Matching Usable TiB in an RFP to the TB on a Vendor Quote
The requirement says the array must deliver 500 TiB usable. Three responses come back and none of them uses that unit: one quotes raw terabytes, one quotes usable terabytes after its own RAID assumptions, and one leads with an effective capacity figure that already bakes in a promised data reduction ratio. Before any of them can be scored, every number has to be pulled back to the same unit and the same definition.
The Spec Fixes the Unit
The Quote Uses the Sales Unit
Three Words, Three Numbers
The unit gap is not a rounding error at procurement scale. On a 500 TiB requirement it is about 50 TB — often a full shelf of drives, and a real price difference between two bids that look identical on the summary page.
Checking a Bid Against the Requirement
Enter the usable figure your specification demands
Type the TiB number from the requirement into the left field. It converts as you type, so the terabyte equivalent is on screen before you open the first response. Comma or dot both work as the separator.
Find the matching line in each response
Compare like with like: the vendor's usable-after-protection number against your converted figure. If a bid gives only raw capacity, note it separately — it will always look generous and it is not what you are buying.
Convert the other way for the vendor's figures
Both fields are editable, so a quoted TB number typed on the right shows what the array will actually report in TiB once racked. The swap button flips the page to TB-first for a stack of responses.
Paste converted figures into the scoring sheet
The copy button on each field gives the bare number with no unit and no thousands spacing, so a comparison matrix or a clarification e-mail gets a clean value. Ctrl+C in a field does the same.
Both unit menus are searchable and list every storage unit, so a response that mixes in GiB per volume or PB for the archive tier is handled by retargeting one side rather than reaching for a second tool mid-evaluation.
Array Usable TiB Against Quoted TB
Each usable requirement below is converted into the terabyte figure a bid will express it in, alongside roughly how much raw capacity has to sit behind it under a common 8+2 erasure-coding layout that loses 20 % to parity. Real quotes also deduct hot spares and a system reserve, so read the raw column as a floor.
| Usable capacity in the RFP | Same figure in TB | Raw TB behind it at 8+2 (before spares) |
|---|---|---|
| 10 TiB | 10.995 TB | 13.744 TB |
| 20 TiB | 21.990 TB | 27.488 TB |
| 50 TiB | 54.976 TB | 68.719 TB |
| 100 TiB | 109.951 TB | 137.439 TB |
| 250 TiB | 274.878 TB | 343.597 TB |
| 500 TiB | 549.756 TB | 687.195 TB |
Read the bottom row as a procurement statement. A 500 TiB usable requirement is a purchase of roughly 687 TB of raw media before a single spare — around 39 drives of 18 TB, realistically 40 or 41 once the spare policy applies. A bid reaching 500 TiB usable from noticeably less raw media is assuming something, and the evaluation should find out what.
Normalise every bid to one unit
Live bidirectional fields let you convert each capacity line as you meet it while reading a response, instead of collecting them all and doing a conversion pass afterwards.
Precision the negotiation deserves
Results carry up to eight decimals, so 250 TiB resolves to 274.877906944 TB rather than a rounded 275 — the detail a contractual capacity clause should be written at.
Nothing about the tender leaves the page
Bid figures are commercially sensitive while an evaluation is open; the conversion runs locally in your browser once the page has loaded and nothing is sent anywhere.
Capacity Questions That Decide a Storage Award
Why does a storage RFP specify usable capacity in tebibytes?
Because TiB is the unit the delivered system will report, which makes the requirement testable. If the specification says 500 TiB usable, the acceptance criterion writes itself: mount the array, read the reported capacity, compare. Written in TB, an argument about measurement convention is available to anyone who comes up short — and that argument is worth almost 10 % of the contract. Specifying TiB also forces each bidder to do the conversion at their end, where they hold the layout details.
What does a raw capacity figure on a quote leave out?
Everything between the drives and the volume you can write to. Data protection is the biggest deduction: mirroring costs half the raw capacity, single parity around a fifth, dual parity roughly a third, and erasure-coding schemes land wherever their data-to-parity ratio puts them. Enterprise arrays then hold back a system reserve of roughly 5 to 15 % for metadata, journals and garbage collection, and the filesystem on top takes a further slice. A quote leading with raw is describing the bill of materials, not the deliverable.
Do hot spares count toward the capacity a vendor delivers?
They are on the invoice and they are not in the usable figure. A spare sits idle waiting for a failure, so its capacity is unavailable by design, and typical policies of about one spare per twenty drives, or one to two per protection group, remove a meaningful slice from a large array. The trap is a response quoting usable capacity with no spares while its own reference architecture mandates them — the delivered system then lands under the requirement through a configuration choice nobody disputed. Ask for the spare policy in writing.
How should a data reduction ratio in a bid be treated?
As a commercial promise, not as capacity. Effective capacity is usable capacity multiplied by an assumed deduplication and compression ratio, and the assumption depends on your data rather than the vendor's. Industry expectations vary enormously — general databases in the low single digits, virtual server estates higher, virtual desktop estates higher still — and incompressible data drags the blended figure down hard: if half your data does not reduce at all, a headline 5:1 on the rest becomes about 1.67:1 overall. If a bid needs reduction to meet the requirement, insist the ratio becomes a guarantee with a remedy attached.
What should acceptance testing measure when the array is installed?
The reported usable capacity, in the unit the contract used, taken from the system itself with the agreed protection layout, spares assigned and the intended volumes created — not from a specification sheet. Record it at handover and attach it to the acceptance document, because it is the baseline for any later shortfall claim. If the contract also carries a reduction guarantee, that needs a second test against real production data after a defined measurement window. Writing both tests into the RFP turns a capacity requirement into something a supplier can be held to.
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