What is the plural of "equipment"?

"Equipment" is nearly always used as a mass noun, not a count noun. There may be a few obscure exceptions, but they would be very rare. Mass nouns don't have plurals (except when they double as count nouns).

So anything you say with "equipments" in it is more than likely to be ungrammatical. I recommend avoiding this word.

Short answer: use 'equipment', do not use 'equipments'

Long answer: in Modern Standard English, 'equipment' is a mass or non-count noun, like 'water' or 'traffic', which describes something that is somehow implicitly plural but doesn't have a plural form. That is, it wouldn't make sense to have more than one equipment, since it describes a set of things already. 'Equipment' is already plural in the sense that it refers to a number of things together. There are instances where one has more than one set of these things, the many waters of the world.

To address some of the controversy mentioned elsewhere:

• 'the OED has 34 entries': if you look at what a current search for 'equipment' returns 34 of, it is 'stemming' (removing suffixes) in some arbitrary fashion. The OED search matches against the word, not any words in the definition or quotes, and the 34 words matched for 'equipments' includes 'equipment' along with 33 other words which are not 'equipments'.

• 'dictionaries leave out the plural': yes, they do but they also don't mark if a word is a mass noun like 'water', so you can't tell if there is no plural form as in a mass noun. A proper dictionary will give the plural form if it is mot simply '-s', e.g. 'ox, n. pl. oxen' or 'daisy, n. pl. -ies'.

• 'the OED has a definition for equipments': yes, it does.

  2 a. concr. Anything used in equipping; furniture; outfit; warlike apparatus; necessaries for an expedition or voyage. Used in the pl. to indicate the articles severally, in the sing. collectively.

  1873 Act 36 & 37 Vict. c. 88 Sched. 1, Equipments which are primâ facie evidence of a Vessel being engaged in the Slave Trade

  This definition might be used but it is a technical usage, like for hardware or military use.

• 'Google gives X million hits', 'Google Ngrams says that it is so': Google Search/Books/Ngrams are easy to misuse and draw several levels of unfounded conclusions from. Google (and other search engines use 'stemming' which sometimes cuts off suffixes. So you can't tell what is being counted. Google ngrams doesn't stem, but you still need to check the results. A comparison of equipment and equipments shows that the plural form does occur but at a much smaller frequency than the plural form. Even given this lesser frequency it looks like 'equipments' was more common in the past, in foreign works, and in technical works.

As a final note of relevance, as a native speaker, I find 'equipments' to sound really wrong. So if you're climbing up a mountain and have a bunch gear to put away, just 'pack your equipment', don't use the plural.

First of all I want to make my point the I have heard equipments, but it was really in very very rare cases. I think that both are possible although they would mean different things.

Equipment is collective noun and as such can be used in plural. Although there are a lot of collective nouns that can be used with -s or without and they mean different things.

For example what is the plural of water- water. But we can say waters when revering to different types of waters in sea, oceans, river and ect. Another example is folk and folks. Folks is collective noun for group of people, while folks referring to collection of different groups of people.

Therefore when we are speaking about different machines for fitness we can call them equipment. When we are speaking about fitness equipment and fishing equipment, we should use equipments.

Don't forget that English is something alive and in different areas in different countries even native born English folks have their own English within their own community , with different way of living, different jobs, they belong to different parts of the society and ect. English is a languages and there is nothing constant in languages it changes and evolves every day.

Equipments has been in continuous use as a plural form of equipment in English for centuries, as this Ngram chart tracking instances of equipments across the years years 1700–2008 indicates:

However, the areas where equipments functions as an acceptable plural are rather narrow: as other answerers have noted, it is extremely unusual (and sounds flatly wrong in U.S. English) for someone to ask "Do you have all the equipments you need?" Microsoft Word automatically marks equipments as a misspelling because in most instances it is not the right word to use.

Historical usage of 'equipments'

Google Books search results for equipments turn up a multitude of matches for the term in publications from the period 1700–1817—something on the order of 170 unique matches. Many of these instances are associated with what most people today would refer to as "military equipment" (using equipment as a mass known). For example, from the court case of Savage v. Gulliver (March term, 1808), in Reports of Cases Argued and Determined in the Supreme Judicial Court, of the Commonwealth of Massachusetts, volume 4 (1808):

The eighteenth section of the statute of 1793. c. 14, declares what arms and equipments each private soldier shall at all times be provided with. In the nineteenth, twentieth, and twenty-fourth sections, penalties were imposed on the neglect of the soldier to appear on the parade, with his arms and equipments. If he neglected to appear, he was liable to a fine of ten shillings, and also to a fine of twenty shillings if his arms and equipments were not on the parade. If part of them were there, then to a fine of such a part of the twenty shillings, as the articles of his arms and equipments absent were in proportion to the whole articles required: which proportion, from the nature of the equipments, it must have been difficult to ascertain. But the justice was bound to exercise his discretion. If the soldier appeared with his arms and equipments not in good order, he was to be fined three shillings: and perhaps a proportion of the twenty shillings for the articles not present. If he was absent, and his arms and equipments complete, he was then subject to a fine of ten shillings only. So that in every case, where he was liable to a fine, he was liable to more than one, except when all his arms and equipments were on the parade and in good order.

Unmistakably, equipments is not a typo here. The term also comes up in connection with outfitting a ship for a voyage, as in this example from "Naval Report," in Niles' Weekly Register (December 10, 1814):

Our ships are excellent, and all in good condition. The classes are few, and so uni form that without inconvenience the masts, spars and equipments of any one of a class will serve indifferently for any other of the same class. All the new ships. All the new ships of each rate are of the same class, and are absolutely similar in all their equipments, and in the dimensional proportions of their hulls, masts, spars &c. This strict similarity should be carefully preserved upon every principle of convenience, economy and efficiency.

'Equipments' today

A Google book search for equipments turns up more than 400 unique matches from the period 1972–2008. Clearly the plural form continues to be used—but where and by whom?

One area where equipments remains in use is in discussions of military history—as for example in the Osprey Military series "Men-at-Arms," which includes such recent titles as Artillery Equipments of the Napoleonic Wars (1979), German Combat Equipments 1939–45 (1991), and British Infantry Equipments (2), 1908–2000 (2000). Likewise army tech manuals continue to use the plural form, as in "Organizational, Direct and General Support, and Depot Maintenance Repair Parts and Special Tools Lists" (January 1968):

g. Fifteen-Day Organizational Maintenance Allowances.

...

(3) Organizational units providing maintenance for more than 100 of these equipments shall determine the total quantity of parts required by converting the equipment quantity to a decimal factor by placing a decimal point before the next to last digit of the number to indicate hundredths, and multiplying the decimal factor by the parts quantity authorized in the 51–100 allowance column. Example, authorized allowance for 51–100 equipments is 12; for 140 equipments, multiply 12 by 1.40 or 16.80 rounded off to 17 parts required.

Also disproportionately represented in the recent Google Books results are books from India. For example, from M.H. Siddiqut, Teaching of Economics (New Delhi, 1993):

CLASSROOM EQUIPMENTS

Classroom equipments constitute the following aspects:

1. The Items of Equipments:

The items in a classroom may be put into three heads which are as follows:

(a) Essential Equipments

...

(b) Desirable Equipments

...

(c) Non-Essential Equipments

And from S.K. Katiyar & H.S. Niranjan, "Topographic Mapping of Hilly Terrain Using Total Station, "in Proceedings of the National Conference on Advances in Civil Engineering: Perspectives of Developing Countries (ACEDEC-2003) (New Delhi, 2003):

Conventionally a map is prepared after doing field survey by using various equipments like chain, tape, theodolite and levels. ... Sometimes these techniques become unsatisfactory due to non-fulfillment of certain requirements like obstruction of line of sight and steep slope of the area. In the modern age of surveying, the electronic equipments like Robotic Total Station, Digital Levels and Laser Levels can make our fieldwork tasks very easy, due to their versatile functionalities. These electronic equipments have so many advantages over the conventional instruments in terms of accuracy as well as speed. Though these equipments have much higher costs as compared to conventional surveying equipments. However if survey fieldwork cost and time is also included, then it is worth to purchase these costly equipments.

Elsewhere in modern English-language publications, however, equipments is very rare indeed.

At first, a couple of general remarks on the count/non-count dichotomy in linguistics. (Incidentally, this dichotomy is language-specific).

What is taught in school about this distinction, whether English is your L1 or L2, is a gross oversimplification. It is done for pedagogical reasons.

However, in linguistics, it has been abandoned for various reasons. On the one hand, as Allan showed in his 1980 paper in “Language”, it is better to think of countability as a continuum, see the following examples (from the most countable to the least countable):

car, oak, cattle, scissors, mankind, admiration, equipment, Himalayas.

One the other hand, as Payne and Huddleston (2002) argue, “many nouns can be used with either a count or non-count interpretation”. They also add that since in some cases “the existence of paired count and non-count senses is entirely predictable, so that it is not necessary for a dictionary to list both”.

To make things even more complicated, some linguists have proposed up to eighteen types of concrete nouns only (e.g. Goddard 2009).

In natural languages, there is obviously a lot of variation. However, the trend is that the form “equipments” is not common. For example, I searched The Times and The Sunday Times, and “equipments” returned zero hits. Same with The Guardian. If you are an L2 learner of English, I’d recommend forgetting about the existence of that form altogether.

Some commenters suggested it might be used in cases when different types of equipment are meant. See, however, the following example from The Times:

Makro is also stronger in non-food items, such as stationery and office equipment.

Now, with The NY Times, things are a bit different.

Searching the archives from 1980 to present and manually going through the results, I got 28 hits only. Six of them were company names (Recognition Equipments Inc., Digital Equipments etc.) and one was obviously a typo:

According to Dr. P. L. Fan of the Council on Dental Materials, Instruments and Equipments, …. (1987) – see the original report here http://www.ncbi.nlm.nih.gov/pubmed/1918680

Another sentence suspiciously looks like a typo, too:

In the face of such enmity, Pakistan's armed forces stand more and more vulnerable as old, mostly American equipments wears [sic!] out, with no ready replacements. (1980)

But:

That will include $800,000 a year in grants for the groups, $100,000 a year in sports equipments, and 15,000 tickets a year to Yankee home games. (2006)

Several ski equipments are presented. (2007)

The microphone was in place, and the synthesizer and other electronic equipments were turned on. (1984)

Now, if you search the archives of the NY Times for 1851-2008, things are a bit different. I got 9983 results, with the highest peak for 1960-1969 – 1524 results.

An important thing about those examples – it seems most of them are classified ads, e.g. “Candidates should have at least four years of maintenance experience on the above equipments” (1962).

Only the first five hits were from the other sections of the newspaper, four of them written by the same person, Burton Crane.

Oils, drugs, office equipments and a number of the high-price stocks known as … (1961)

Now about the OED examples. There are 30 occurrences in quotations (I excluded definitions).

one – 18th century

eleven – 19th century

seventeen – 20th century (most of them are used in engineering or military contexts)

one – 21th century

If you search the BNC, you can see that in British English the form “equipments” is mostly used by the military:

Total number of occurrences: 27

7 of those come from ABA Britain's defence dilemma. 1990.

6 come from JNM House of Commons Select Committee for Defence: meeting (Public/institutional). Recorded on 9 February 1994

3 come from JNN House of Commons Select Committee for Defence: meeting (Public/institutional). Recorded on 9 February 1994

etc.

With dictionaries, you need to remember one very important thing - they are a good place to start your research but you shouldn't base your answer solely on the OED (OED-thumping) or some other dictionary.