and What Drive Size Do You Need?

This wizard works out how many NAS bays and what size drives you need from the data you have now, how fast it grows, the years you plan for, the free space you want to keep and the drive protection you choose. It shows the cheapest drive plan for 2, 4, 5, 6 and 8 bays with drive prices from NTK's market index, and hands the result to Find My NAS to pick a model.

The plan uses decimal TB (1 TB = 1,000 GB) and the same usable-space engine as the RAID Usable Capacity Calculator, including the space Synology and QNAP reserve for themselves when you pick one.

Your data

How much data do you have now?

Include photos, video, documents and backups you will keep on the NAS.

How fast does it grow? The presets are NTK starting points, not measured averages. GB a month adds the same amount each year; a percentage compounds.

How many years should it last?

2 years3 years5 years7 years

When you would want to reassess, not when the hardware wears out.

Free space to keep at the end

NTK planning default 25%. Space needed = data at the end / (1 minus this share).

Drive protection

One drive: two drives mirrored, or single parity from 3 drives (SHR on Synology, RAID 5 elsewhere). Two drives: dual parity from 4 drives (SHR-2 or RAID 6). Neither is a backup.

NAS platform Changes the reserved space and how the plan can grow later.

Plan for

Main use Carried to Find My NAS with your plan.

Your NAS plan

NAS size-

Drives-

Layout-

Raw capacity-

Usable space-

Data at the end-

Space needed-

Free at the end-

Drive cost at market prices

Find NAS models for this plan →

Find My NAS opens with this plan filled in and shows only models that can hold it.

Every bay count for your data

Bays	Cheapest drive plan	Usable	Drives cost

Your data over the plan

Growing it later

How this wizard works

Data at the end of the plan = your data now plus the growth: GB a month adds the same amount every year (straight-line, not compounded on the larger total); a percentage a year compounds. Space needed = that figure divided by (1 minus the free share you keep), so the free share at the end is exactly what you chose. For each bay count the wizard tries every drive size with a market price and every drive count that gives your protection, keeps the plans whose usable space covers what you need, and shows the one with the lowest drive cost.

Usable space is the layout (two drives mirrored, single parity from 3 drives, dual parity from 4) minus what the platform reserves, from the same engine as the RAID Usable Capacity Calculator. Drives in a plan are all one size. Method reviewed 2026-10-08.

Worked example

2 TB growing 50 GB a month for 3 years is 3.8 TB at the end; keeping 25% free needs 5.07 TB of usable space. With one drive of protection and no platform reserve:

Bays	Cheapest drive plan
2	2 x WD Red Pro 6TB (6 TB usable), about $509 each
4	4 x WD Red Plus 2TB (6 TB usable), about $249 each
5	4 x WD Red Plus 2TB (6 TB usable), about $249 each
6	4 x WD Red Plus 2TB (6 TB usable), about $249 each
8	4 x WD Red Plus 2TB (6 TB usable), about $249 each

NAS drive prices the wizard uses

The cheapest NAS-line drive at each size with an NTK market price (23 October 2026): the median of the latest prices from at least three Australian retailers. A size appears here when the index prices it.

Size	Drive	Market price	Retailers
2 TB	WD Red Plus 2TB	$249	5
4 TB	WD Red Plus 4TB	$336	9
6 TB	WD Red Pro 6TB	$509	4
8 TB	WD Red Plus 8TB	$594	9
10 TB	WD Red Plus 10TB	$789	8
12 TB	WD Red Plus 12TB	$899	7
16 TB	Seagate IronWolf 16TB	$1225	8
20 TB	Seagate IronWolf Pro 20TB	$1538	11

Check your NAS maker's compatibility list for the exact drive before you buy.

How each platform grows a pool

Layout	Adding a drive	Replacing with bigger drives
Synology SHR-1 and SHR-2	Synology allows expansion by adding drives on JBOD, RAID 5, RAID 6, RAID TP, RAID F1, SHR-1 and SHR-2 (not RAID 0, RAID 1 or RAID 10). Source A drive added to a Synology SHR pool must be at least as large as the smallest drive in the pool. Source Adding a second drive to a single drive SHR pool adds redundancy only; capacity grows from the third drive. Source	On an SHR pool of equal sized drives, capacity grows only after at least two drives (SHR-1) or four drives (SHR-2) have been replaced with larger ones, one at a time. Source On an SHR pool of mixed sizes, Synology says to replace the smallest drives with drives at least as large as the largest drive already in the pool. Source Synology's Replace Drive option needs a storage pool of at least two drives. On one-bay models without it, Synology says to back up your data to another place before replacing the drive. Source
Synology RAID 5 and RAID 6	Synology allows expansion by adding drives on JBOD, RAID 5, RAID 6, RAID TP, RAID F1, SHR-1 and SHR-2 (not RAID 0, RAID 1 or RAID 10). Source	On classic RAID (not SHR) with equal sized drives, every drive must be replaced with a larger one before capacity grows. Source On classic RAID with mixed sizes, replacing a drive other than the smallest does not change capacity; the smallest must be replaced first. Source
Synology RAID 1	Adding drives to a Synology RAID 1 pool adds mirror copies, not capacity (Synology RAID 1 is an N-way mirror). Source	On classic RAID (not SHR) with equal sized drives, every drive must be replaced with a larger one before capacity grows. Source
QNAP QTS RAID	QNAP can add disks to RAID 1, 5, 6, 50 and 60 groups while the pool stays online; each added disk must be at least as large as the smallest in the group. Source Adding disks to a QNAP RAID 1 group converts it to RAID 5. Source	QNAP supports expanding by replacing disks on RAID 1, 5, 6 and 10; capacity then follows the smallest drive. Source On QNAP the Expand Capacity step becomes available only after every disk in the RAID group has been replaced and rebuilt. Source
Unraid array	Adding a data disk to an Unraid array expands its capacity. Source A data disk added to an Unraid array must be no larger than the parity disks; a larger disk means upgrading parity first. Source	Replacing an Unraid data disk with a larger one rebuilds onto it and the file system grows to the new disk's size. Source A parity swap procedure exists for replacing a data disk with one larger than the current parity disk. Source
ZFS (TrueNAS and other OpenZFS)	RAIDZ expansion (adding a disk to an existing RAIDZ vdev) is a feature of OpenZFS 2.3.0. Source RAIDZ expansion keeps the parity level (a RAIDZ2 stays a RAIDZ2), can be repeated, and the added disk must be at least as large as the smallest disk in the vdev. Source Attaching a disk to a ZFS mirror adds another copy (two way becomes three way); it adds redundancy, not capacity. Source	When ZFS drives are replaced with larger ones, the new space appears only after every disk in that mirror or RAIDZ group is larger (autoexpand, off by default). Source

Platform facts checked 2026-10-07.

Frequently Asked Questions

How many bays do I actually need?

Enough to hold your data at the end of your plan plus the free space you want to keep, with the drive protection you choose. The wizard works that out for 2, 4, 5, 6 and 8 bays with every drive size that has an NTK market price, and shows the cheapest plan for each. For example, 2 TB growing 50 GB a month for 3 years, keeping 25% free, needs 5.07 TB of usable space; the cheapest plan is 4 bays with 4 x WD Red Plus 2TB (6 TB usable), about $249 each. No bay count suits everyone: a 2-bay mirror suits data that will not outgrow it, while 4 or more bays leave room to add drives and give more of the drive space to your files with single parity.

Can I start with two drives and add more later?

It depends on the NAS platform, so plan for the one you will buy. On Synology: Synology allows expansion by adding drives on JBOD, RAID 5, RAID 6, RAID TP, RAID F1, SHR-1 and SHR-2 (not RAID 0, RAID 1 or RAID 10). Adding drives to a Synology RAID 1 pool adds mirror copies, not capacity (Synology RAID 1 is an N-way mirror). So a pool you plan to grow should start as SHR or RAID 5, not RAID 1. On QNAP: QNAP can add disks to RAID 1, 5, 6, 50 and 60 groups while the pool stays online; each added disk must be at least as large as the smallest in the group. Adding disks to a QNAP RAID 1 group converts it to RAID 5. For other makers, check how their storage manager grows a pool before you buy.

Should I add drives or replace them with bigger ones?

If you have an empty bay and your platform lets you add a drive to the pool, adding one grows the space without replacing drives that still work. If every bay is full, replacing drives with bigger ones is the way to grow. On an SHR pool of equal sized drives, capacity grows only after at least two drives (SHR-1) or four drives (SHR-2) have been replaced with larger ones, one at a time. On classic RAID (not SHR) with equal sized drives, every drive must be replaced with a larger one before capacity grows. On QNAP the Expand Capacity step becomes available only after every disk in the RAID group has been replaced and rebuilt. Synology Storage Manager replaces one drive at a time. The wizard shows how many bays your plan leaves empty, and the NAS Capacity Growth Planner shows the space after each step for your platform.

How much space does drive protection cost?

Synology states pool capacity as: RAID 5 = (drives minus 1) x smallest drive; RAID 6 = (drives minus 2) x smallest; RAID 10 = (drives / 2) x smallest. Which to choose is a trade-off, not a rule: two-drive protection keeps your data if a second drive fails while the first is rebuilding, at the cost of another drive. RAID and SHR protect against a failed drive, not against deletion, ransomware or a lost NAS, so keep a backup as well.

Can I mix drive sizes?

Classic RAID on Synology sizes every member to the smallest drive in the pool; space above it is unused. On an SHR pool of mixed sizes, Synology says to replace the smallest drives with drives at least as large as the largest drive already in the pool. The wizard plans with drives of one size.

Does the processor matter if I only store files?

For storing files, the bays and drives decide the space. The processor and memory matter for the apps you run. For Plex, hardware transcoding depends on the exact model, and Plex's own NAS compatibility list is the source; Find My NAS reads it. For other apps, check each app's own requirements.

Which drives should I buy?

The wizard prices drives from the NAS drive lines in NTK's market index (Seagate IronWolf, Seagate IronWolf Pro, WD Red Plus and WD Red Pro), the cheapest of them at each size. Check your NAS maker's compatibility list for the exact drive model and size before you buy, and keep the drives in one pool the same size.

What about expansion units?

Synology expansion units are supported on specific models only. QNAP says a NAS storage pool cannot be expanded onto TR or TL series expansion units, except TL series SAS JBOD models such as the TL-R1620Sep-RP and TL-R1220Sep-RP. Find My NAS can require a model that takes one.

Why does the plan keep space free?

So the NAS is not full at the end of your plan. The wizard keeps the share you choose free (25% unless you change it): space needed = your data at the end of the plan divided by (1 minus that share). QNAP's pool and volume usage alert threshold defaults to 80% and can be changed; it is an alert, not a limit. TrueNAS calls 80% full a starting point, not a hard limit: write once archival pools can reasonably run to 90 or 95%, while heavy random I/O pools may need to stay below 80%.

Why is usable space less than the drives add up to?

Usable space is the drive layout minus protection and what the NAS platform reserves for itself, in decimal TB (1 TB = 1,000 GB). For Synology and QNAP the wizard takes the reserves those makers publish; the RAID Usable Capacity Calculator shows each one.

Can I share my plan or take it to Find My NAS?

Yes. Copy permalink puts every input in the link. Find NAS models for this plan opens Find My NAS with your space, bays and protection filled in, so you do not answer them twice. Links to this wizard from before it was rebuilt carried no settings, so they open with the defaults.

Next steps

Pick a model for your plan: Find My NAS. Space after each upgrade step: NAS Capacity Growth Planner. Every layout in detail: RAID Usable Capacity Calculator. Rebuild times for big drives: RAID Rebuild Time Estimator. Footage storage: NVR Storage Calculator. Plex streams: Plex Stream Planner. Reaching your NAS from outside: NBN Remote Access Checker. Drives: NAS drives guide.