Storage is quoted in effective capacity, which depends on a data reduction ratio. Put in the usable capacity, the ratio you have been offered and your own data mix, and see how far apart the claim and a planning figure might be.
Updated 12 October 2026
C4C is an independent technology consultancy that partners with several storage vendors, including Dell, NetApp, HPE and VAST Data, and is tied to none. Our founders spent over a decade at EMC building exactly these effective capacity figures. How to read reduction claims.
Usable capacity is after protection, before data reduction. The ratio is the one in the proposal or the vendor guarantee.
From the quote or proposal you are assessing.
Share of your data by type. These should add up to 100; the calculator normalises them if not.
Typical planning assumptions, replace with your own measured ratios. Kept deliberately conservative.
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The calculator treats your data as a mix of types, each with its own planning reduction ratio. The blended ratio is the capacity weighted harmonic mean: for each type, its share of the data divided by its ratio, added together, then inverted. That is how much physical capacity a mixed data set really needs.
Planning effective capacity is usable capacity times the blended ratio. Claimed effective capacity is usable capacity times the ratio in the proposal. The gap is the difference, and dividing it by the blended ratio shows the extra usable capacity you would need to close it.
These are the publicly available data reduction guarantees and claims from the vendors we work with, as checked on 12 October 2026. A guarantee is the level a vendor commits to in writing, not a forecast of what your data will achieve: real world reduction is often well above it, sometimes below it, and it depends entirely on your data. Programmes change, so always ask for the written terms and the effective date that apply to your order.
| Platform | Published ratio | Conditions worth knowing |
|---|---|---|
| Dell PowerStore | 6:1 average, guaranteed for reducible data | Dell lists host compressed and host encrypted data, audio, images, PDFs and video as not reducible. At least 50 percent of the physical drive space must be written before the guarantee is assessed, and a shortfall goes through a formal remediation request (Dell, Dell KB). |
| Dell PowerMax | 5:1 for open systems block data, 3:1 for mainframe data | The same unreducible data types and 50 percent threshold, on PowerMaxOS 10.1 or later (Dell, Dell KB). |
| Dell PowerScale | 2:1 | Dell notes that actual reduction varies; check the detailed terms (Dell). |
| NetApp AFF and ASA | Up to 4:1 for SAN workloads, 1.5:1 for NAS workloads | NetApp says it will make an unmet efficiency goal right at no cost to you (NetApp, NetApp). |
| HPE Alletra Storage MP B10000 | A tailored effective capacity guarantee based on 5:1 for reducible data | Valid for one year, with full terms from HPE or a partner. In 2024 the programme offered at least 4:1 for the support term, which is why the version and date matter (HPE). |
| VAST Data | Markets a minimum of 3:1 on any data, including pre compressed and encrypted files | Reduction combines deduplication, compression and similarity based reduction. Ask how the ratio is measured for your data (VAST Data). |
Notice the different wording: reducible data, average, up to, minimum. A 6:1 guarantee on reducible data and a 4:1 guarantee on SAN workloads are not measured on the same basis, so compare the conditions, not just the numbers.
Effective capacity needs a case by case look. We work with you and the vendor to understand what your data is likely to achieve, size the array on that rather than on a headline figure, and make sure the ratio in your proposal is one the vendor will stand behind in writing. Having built these figures on the vendor side for years, we know which claims are backed and which questions to ask. Talk to us about your proposal.
Data reduction ratio is the logical data written divided by the physical capacity it uses after deduplication and compression. Total efficiency or overall efficiency is usually a larger number, because it can also count thin provisioning, snapshots and clones, space the data never actually needed. Effective capacity is usable capacity multiplied by an assumed ratio, and some effective capacity figures also include savings on data protection overhead. Even the denominator varies: Dell’s OneFS documentation measures its data reduction ratio against usable capacity before protection, and its efficiency ratio against raw capacity after protection (Dell).
It is a common point of confusion. One PowerStore user on Reddit saw four figures in the management console, overall data reduction, overall efficiency, snapshot savings and thin savings, and asked which one to compare with the sales promise. The answer is the one written in the guarantee, which is almost always the data reduction ratio on reducible data.
For each proposal, ask whether the quoted ratio includes deduplication, compression, zero detection, thin provisioning and snapshots, which data is excluded, what the measurement threshold is, and what the remedy is if the ratio is missed.
Effective capacity is the amount of data an array is expected to hold after data reduction such as deduplication and compression. It is usable capacity multiplied by a reduction ratio. Because the ratio depends on your data, effective capacity is an estimate, while usable capacity is a fact.
Data reduction ratio counts only deduplication and compression of the data actually written. Total efficiency usually also counts thin provisioning, snapshots and clones, so it is a larger number. A guarantee is almost always written against the data reduction ratio, so that is the figure to compare.
Plan on your own measured data where you can. As a rough guide, databases and virtual machines usually reduce well, file and unstructured data less, and data that is already compressed or encrypted, such as media and backups, barely at all. A single headline ratio across a mixed estate is usually optimistic.
Because it has little redundancy left to remove. Dell, for example, lists host compressed and host encrypted data, audio, images, PDFs and video as not reducible under its PowerStore and PowerMax guarantees. A large share of such data pulls the blended ratio down sharply.
As published on 12 October 2026: Dell guarantees a 6:1 average on reducible data for PowerStore, 5:1 for PowerMax open systems block data and 2:1 for PowerScale. NetApp offers up to 4:1 for SAN and 1.5:1 for NAS workloads on AFF and ASA. HPE offers a tailored effective capacity guarantee for Alletra Storage MP B10000 based on 5:1 for reducible data, valid for one year. These are publicly available guarantees, the level each vendor commits to; real world reduction is often higher but depends on your data, so it needs a case by case look and the written terms.
Yes, and get it in writing. A guarantee is only useful if it states the ratio, how it is measured, which data types are excluded, the threshold before it is assessed, and the remedy if it is missed. Read the exclusions carefully, because they often cover exactly the data that reduces least.
No. It uses your inputs and generic planning ratios that you can change. C4C partners with several storage vendors and is tied to none, so the point is to compare proposals on the same honest basis.