NAS and external storage planning

RAID Calculator

Compare usable storage across drive sizes and RAID types.

Start calculating

Estimate usable NAS and external RAID storage with our RAID Calculator. Choose your enclosure size, set the capacity of each HDD or SSD, and compare RAID 0, 1, 5, 6, 10, SHR and SHR-2. The calculator separates usable storage, drive-failure protection, unused capacity and your snapshot allowance.

How to use this RAID Calculator
  1. Choose your enclosure and drive type. The starting example has eight bays, four 4 TB HDDs and four empty bays.
  2. Select a bay, then set its capacity. Choose Empty bay for an unused slot. You can enter different sizes in different bays.
  3. Compare RAID types and the colour bar. Review capacity after RAID and after the snapshot allowance. Adjust the allowance under Advanced settings.
  4. Save your plan by email. Choose Email my results, enter your email and select Send my results. Choose Ask ACE for advice if you would like a consultation.
Front view of a Ubiquiti UNAS Pro seven-bay rackmount NAS
Hardware example: Ubiquiti UNAS Pro, 7 bays. Select your own enclosure size below.
4 of 8 bays usedSelect a bay to change its drive.
Bay 1 · HDD
Set several drives at onceOptional
Advanced settingsOptional
Automatic splits RAID 5/6 into groups of up to 16 drives. Capacity shown is the sum of the separate groups.
Installed capacity
16 TB
After RAID
12 TB
Usable after 5% snapshots
11.4 TB
10.37 TiB
New-pool estimates · System/filesystem overhead excluded · Model and drive compatibility require checking · RAID is not a backup

Save your results or get advice

Keep your storage plan handy, or ask our team to review it with you.

How to calculate RAID capacity

For a conventional RAID 5 group, capacity before system overhead is the number of drives minus one, multiplied by the smallest drive's capacity. RAID 6 subtracts two drives. The default four 4 TB drives provide 16 TB installed and approximately 12 TB after RAID 5, before system overhead and any snapshot allowance. The four empty bays add no capacity.

With four 4 TB drives in RAID 5 and a 5% snapshot allowance, this calculator shows 11.4 TB for data, approximately 10.37 TiB. Set the snapshot allowance to 0% if it does not apply to your layout. System partitions and filesystem metadata can reduce the final capacity shown by your device.

For mixed drive sizes, set every bay individually. The unused-capacity bar shows space that the selected layout cannot use. TB uses decimal units; TiB uses binary units. These unit differences are separate from capacity used by RAID or the filesystem.

RAID planning for popular NAS brands

Synology: Compare standard RAID with SHR or SHR-2 when those options are supported by your NAS. SHR can make better use of some mixed-capacity combinations. This calculator estimates a new storage layout; it does not determine whether an existing pool can be expanded or migrated. Synology also reserves system and filesystem capacity, so the final number can differ from this estimate.

QNAP: Compare HDD storage and SSD storage separately in Hybrid mode. SSDs assigned as cache do not add to the storage result. Automatic grouping divides RAID 5/6 layouts into groups of up to 16 drives; this is a modelling choice aligned with the limit described in QNAP's calculator, not a universal limit for every NAS. Check your exact QNAP model, operating system and storage-pool layout.

ASUSTOR: Select a standard RAID layout supported by your exact model, then enter its planned drives. Check the product's supported RAID types, drive compatibility and volume limits before purchasing. The calculator's capacity result is not a model compatibility check.

Ubiquiti UniFi UNAS: Choose the standard RAID layout that matches your device's protection setting. Supported options and group sizes vary by model. For example, Ubiquiti documents RAID 5 for Basic Protection and RAID 6 for Advanced Protection on supported UNAS configurations. Confirm the drive count and protection mode in your device settings; this calculator does not reproduce every UniFi storage policy.

TerraMaster and UGREEN NASync: Use the standard RAID mode supported by your chosen model. Confirm enclosure limits and drive compatibility before purchasing. Vendor-specific automatic storage modes are not simulated by selecting a standard RAID mode here.

G-DRIVE, G-RAID and LaCie external storage

G-DRIVE and G-RAID: A single-drive G-DRIVE is external storage, not a multi-drive NAS or RAID array. For that layout, choose one bay and No RAID. G-RAID multi-drive products can be estimated using their supported RAID mode and the capacity of each internal drive. Check the exact model and set snapshot allowance to 0% if it is not used.

LaCie: Multi-drive products such as 2big RAID are direct-attached storage enclosures. Choose the RAID mode supported by the exact enclosure and enter each internal drive separately, rather than entering the advertised total capacity into every bay. Single-drive LaCie products do not gain RAID protection from this calculator.

RAID Calculator questions

Can I mix different drive capacities?

Yes, the calculator accepts a different capacity in each bay. Conventional RAID 5 and RAID 6 calculations use the smallest drive in each group, leaving some capacity on larger drives unused. SHR and SHR-2 may use more of that space on compatible Synology systems. Actual capacity depends on the full combination of drives.

Is No RAID the same as JBOD?

In this calculator, No RAID means separate, unprotected drives. JBOD means a combined, unprotected storage space. Both lack drive-failure protection, but they organise the storage differently. Confirm the terminology used by your device.

Can I use HDDs and SSDs together?

Use Hybrid mode to plan HDD and SSD storage separately. If SSDs are used for cache, their capacity is excluded from usable storage. The calculator does not model every vendor's tiering, cache or performance behaviour.

Does choosing RAID 5 mean it is the best choice?

No. RAID 5 is our initial calculator setting and needs at least three drives per group. The appropriate layout depends on the NAS, workload, recovery requirements and drive count. RAID protection also does not replace an independent backup.

Does a 40 TB option mean my NAS supports a 40 TB drive?

No. The calculator includes planning capacities up to 40 TB per HDD and enclosures up to 48 bays. These options do not confirm product availability, supported drive models or the capacity limits of a particular enclosure.

Browse related storage products

Explore storage hardware from Synology, QNAP, ASUSTOR and Ubiquiti, or browse our hard drives.

Need help choosing storage?

Use Ask ACE for advice to discuss your NAS, RAID layout and drives. Let us know if you also need cabling or installation. For surveillance retention planning, use our NVR Calculator.