NAS RAID Usable Capacity & Drive Upgrade Planner

When buying a NAS for family photos or business records, adding the capacity printed on every drive is only the starting point.
Mirrored copies and parity consume space, followed by system and filesystem overhead.
Future photos, video and operating headroom also matter when deciding how much capacity to buy.

This NAS RAID usable capacity calculator separates nominal RAID capacity from a planning storage limit.
Enter current data and annual net growth to see the data expected at your target horizon, the shortfall in your current configuration and the time until your operating limit is reached.
Results are input-based estimates, not a replacement for the actual storage-manager display.

The distinction is especially useful when reusing mixed-size drives.
Replacing one small drive with a larger one may leave nominal capacity unchanged.
Separating unused mixed-size space from parity and mirrors explains that outcome.
Save a configuration report and use it to check hardware compatibility and expansion procedures before ordering.

Bays and hot spares

Bay count means the drive slots included in this comparison.
Enter the labelled TB for each installed drive and zero for an empty slot.
Exclude physical drives or bays reserved for hot spares.
The operating reserve in this calculator is logical space left unfilled; it is not a spare physical drive.

Current data and net growth

Current usage is stored data, while annual net growth is new data less deletions.
If you retain another full backup on the same NAS every year, include that growth too.
New recording volume alone can overstate demand if files are regularly overwritten or expire.
Planned increases in video resolution can have the opposite effect if omitted.

TB and TiB

TB means 10^12 bytes and TiB means 2^40 bytes.
Drive inputs always use manufacturer TB; current data and annual growth share a selectable TB or TiB unit.
Switching the unit converts the existing values while preserving their physical quantity.
Check what your operating system actually reports.
A change of units is not lost storage and must not be counted again as RAID or formatting overhead.

RAID 0 sum convention

Let N be the active drive count, S the smallest drive size and R the sum of raw drive capacities.
RAID 0 needs at least two drives.
This calculator uses R as nominal capacity, following the Synology sum convention.
Some controllers constrain mixed-size RAID 0 to the smallest member; verify the documented convention for your device.

Mirror and parity capacity

RAID 1 models a two-to-four-drive mirror with nominal capacity S.
RAID 5 needs at least three drives and provides (N−1)×S.
RAID 6 needs at least four and provides (N−2)×S.
RAID 10 requires an even count of at least four drives arranged as paired mirrors, using (N/2)×S.
Odd-count implementations and unequal mirror-group arrangements are outside this model.

Failure tolerance conditions

RAID 5 is designed to tolerate one drive failure and RAID 6 two.
In the RAID 1 model, every active member holds the same data and one surviving member retains a copy.
For RAID 10, losing both drives in the same mirror pair can cause data loss.
Do not interpret half the drive count as a guarantee for arbitrary simultaneous failures.
These structural descriptions do not estimate recovery success probabilities.

Unused mixed-size space

For RAID 1, 5, 6 and 10, the model uses S from every active drive, so unused mixed-size space is R−N×S.
Subtract mirrors or parity from that participating capacity to obtain nominal capacity C.
RAID 0 shows zero mixed-size waste under the sum convention.
SHR automatically manages mixed sizes differently and must not be evaluated with this model.

Sequential allowances

With a system/filesystem allowance f and an operating reserve r, the planning limit is C×(1−f/100)×(1−r/100).
Do not simply add the percentages and apply them to raw capacity.
Reserve is calculated after the system allowance.
The result reconciles raw total to mixed-size unused space, mirrors/parity, system allowance, operating reserve and the remaining planning limit.

Product-specific assumptions

The default 4% allowance and 20% reserve are editable illustrations, not universal NAS specifications.
Check system partitions, filesystem metadata, application storage and snapshot retention on your product.
Including the same storage in both current usage and an allowance double-counts it.
Compression and deduplication savings are not assumed.

Target data and shortfall

For current data U, annual net growth G and target years T, future data equals U+G×T.
The calculator converts data to TB internally before comparing it with planning limit B.
Shortfall is max(0,U+G×T−B), and current headroom is B−U.
Negative headroom remains visible when you have already exceeded the chosen operating limit.

Time to operating limit

When current data is below the limit and growth is positive, time to the limit is (B−U)/G years.
Zero growth below the limit is shown as no growth.
Already being at or above the limit gives zero years, including when growth is zero.
This measures how long you can preserve the chosen reserve, not the date the disk becomes physically full or fails.

Limits of linear growth

The forecast assumes a constant amount of added data each year.
Recalculate after a camera upgrade, another business location or a retention-policy change.
A zero-year target checks current usage alone.
Annual growth is a capacity, not a percentage.
Treat the output as a planning horizon for purchasing and migration, not a precise calendar deadline.

Purchase combinations examined

Enter three candidate drive sizes.
For each size, the planner purchases only that size, replaces the smallest eligible existing drives and considers adding drives to empty bays.
It never replaces a drive with a smaller one or automatically changes RAID level.
Final layouts that violate drive-count requirements or the even-count RAID 10 rule are excluded.

Ordering and scope

Qualifying configurations are ordered first by drives purchased, then by total purchased TB, then by excess capacity above the target.
Up to six are displayed.
A no-purchase option is included when the current configuration is sufficient.
Because no prices are entered, the first option is not a lowest-cost recommendation.
The search does not cover every mixture of newly purchased sizes.

When no option qualifies

If none qualifies, the planner shows the maximum-capacity layout within the search range and its remaining shortfall.
Consider larger candidates, another bay count or a revised retention policy.
Increasing bay count in this comparison does not establish support for an expansion unit or new chassis.
Final-layout capacity and the ability to migrate an existing pool to that layout are separate questions.

Current layout: 9.216 TB

Four 4 TB drives in RAID 5 have 16 TB raw capacity, 4 TB parity and 12 TB nominal capacity.
A 4% allowance leaves 11.52 TB.
Preserving another 20% as operating reserve gives 9.216 TB, approximately 8.38 TiB.
Starting with 6 TB and adding 2 TB per year requires 12 TB after 3 years, leaving a 2.784 TB shortfall.
The operating limit is reached after approximately 1.608 years.

Replace all: 18.432 TB

With all four bays occupied, replacing only one drive with 8 TB leaves the smallest size at 4 TB.
RAID 5 nominal capacity stays at 12 TB and mixed-size unused capacity becomes 4 TB.
Replacing all four with 8 TB gives a final nominal capacity of 24 TB and a planning limit of 18.432 TB.
The gain is 9.216 TB, and the same growth assumption reaches the new operating limit after 6.216 years.

Add to a free bay: 12.288 TB

If a fifth bay is available, adding one 8 TB drive gives final nominal capacity of 16 TB and a planning limit of 12.288 TB.
It covers the 12 TB target with one purchase, so it can rank before replacement options.
This arithmetic does not establish that your NAS and pool support adding a drive.
These are verification examples, not product recommendations.

1. Verify the current layout

First inspect your storage manager for the actual RAID level and pool members.
Do not count cache SSDs or hot spares as data drives.
Enter installed capacities in bay order so replacement bay numbers can be checked against the chassis.
Reducing the bay count removes trailing entries, so verify the remaining layout afterwards.

2. Set demand and candidates

Confirm the unit of current usage, then enter annual net growth and target years.
Adjust allowances without double-counting and enter three drive sizes you could purchase.
Calculate to see current sufficiency, a RAID comparison and final purchase configurations.
Editing inputs clears the previous result and download button until you calculate again.

3. Save the TXT report

The TXT report contains inputs, allowances, current capacity breakdown, target shortfall, addition and replacement counts, replacement bays and final capacities.
Attach it to a quotation or an administrator query to discuss the same assumptions.
It records a snapshot of your inputs; recalculate when usage or supported product features change.

Family photos and video

For family photos, review how much photo and video storage increased over recent years.
Distinguish originals, edited copies and duplicate family uploads to estimate net growth.
Instead of lowering reserve to zero just to make an option qualify, examine deletion policies and a separate backup destination.

Small-business records

For a small business, estimate original records, accounting data, shared folders and version history separately.
Camera storage demand depends on recording settings and retention duration: estimate the data first, then compare RAID layouts here.
Video outage costs and the staff-time benefits of document management belong in the related cost calculators.

Reusing mixed-size drives

Reusing mixed-size drives may avoid purchases even when some capacity is unused.
Matching every drive size can simplify the layout but increase spending.
This tool does not assess price, electricity, performance or warranties.
Combine the capacity report with actual quotations and operating requirements before deciding.

Online expansion support

Online expansion depends on the NAS model, operating-system version, RAID level, pool health and documented procedure.
For example, the Synology guide states that a RAID 0 pool cannot grow online by adding drives or replacing them with larger ones.
A larger final capacity in this calculator is not an executable expansion procedure.
Recreating a pool and restoring data may require separate storage.

Independent backup and restoration

RAID offers some continuity against drive failures but is not an independent backup for accidental deletion, ransomware, theft or fire.
Snapshots retained only on the same device remain exposed to losing the whole device.
Before expansion, check a separate backup and your ability to restore the needed files.
This calculator does not design backup policies or guarantee recovery.

Hardware and model limits

Check drive compatibility, supported sizes, maximum single-volume and pool capacity, and expansion-unit conditions.
Input support for 2–24 bays and up to 100 TB per drive is a calculator range, not a product certification.
Drive technology, sector formats, degraded states, rebuild duration, compression and transfer performance are excluded.