Read on 2026-10-16. The sizing formulas come from a guide written for offices. The calculator below runs the same sums on your numbers; it does not know your NAS model's own rules.
Add up what you store today
Backblaze, in its NAS buying guide published May 30, 2024, says that you can quickly estimate how much storage you'll need by adding up the hard drives and external drives of all the systems you'll be backing up, adding the amount of shared storage you'll want to provide to your users, and factoring in any growing demand you project for shared storage. It adds that if you have any historical data under management from previous years, you can calculate a simple growth rate. It also says that generally, plan for systems that are two to four times your current data capacity. The same post says that for example, if your total storage needs amount to 20TB, double that to 40TB to account for growth.
Turn capacity into drives
Backblaze, in the same guide, says that its second formula is ((NAS Storage Needed / Hard Drive Size) + 2 Parity Drives) = Drive Bays Needed. It adds that in its worked example, a need of 36TB with 12TB drives and two additional drives for RAID 6 comes to a NAS that can support five or more drive bays of 12TB hard drives. It also says that you can partially fill the drive bays.
Backblaze, in its post “NAS RAID Levels Explained”, says that mirroring requires twice as many disks for the same total storage capacity. It adds that 67% of a three-drive RAID 5 array's disk space is available for data, compared with just 50% of the RAID 1 array's disk space. It also says that with RAID 1+0, only 50% of the total disk space is usable.
Leave room for the other copies
Backblaze, in its 3-2-1 post, says that the three copies of your data in a 3-2-1 backup include your original data (also called production data), plus two more copies.
Storage calculator
Drives for your data
Enter your own numbers. Growth factor 2 doubles today's data; the source suggests two to four times.
- Capacity to shop for
- Drives in the box
- Usable space after protection
- Each extra backup copy
Capacity to shop for = data x growth. Drives = capacity / drive size, rounded up, plus one drive for RAID 5, two for RAID 6, and doubled for RAID 1 and RAID 10. A 3-2-1 plan needs two more copies of the data. Check the minimum number of drives your NAS needs for a level. Nothing you type leaves your browser.
Sources
Checked on 2026-10-16 against Backblaze: NAS Buying Guide, Backblaze: NAS RAID Levels Explained, Backblaze: The 3-2-1 Backup Strategy. Read the source again before you rely on a number, because posts get updated and drive models change.
Related guides
- RAID levels explained: Striping, mirroring and parity, and what each level tolerates.
- NAS or external hard drive: What each one is and what neither protects against.
- SSD vs HDD for home storage: How the two drive types differ, from the source's own comparison.