Capacity math
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NAS storage planning console
Plan usable NAS storage before you buy disks.
Calculate RAID and ZFS capacity, compare failure tolerance, follow the buying guide path, and turn a drive-count idea into a safer NAS hardware shortlist.
Planning snapshot
Capacity, risk, then purchase
Capacity
VisibleRaw, usable, reserve, and efficiency stay separate.
Risk
ExplicitDisk tolerance is not presented as a backup plan.
Next step
GuidedMove from capacity math to hardware checks.
01
Calculate usable TB
Model raw capacity, parity overhead, reserve, and failure tolerance.
02
Choose RAID level
Compare RAID 5, RAID 6, RAID 10, RAID-Z1, and RAID-Z2 by workload.
03
Build buying list
Match bay count, CMR drives, UPS, backup, and expansion path.
Start the 8-step NAS guide path
Use the calculator result, then walk the buying decisions in order.
The guide path turns a raw capacity estimate into a safer NAS purchase plan: RAID level, bay count, CMR drives, TrueNAS choices, backup, and UPS.
Trust layer
NAS planning guidance users can audit before buying disks
This layer explains what the calculator actually estimates, where RAID planning stops, and which hardware checks should happen before opening product searches.
Risk language
RAID is framed as availability, not backup
The page repeatedly separates disk failure tolerance from deleted files, ransomware, fire, theft, and restore testing.Purchase filters
Hardware choices are tied to workload constraints
Bay count, CMR drive class, UPS, backup target, noise, warranty, and expansion path are presented before product-category searches.Method
How the capacity decision works
Raw capacity Drive count multiplied by per-drive TB gives the starting pool size.
Parity overhead RAID/ZFS mode determines how many disks are reserved for mirror, parity, or striped layout behavior.
Usable reserve A planning reserve keeps the result from pretending every formatted terabyte should be filled.
Failure tolerance The output separates usable capacity from how many drives can fail before data is at risk.
Pre-action checks
Check these before trusting the array
- Prefer CMR NAS drives for RAID and ZFS pools; avoid surprise SMR drives for parity rebuild workloads.
- Budget for at least one independent backup target because RAID protects availability, not deleted files or ransomware.
- Check bay count, expansion path, power draw, noise, network speed, and replacement-drive availability before buying disks.
- Keep filesystem reserve and snapshot growth in the capacity plan instead of filling the pool to the headline usable TB.
- Add UPS protection early, especially for parity arrays and ZFS pools that should shut down cleanly.
NAS links are currently neutral product-category searches. Treat them as comparison prompts, then verify CMR/SMR status, bay count, warranty, UPS support, backup target, and restore process before buying.
DisclosureEditorial method
What this calculator can—and cannot—decide
The capacity model makes drive count, drive size, RAID layout, and reserve visible. It is a planning aid: it does not predict exact performance, rebuild duration, hardware compatibility, or the probability of data loss for a specific system.
Last reviewed: July 10, 2026. Product links remain neutral category searches until a partner relationship and page-level disclosure are in place.
OpenZFS checksums and scrubs Background on checksum verification, scrubs, and repair in suitably redundant ZFS pools. Read primary source ↗ TrueNAS data protection Official overview of scrubs, S.M.A.R.T. tests, snapshots, replication, and backup tasks. Read primary source ↗ CISA ransomware guidance Why recoverable, regularly tested backups should be separate from array redundancy. Read primary source ↗
NAS Cluster Guides
Use the learning path after the calculator result
Step 1
Buying Checklist
A practical NAS RAID buying checklist for choosing drive count, bay count, CMR drives, UPS protection, backup targets, and RAID level before purchasing storage hardware.
Step 2
RAID 5 vs RAID 6
Compare RAID 5 and RAID 6 for NAS usable capacity, rebuild risk, parity overhead, drive count, and home-server buying decisions.
Step 3
RAID 10 vs RAID 5
Compare RAID 10 and RAID 5 for home servers, including usable capacity, rebuild behavior, random I/O, drive failure tolerance, and budget tradeoffs.
Step 4
RAID-Z1 vs RAID-Z2
Compare RAID-Z1 and RAID-Z2 for TrueNAS and ZFS pools, including usable capacity, vdev planning, rebuild exposure, scrubs, checksums, and expansion tradeoffs.
Step 5
4-Bay Planning
Plan a 4-bay NAS by comparing RAID 1, RAID 5, RAID 6, RAID 10, RAID-Z1, and RAID-Z2 capacity, redundancy, and upgrade constraints.
Step 6
6-Bay RAID-Z2
Plan a 6-bay NAS or TrueNAS RAID-Z2 build with usable capacity, dual parity, CMR drive selection, UPS, backup, and future expansion checks.
Step 7
CMR vs SMR
Understand why CMR and SMR hard drives matter for NAS RAID, ZFS pools, parity rebuilds, write workload behavior, and safer drive shopping.
Step 8
Backup vs RAID
Learn how NAS backup and RAID differ, why redundancy is not backup, and how to plan local, offsite, snapshot, and restore checks for home storage.
High-intent NAS decisions
Answer buying questions before visitors choose hardware
These pages target decision searches like RAID 5 vs RAID 10, RAID-Z1 safety, SHR expansion, and CMR vs SMR drive buying risk.
4-bay RAID 5 vs 10
4-Bay NAS RAID 5 vs RAID 10
Choose RAID 5 or RAID 10 for a 4-bay NAS by usable capacity, rebuild behavior, random I/O, backup needs, and drive buying trade-offs.
RAID-Z1 8TB safety
Is RAID-Z1 Safe for 8TB Drives?
Decide whether RAID-Z1 is safe enough for 8TB NAS drives by rebuild exposure, backup quality, pool width, downtime tolerance, and RAID-Z2 alternatives.
SHR vs RAID 5
Synology SHR vs RAID 5 for Home NAS
Compare Synology SHR and RAID 5 for home NAS buyers by mixed drive sizes, usable capacity, expansion path, portability, and backup requirements.
CMR vs SMR decision
CMR vs SMR NAS Drive Buying Decision
Choose CMR or SMR drives for NAS RAID and ZFS by rebuild behavior, write workload, parity risk, price, and vendor specification checks.
2-bay vs 4-bay NAS
2-Bay vs 4-Bay NAS for Home Backup
Choose a 2-bay or 4-bay NAS for home backup by usable capacity, RAID options, upgrade path, noise, cost, and independent backup needs.
Plex NAS RAID
Best RAID for a Plex Media Server NAS
Choose a NAS RAID layout for Plex media by usable capacity, rebuild risk, streaming workload, metadata growth, backup priority, and drive buying strategy.
6-bay RAID 6 vs Z2
RAID 6 vs RAID-Z2 for a 6-Bay NAS
Compare RAID 6 and RAID-Z2 for a 6-bay NAS by platform choice, usable capacity, checksums, snapshots, rebuild behavior, and buying constraints.
NAS UPS decision
NAS UPS Buying Decision for RAID and ZFS
Decide when a NAS needs a UPS by write safety, clean shutdown, ZFS behavior, RAID rebuilds, backup jobs, runtime, and USB shutdown support.
Three-tier buying list
Pick a NAS shopping path after the capacity result
Before checkout
These are non-affiliate category search links for now. The goal is to guide users toward safer product categories, then replace the URLs later when Amazon Associates is ready.
- Prefer CMR NAS drives for RAID and ZFS pools; avoid surprise SMR drives for parity rebuild workloads.
- Budget for at least one independent backup target because RAID protects availability, not deleted files or ransomware.
- Check bay count, expansion path, power draw, noise, network speed, and replacement-drive availability before buying disks.
- Keep filesystem reserve and snapshot growth in the capacity plan instead of filling the pool to the headline usable TB.
- Add UPS protection early, especially for parity arrays and ZFS pools that should shut down cleanly.
Starter
4-bay NAS baseline
A compact first NAS plan for backups, media, documents, and a small protected home-storage setup.
First NAS or light homelab storage
- • Use the calculator before choosing between RAID 5, RAID 6, RAID 10, or RAID-Z2.
- • Keep one backup target outside the NAS before storing irreplaceable files.
- • Confirm drive trays, memory ceiling, network speed, and noise expectations.
Recommended
6-bay dual-parity plan
A more durable path for RAID 6 or RAID-Z2 buyers who want usable capacity without relying on single parity.
RAID 6, RAID-Z2, media libraries, and backups
- • Model usable capacity after two parity drives and at least 10% reserve.
- • Prefer CMR NAS drives for parity rebuild, scrub, and resilver workloads.
- • Plan the next expansion before all six bays are already occupied.
Resilient
TrueNAS or rebuild-safe setup
A cautious build path for larger pools, ZFS users, VM storage, long rebuild windows, and serious backup planning.
TrueNAS, ZFS, VM storage, and critical files
- • Decide vdev shape and backup destination before buying a full drive set.
- • Budget for UPS shutdown support, spare drive access, and tested restore workflow.
- • Use RAID-Z2 or RAID 6 as a baseline when rebuild exposure feels unacceptable.
NAS sizing console
Model the array, then pressure-test the buying decision.
Start with drive count, drive size, RAID/ZFS mode, and reserve. The console keeps inputs, usable TB, fault tolerance, and RAID 10 vs parity comparison in one decision surface.
1. Array Inputs
Quick Presets
2. Capacity Result
Raw Capacity
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Usable Capacity
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Fault Tolerance
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Efficiency
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3. RAID 10 vs RAID 5 Snapshot
| Mode | Usable | Tolerance | Efficiency |
|---|
Common capacity examples
Start with a familiar array shape, then return to the calculator to change the inputs for your own hardware plan.