NAS Storage Capacity Growth Planner

2-Bay vs 4-Bay: Which One Your Data Needs

Neither is the right answer for everyone. A 2-bay NAS suits data that will not outgrow it within the years you plan to keep it: it is smaller, quieter to place and a mirror is simple to run. When it fills, you grow it by replacing both drives with bigger ones, one at a time.

A 4-bay NAS costs more up front and holds more drives. It leaves room to add drives later, and with single parity more of its drive space holds your files: four 8 TB drives give 24 TB of layout capacity, two 8 TB drives mirrored give 8 TB.

The planner answers the question for your own numbers. With your data and growth, this is when each would reach your planning threshold:

Enter your data above to compare.

Expansion rules by platform

How each platform grows a pool, in the vendors' own terms. The planner applies these rules step by step.

Expansion units

An expansion unit adds bays to some NAS models without replacing the NAS. Synology expansion units are supported on specific models only. (Synology, checked 2026-10-07) 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. (QNAP, checked 2026-10-07) Check your own model before you plan on one; this planner does not model expansion units.

Worked examples

A forecast: Synology SHR, 4 x 4 TB, 5 TB stored

Four 4 TB drives in SHR-1 with Btrfs leave 11.48 TB available after Synology's system partitions and file system metadata. An 80% planning threshold is 9.19 TB.

• Adding 1 TB a year, 5 TB reaches the threshold in about 4.2 years and is full in about 6.5 years: 2.3 years between the two.
• If growth is 20% higher (1.2 TB a year) the threshold comes in about 3.5 years; 20% lower, about 5.2 years.
• Growing 10% a year instead, the same 5 TB reaches the threshold in about 6.4 years and is full in about 8.7 years.

Upgrading drives one at a time

Layout capacity before platform reserves, decimal TB. A 12 TB data drive is refused on the 8 TB parity array until parity is at least as large: that is the Unraid parity rule. Each replacement is one rebuild; an 8 TB drive at 50 to 100 MB/s takes about 22 h to 44 h.

Footage kept for a fixed period

Footage recorded at 7 GB a day and kept 30 days levels off at 0.21 TB once the first 30 days are recorded, and stays there however many years pass. Counting it as yearly growth instead would add the same footage again every year.

NTK starting points: what 5 years looks like

The presets in the planner are NTK starting points for readers with no history to measure, not averages measured from real NAS owners. Replace them with your own growth as soon as you can. This table runs each one through the planner's own formula: data after 5 years, and the space available you would need to stay under an 80% planning threshold.

Space needed = data after 5 years / 80%. It is space available for your data, after redundancy and platform reserves; the RAID Usable Capacity Calculator turns it into drives.

How we calculate

1. Growth. From two readings: (data now minus data then) x 12 / months between them, per year. Or the yearly amount you type, or a percentage per year (data x (1 + rate) to the power of the years).
2. Footage kept a fixed period grows at the daily amount until the retention period is full, then stays at GB a day x days kept. It is added to the growing data, never multiplied by the years.
3. Space available comes from the shared NTK capacity engine: the RAID layout (SHR tiers, RAID 1, 5, 6 and 10 at the smallest drive, an Unraid array as the sum of its data drives, one ZFS vdev as ZFS reports it), minus the platform reserves the vendors publish. A QNAP reserve is a range, so the planner uses the lower figure.
4. Planning threshold date is when the forecast data first reaches your threshold x space available; full date when it reaches the space available. Both are counted from today. The forecast looks up to 50 years ahead, an NTK planning assumption.
5. Sensitivity repeats the forecast with growth 20% higher and 20% lower, an NTK planning assumption. It is a range of scenarios, not a probability.
6. Upgrades apply each replacement or added drive in turn with the platform's rule, recalculate the capacity after every step, count the rebuilds and rerun the forecast on the final space available.

Assumptions NTK made

• The presets and the 1,000 GB a year default are NTK starting points, not measured averages.
• Rebuild speed ranges are NTK planning assumptions shared with the RAID Rebuild Time and Risk Estimator; a rebuild is timed as the new drive's full capacity.
• RAID 0 counts every drive in full, as Synology's RAID Calculator does. On Unraid your largest drives become parity.
• QNAP is modelled as QTS with ext4; QuTS hero (ZFS) is not modelled. ZFS is one vdev; for several vdevs use the ZFS vdev calculator.
• A larger NAS is worked out as a new pool you copy your data into; whether an existing pool can move into another chassis depends on the models.

Method last reviewed: 2026-10-14. Vendor rules checked at source: 2026-10-06 to 2026-10-14. Drive market prices last changed: 23 October 2026.

Sources this planner uses

Frequently Asked Questions