NAS buying / backup planning

Compare RAID capacity with the backup workload you need to retain.

Estimate RAID 0, 1, 5, 6, or 10 usable capacity in decimal TB and TiB, then compare it with a user-entered backup retention plan.

Your inputs stay in your browser

Your plan

01 / INPUTS

Drive countdrivesNumber of same-capacity drives for this simplified estimate. RAID 10 uses an even count here.Capacity per drivedecimal TBUse the advertised decimal capacity per drive. Mixed capacities are not modeled.RAID layoutSelect the layout you are comparing. Minimum-drive rules are validated.NAS free-space reserve%Capacity you do not plan to fill, for filesystem behavior, snapshots, and operational headroom.Full backup datadecimal TBData set size for one full backup copy.Changed data per dayGB/dayAn estimate for daily changed data retained by your backup method.Retention windowdaysNumber of daily change increments included in the estimate.Independent backup copiescopiesHow many copies of the estimated backup set to plan for. RAID members are not counted as independent copies.Backup overhead%Reserve for metadata, versioning, or the behavior of your chosen backup software.

Illustrative example

Keep a copy of your inputsNo automatic saving. Files and photos are not stored.

ArrayKeep Storage Planner

Calculated from user-entered inputs. Review assumptions and limitations.

Plan summary

02 / RESULTS

Live comparison

Backup capacity plan

Estimated backup requirement6.05 decimal TB

Usable NAS capacity (decimal TB)21.6 decimal TB

Space after backup estimate15.55 decimal TB

Shared scale: 0 to 21.6 decimal TB. Values may overlap; bars are comparisons, not parts of a total. Review the details below.

Usable NAS capacity (decimal TB)21.6

Usable NAS capacity (TiB)19.65

Estimated backup requirement6.05

Space after backup estimate15.55

Raw RAID capacity24

Selected RAIDRAID 5

Capacity and retention estimate
Item	Value	Unit
Drive set	4	drives
Per-drive capacity	8	decimal TB
RAID usable before reserve	24	decimal TB
Reserve applied	10	%
Full backup data	4	decimal TB
Changed data in retention window	1.5	decimal TB
Independent backup copies	1	copies
Backup overhead	10	%
Estimated backup requirement	6.050000000000001	decimal TB

Selected drive count across RAID layouts
RAID	Minimum drives	Usable decimal TB	Usable TiB	Status
RAID 0	1	28.8	26.193447411060333	Estimate
RAID 1	2	7.2	6.548361852765083	Estimate
RAID 5	3	21.6	19.64508555829525	Estimate
RAID 6	4	14.4	13.096723705530167	Estimate
RAID 10	4	14.4	13.096723705530167	Estimate

RAID redundancy is not a backup. Keep an independent backup copy and test restoration.

Decimal TB uses 1,000 GB per TB. TiB is shown separately using 1 TiB = 1,099,511,627,776 bytes.
The estimate reserves 10% of RAID capacity before comparing the backup workload.
Retention arithmetic treats daily changed data as 50 GB per day and multiplies the resulting backup set by 1 independent copies.
RAID level behavior, filesystem overhead, snapshots, compression, deduplication, and mixed drive sizes can change actual capacity.

Know the boundaries

This simplified model assumes equal-capacity drives and does not verify NAS bays, drive support, filesystem behavior, snapshots, compression, or backup software.
RAID improves availability or fault tolerance depending on layout, but it is not an independent backup and does not guarantee recovery.
No live drive inventory, prices, affiliate catalog, or paid API is used.

Examples are illustrative. Results depend on the assumptions and measurements you enter.

Method, not guesswork

ArrayKeep connects a drive purchase question to a retention workload. First calculate a conservative usable array capacity, then model full data, daily changes, copies, and overhead. Keep the backup distinction visible throughout.

Read the methodology and worked examples

From inputs to a useful decision

All guides

Practical guide

Estimate usable NAS capacity from a RAID layout

Use equal-drive assumptions and a free-space reserve to compare raw RAID capacity with usable planning space.

Read guide Practical guide

Compare NAS capacity with a retention workload

Model full backup data, daily changes, copies, and overhead before deciding how much space the plan needs.

Read guide

Common questions

How does RAID 5 capacity work here?

For equal drives, the simplified model uses drive count minus one multiplied by drive capacity, then applies the entered reserve percentage.

How is backup requirement estimated?

The engine adds daily changes across retention days to full data, multiplies by copies, and applies backup overhead.

Is RAID counted as a backup copy?

No. The planner keeps array capacity and backup workload separate so an independent backup and restoration review remain visible.