RAID Usable Capacity Calculator

How this is calculated

USABLE CAPACITY DEPENDS ENTIRELY ON THE LEVEL, and the differences are large. RAID 0 gives you everything and protects nothing. RAID 5 gives up one drive's worth to parity. RAID 6 gives up two. RAID 10 gives up half. RAID 1 gives you the size of a single drive no matter how many you mirror. ⚠️ RAID IS NOT A BACKUP, and this is the single most expensive misunderstanding in storage. It protects against a DRIVE failing. It does nothing about deleting the wrong folder, ransomware, a corrupt file written correctly to every disk, a failed controller, theft, fire or flood - and in every one of those cases the redundancy faithfully replicates the problem. An array with no separate backup is one mistake from total loss regardless of its level. ⚠️ YOUR OPERATING SYSTEM WILL REPORT LESS THAN THIS AND NOTHING IS WRONG. Drives are sold in decimal terabytes of a trillion bytes; Windows reports TEBIbytes of 1,099,511,627,776 bytes and labels them TB. That is a fixed 9.1% gap, so 20 usable TB shows up as roughly 18.2. Formatting, filesystem metadata and any reserved space come off after that, so expect a little less again. MINIMUM DRIVE COUNTS ARE ENFORCED HERE. RAID 0 and 1 need two drives, RAID 5 needs three, and RAID 6 and 10 need four. Ask for fewer and the capacity outputs return a dash rather than a plausible-looking number, because an array that cannot exist has no capacity. RAID 10 WITH AN ODD NUMBER OF DRIVES ROUNDS DOWN. Mirrored pairs need an even count, so the spare drive contributes nothing to capacity - it is only useful as a hot spare. The tool floors it rather than reporting half a drive of storage. THE DRIVES-CAN-FAIL FIGURE IS THE GUARANTEED ONE. RAID 10 can sometimes survive more than one failure if the losses fall in different mirror pairs, but it can also die on the second drive if you are unlucky, so one is the number to plan against. RAID 1 across many drives is the exception - it genuinely survives until one drive is left. ⚠️ COST PER USABLE TERABYTE IS THE HONEST COMPARISON BETWEEN LEVELS, and it is why RAID 6 looks worse than it is. Fewer, larger drives usually beat more small ones on this figure, but they also make a rebuild slower and riskier - which is a separate calculation and the reason the rebuild time tool exists.

Common questions

Is RAID a backup?

No, and treating it as one is the most expensive misunderstanding in storage. RAID protects against a DRIVE failing - the array keeps serving data while you replace the dead disk. It does nothing whatsoever about deleting the wrong folder, ransomware encrypting your files, a corrupt file being written correctly to every disk, a controller failure, theft, fire or flood. In every one of those cases the redundancy works exactly as designed and faithfully replicates the problem across all your drives. An array without a separate backup, ideally one that is off-site or offline, is one mistake away from total loss no matter which level you picked.

Why does my OS report less space than this?

Because drives are sold in decimal terabytes of exactly a trillion bytes, while Windows reports tebibytes of 1,099,511,627,776 bytes and labels them TB anyway. It is a fixed 9.1% gap, so 20 usable TB appears as roughly 18.2 - and that is the same number of bytes described in two different units, not missing space. Formatting overhead, filesystem metadata and any reserved capacity come off after that, so expect a little less again. The tool reports both figures side by side so the number you see in the operating system is not a surprise.

RAID 5 or RAID 6?

Compare them on cost per usable terabyte and the decision gets much easier. On six 4 TB drives, RAID 5 gives 20 TB usable and RAID 6 gives 16 - which sounds like a big sacrifice until you price it: 30 against 37.50 per usable terabyte at 100 a drive. Twenty-five percent more, for double the fault tolerance. The reason that premium is usually worth paying is the rebuild: when a drive dies in a RAID 5 array, every remaining drive must be read completely to reconstruct it, and a second failure during that window loses everything. With large modern drives that window is long, which is what our RAID rebuild time calculator is for.

Why does RAID 1 give me so little?

Because every drive holds a complete copy, so the usable capacity is the size of ONE drive no matter how many you add. Six 4 TB drives in RAID 1 gives 4 TB usable at 150 per usable terabyte - five times the cost of RAID 5 on identical hardware. What you buy for that is extreme fault tolerance: five of the six drives can fail and the data survives. That trade is sensible for two drives holding something critical and almost never sensible beyond that, which is why RAID 10 exists - it mirrors in pairs and stripes across them, keeping half your capacity instead of a sixth.

What happens with an odd number of drives in RAID 10?

The spare one contributes nothing to capacity, and the tool rounds down rather than reporting half a drive of storage. Five 4 TB drives in RAID 10 gives 8 TB usable, exactly the same as four would - the fifth is only useful as a hot spare that automatically takes over when one of the others dies. That is not a bad use for it, but it should be a deliberate choice rather than a surprise, and it is worth knowing before buying an odd number of disks.

How many drives can actually fail?

The figure shown is the GUARANTEED number, which is the one to plan against. RAID 5 survives one, RAID 6 survives two, and RAID 0 survives none - lose any drive and the whole array is gone. RAID 10 is the interesting case: it might survive several failures if they happen to fall in different mirror pairs, but it can also die on the second drive if both losses hit the same pair, so one is the honest number. RAID 1 is the exception where the count really does scale, surviving until a single drive is left holding the data.

Are fewer large drives better than more small ones?

On cost per usable terabyte, usually yes - larger drives are generally cheaper per terabyte and a parity level gives up a fixed NUMBER of drives rather than a fixed share, so a parity drive costs proportionally less in a bigger array. But capacity is not the only axis. Larger drives take longer to rebuild after a failure, and a longer rebuild is a longer window in which a second failure is fatal, so the level that looked adequate on a small array may not be on a large one. That trade is a separate calculation, and it is the reason to look at rebuild time before settling on a layout.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the RAID Usable Capacity Calculator at https://www.bfcbrilliance.com/tools/raid-usable-capacity-calculator.

What I entered:
- RAID level: ___
- Number of drives (drives): ___
- Size of each drive (TB): ___
- Price per drive ($): ___

What it calculated:
- Usable capacity: ___
- What the OS will report: ___
- Raw capacity you paid for: ___
- Given up to redundancy: ___
- Share of what you bought that is usable: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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