How a RAID Calculator Works
You build a storage array by combining several drives into one logical unit. A raid calculator takes the number of disks you plan to use, the size of each disk, and the RAID level you pick. It then returns the usable space you actually get to store files on. Raw capacity is the simple sum of every disk. Usable capacity is what remains after the array reserves room for mirroring or parity.
Start by sizing your array on paper. Work out how much space your photos, videos, and backups need today, then add room for growth over the next two or three years. Once you know the target capacity, head to bestdiskprices.com and check live NAS and hard drive prices so you can buy the right disks at the lowest cost.
The math behind every calculator is simple once you see the pattern. Most levels follow one formula based on the count of drives, which we write as N, and the size of each drive, which we write as D. The examples below use four 4TB disks, so N equals 4 and D equals 4TB.
RAID 0: Full Speed, Zero Safety
RAID 0 stripes your data across all disks. It writes block one to disk one, block two to disk two, and keeps rotating. This gives you the fastest possible read and write speeds and the most space.
The formula is N multiplied by D. With four 4TB drives you get 4 x 4TB, which equals 16TB of usable space. Nothing is held back for protection. The catch is brutal: if any single disk fails, you lose every file on the array. RAID 0 is the right choice only for scratch space, video editing caches, or temporary renders where you already keep the source elsewhere.
You should never use RAID 0 for a backup target. The moment one drive dies, the whole volume is gone. If you want speed and still care about your data, look at RAID setups built for content creators instead.
RAID 1 and RAID 10: Mirroring for Protection
RAID 1 mirrors your data. Every disk holds an exact copy of every other disk. With two drives of 4TB you get just 4TB of usable space, because the second disk is a clone. If you run four 4TB disks in a pure mirror, the calculator still returns 4TB usable, but you can lose up to three disks and keep your files as long as one survives.
RAID 10 fixes the wasted space problem by pairing disks and then striping the pairs. You take N drives, split them into mirrored pairs, and keep half the raw total. The formula is (N divided by 2) multiplied by D. Four 4TB disks become two mirrored pairs, so you get (4 / 2) x 4TB, which equals 8TB usable. You can lose one disk from each pair and the array stays alive.
RAID 10 is popular on databases and virtualization hosts because it is fast and resilient. It needs an even number of disks. The cost is that you give up half your raw capacity to mirroring.
RAID 5 and RAID 6: Parity That Saves Space
RAID 5 uses distributed parity. It spreads checksum data across all disks so that any one disk can fail without losing the volume. The formula is (N minus 1) multiplied by D. Four 4TB drives give you (4 - 1) x 4TB, which equals 12TB usable. You keep three quarters of your raw space and still survive a single disk failure.
This is the setup most home NAS owners pick, and it is the one people mean when they search calcul raid 5. The parity overhead is small, so you get both capacity and safety. Rebuilding after a failure is slower on large disks, but the trade is worth it for most users.
RAID 6 adds a second parity block. The formula is (N minus 2) multiplied by D. Four 4TB drives give you (4 - 2) x 4TB, which equals 8TB usable. You now survive two disk failures at once. That matters when you run big 20TB or 24TB drives, because a rebuild can take days and a second failure during that window would wipe a RAID 5 volume. If you want to dig deeper into how each RAID level works, our full guide covers the tradeoffs in detail.
Worked Example: Four Drives at 4TB Each
Let us run the numbers side by side so you can see the real cost of protection. You buy four 4TB disks. Your raw total is 16TB no matter what. Here is what each level leaves you to use.
| RAID level | Formula | Raw capacity | Usable capacity | Drive failures tolerated |
|---|---|---|---|---|
| RAID 0 | N x D | 16TB | 16TB | 0 |
| RAID 1 | D | 16TB | 4TB | Up to 3 |
| RAID 5 | (N-1) x D | 16TB | 12TB | 1 |
| RAID 6 | (N-2) x D | 16TB | 8TB | 2 |
| RAID 10 | (N/2) x D | 16TB | 8TB | 1 per pair |
The table shows the heart of the decision. RAID 0 gives you all 16TB but no safety. RAID 5 keeps 12TB and survives one failure, which is the best balance for most people. RAID 6 and RAID 10 both land at 8TB, but they protect you differently. RAID 6 tolerates any two disks dying. RAID 10 tolerates one disk in each mirror pair, which can mean two failures if they hit separate pairs.
Now apply the same formulas to a bigger build. Imagine eight 8TB disks. RAID 0 returns 64TB. RAID 5 returns (8 - 1) x 8TB, or 56TB. RAID 6 returns (8 - 2) x 8TB, or 48TB. RAID 10 returns (8 / 2) x 8TB, or 32TB. The gaps grow with disk count, so your choice shapes both your budget and your risk.
Real disk sizes never match their label exactly. A 4TB drive shows about 3.64TB in your operating system because vendors count in decimal terabytes while computers count in binary tebibytes. Plan for roughly 9% less than the box claims. A calculator that uses true binary math will show that gap for you.
Pick Your Drives and Check Live Prices
You now know how to calculate storage capacity for every common RAID level. The next step is buying the disks. Sizing first protects you from two mistakes: buying too little space and having to rebuild, or buying far too much and wasting money.
Decide your level, run the numbers above, and write down the usable total you need. Then visit bestdiskprices.com to compare live prices on NAS-grade hard drives and full enclosures. The site tracks deals across retailers every day, so you can match your capacity plan to the cheapest healthy disks on the market. Pair that with a read of our ultimate NAS guide and our home NAS optimization tips to finish the build with confidence.
One last rule worth repeating: RAID is not a backup. Mirroring and parity keep your array running through a disk failure, but they will not save you from ransomware, theft, or a flooded office. Keep an offsite or cloud copy of anything you cannot replace. Use the calculator to size the primary array, then spend the savings on a real backup.
