There are 10 sentences summarizing the history, but only 4 sentences summarizing all the rest of the article. In order of vitality, the History section would fall well down the list. The lead is quite short to boot. We have 8 level two sections, 22 total content sections and subsections. If one wrote 2 lines per content section, that's 44 lines; at 6 lines/paragraph, that's 7 paragraphs. That'd be long-ish, but not unbearably long. The typical lead is 4-5 paragraphs of 5-6 lines, about 25 lines. However, this is a longer article, and longer articles need longer leads. Maybe some sections don't need summarizing.

I think the article, though it's GA, suffers some from design by committee - it's replete with detail, and has comprehensive coverage, but it's hard to find anything quickly. I'd suggest a true topical enumerative summary section at the top. Most professional papers have both an abstract and an introductory section, expanding on the abstract.

• The History section is long enough to be its own article, but we've already got an article on History of the metric system. Maybe we'll call it Brief history of the metric system. Then we write 4-6 paragraphs of interesting and useful historical events.
• SI brochure and conversion factors section seem stuck in the middle of nowhere just after History, as if it were the most important thing to know; it's administrative trivia and could be in a sidebar, or even removed to a footnote.
• International System of Units#Post-1960 changes section surely belongs under History; let's see, by my reckoning, that's ~60 years ago. My mother wasn't even born yet - that's History.
• A kilogram is defined (in English text sense) as "The mass of the International Prototype Kilogram (Le Grand K)". Do we know what that is? It's a lump of coal. Maybe it's a litre of water. The words are accurate but negatively enlightening. Be picaresque: "The kilogram is the mass of a small nearly square^[1] cylinder (diameter = height) of ~47 cubic centimeters of platinum-iridium alloy kept in a laboratory in France. Also, any of numerous official copies of it." I don't think we need to know the name of it - it's not a painting.
• International System of Units#Writing unit symbols and the values of quantities section is a collection of lexicographic trivia. What about German (we mention this marginally), Greek, Latin, Russian, etc? We can't possibly cover the world's languages here. This kind of thing should be in a comprehensive appendix by language. When one considers how big it would have to be to be useful as a reference in that language, we go... GULP! What should be in this section? We start out with noting the English triad milli-, micro-, mega-. So we need to carefully distinguish upper and lowercase letters M(ega )and m(illi) and one more, a Greek mu (micro). We fail to mention the character-set issues with Greek letters on terminal input devices and teletype output devices, so that mu is often printed or typed as an English letter 'u'. In many character sets they're almost indistinguishable anyway ( μ u ). Besides grammatical issues, I don't think we want to go into character set issues much, either.
• The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre. So there's a 'new candle' and an old one? But I don't know what an old one was. 'Full radiator"... what's that mean exactly? How hot is molten platinum? Bah, humbug.
• I added the 7 base units to the lead and sidebox, but we don't know what they are. Probably, a large number of people won't know what a candela or mole are, some may assume that Kelvin is another name for Centigrade, and others may not be too sure than an "amp" is an "ampere". It'd be a paragraph worth of text; clutter or essential info?

Sbalfour (talk) 02:46, 18 December 2017 (UTC)

The true primary units are Meter, Gram, and Second. These should be defined here.

Decimally defined units (such as Kilogram, Liter, Hectares, Tonnes, and so on) should be a section below. A separate section below that should define the Coherent units.

Coherent units (weight, force, acceleration, energy, pressure, and temperature) should be defined in a later section.

Non-Coherent units are not basic units, but convenience units. SI electrical units (Coulomb, Volt, Ampere, Watt, Ohm, Mho, Joule), Atmospheres, and Candelas and their derivatives should be defined separately, as they are not coherent with MKS. All of the electrical units are derived from the Coulomb, Ohm, and Second (Ohm and Volt are interrelated but otherwise arbitrary, likewise Ampere, Volt, or Watt can be substituted for Coulomb, but Coulomb is the most basic). A Candela is an arbitrary unit of energy, used only for photons. An atmosphere is an arbitrary measure often defined by MKS pressure units. The mole (actually Avogadro's Number) is also a non-coherent unit. Drbits (talk) 00:51, 25 February 2018 (UTC)

re typesetting issues, we could at least point out a) that they exists and b) that there are international standards (or at least one international standard that I know of - I think it's in IEEE Std 260.1-2004 - see talk page) for dealing with them. Dondervogel 2 (talk) 18:07, 18 December 2017 (UTC)

A Kilogram has always been defined as 1000 grams - standard killogram samples have been created, and the gram derived from that, but a kilogram is not a primary unit. A gram is defined as the mass of a cubic centimeter of pure water (at 4.0 degrees C and 1 ATM pressure). A centimeter is defined as 1/100 meter. A meter is defined based on the wavelength of light from a specific element.The physical standards (a platinum rod to define the meter, the mass of a specific item to define a kilogram, and so on) are only examples, not the basis of the SI definitions. The use of these items as standards are being phased out. These standards are physical objects and were found to be subject to change with aging and use. Drbits (talk) 22:55, 7 February 2018 (UTC) Drbits (talk) 23:47, 24 February 2018 (UTC)

The kilogram (note correct capitalization) is still the mass of the International Prototype Kilogram. Work is under way to redefine it in a way that does not depend on one particular physical artifact, but that work is not complete; the work will not be complete until the new definition is approved at a meeting of the General Conference on Weights and Measures. The water-based definitions were superseded long ago. Jc3s5h (talk) 14:26, 8 February 2018 (UTC)

I agree that the article is too long, that the lead does not give a comprehensive introduction and that it should be much reduced and that what is left could be better and more informatively written. I think that all of the "History", "Post-1960 changes" and "Global adoption" sections could be dropped and "History of the metric system" and "Metrication" added to the "See also" section instead, as they only of incidental relevance. -- DeFacto (talk). 19:06, 18 December 2017 (UTC)

Whew! I'm in agreement, but playing hack-and-slash on a GA article is a good way to get into a nasty conflict. I'll do some of the basic reorg, supply a lead, trim the fat in little pieces, and wait for other editors to weigh in. Maybe when they see it shaping up, they'll cut me some slack. Sbalfour (talk) 20:57, 18 December 2017 (UTC)

Actually, the initial text (before first subsection) of History section seems to be a gloss-over of the entire history section, and with slight augmentation, is all we really want here; they can go to the history article if they want more. Sbalfour (talk) 03:17, 20 December 2017 (UTC)

There are 16 subsidiary main articles, linked by hatnotes, one per every major level 2/3 section. And of course, there's a separate article on each of the 7 base units and 22 derived units. It's a symptom of just how crab-like the encyclopedia has become. They are rife with duplications. In a printed encyclopedia, the information would be collected into one place in a larger article, and no duplication.

The existing article is bloated in some ways, threadbare in others. It seems expeditious to merge some of those in (i.e.out of existance), and to merge some sections of this article out, usually into a longer main article on the topic, thereby deleting them from the article. What it makes sense to do depends on a sense of proportion: as this article grows, it can absorb increasingly larger chunks. When the article gets "too large", it'll make sense to split out one or more large sections into their own substantial article. I added one merge-from tag to the article, and many more could be added. They just clutter up, so I demur. Those articles collectively (except for really big ones like Metrication, History of the metric system and Metric system) are the logical and substantive content missing from this one.

If most of an article is considered to have been split out, the remaining portions become essentially an outline article, and we have one for the metric system (ha! ha! someone even thought of that - it's the road map of the crab). If this article is parceled out, it would become an outline article, and since there is one, it'd just evaporate, its shadow remaining as part of the Metric system article. Then someone would create a new article with the name of this one, and the process would start all over again, like a supernova spawning stars. It's already happened with 'History of the metric system'.

I think most of the 7+22 individual articles on the unit names could be merged into this one, duplicate and superfluous text eliminated, and collated into a 29 parallel-paragraph section. That's big, but it's the feature content here, so ok. Some units like gray and sievert or radian and steradian would be dealt with together.

I'm going to consider the other possible mergers one-by-one.

• MKS system is a boffo article; I think one sentence could summarize it here, then redirect the page to this article's History#Early metric systems section.

Sbalfour (talk) 22:45, 24 December 2017 (UTC)

@Sbalfour: I agree with the general sentiment, but tagging of the #History section was off-base, per WP:SUMMARY. We're supposed to briefly encapsulate much larger side articles (WP:SPINOFFs / WP:SPINOUTs) as short summary sections in a "main" article. That said, we do not need this many side articles, by any means. It would be a good idea to merge the shortest ones directly into this article as sections (and redirect their old titles to the sections). Retain as stand-alone articles only a) well-developed side articles, and b) side articles that topically are not good merge candidates because of their specificity or their perhiperhalness to the main subject of the ISU. This article is nowhere near "too long" for some merging and for some expansion with properly-done WP:SUMMARY-style abstracts of the larger side articles. (These should read a lot like the lead sections at those side articles, but adjusted for their context in the main article, and without reader-frustratingly repeating the exact wording of those leads.)

In the course of consolidating and cleanup, look for a) WP:POVFORKs and b) pointless repetition that isn't within the expectations of WP:SUMMARY and which is apt to annoy readers going over the entire set of articles. This requires some word-smithing, to present and reinforce information in a contextually sensible way and incrementally expanding way, while doing violence to neither the subtopical articles nor the overview at the main article. Finally, "ISU" and "metric system" are not 100% synonymous, and we need to keep that in mind. The ISU is the modern, internationally standardized system developed from the looser, original metric system[s] developed incrementally by various people and bodies, with plus other, later stuff added. In most contexts they can be treated as essentially the same thing, but with regard to history, and to application at a particular time frame, this will not always be true, so we need to avoid anachronisms.
 — SMcCandlish ☏ ¢ 😼  01:12, 23 February 2018 (UTC)

Also pinging Dondervogel 2.  — SMcCandlish ☏ ¢ 😼  01:14, 23 February 2018 (UTC)

And Quondum.  — SMcCandlish ☏ ¢ 😼  01:16, 23 February 2018 (UTC)

Though I agree with the sentiment, I feel that saying that "most of the 7+22 individual articles on the unit names could be merged into this one" is problematic. I am an avid fan of culling duplication, but looking at some of those articles suggests that merging any such article here would place an undue burden on this article. I like the current format where essentially only one line in a table is devoted to each derived unit, and not much more to each base unit. The history section can be reworked and duplication with other articles minimized (yep, too much on outline, history, derived units, etc.). Each of the articles Second, Metre, Kilogram etc. may be on the bulky side, but it should be remembered that many of these units existed before the SI system was formalized. And even those that were born inside the SI bear discussion that I would not like to see bloating this article. And though not many people may concur, I prefer concise, short articles; scrolling around an article to find the relevant material should be easy, not a hunting expedition. This article is a nice length. —Quondum 02:54, 23 February 2018 (UTC)

In my opinion Metric units and SI units should be separate articles (why does metric units re-direct here?!?). SI is clearly important enough to have its own article but metric units is a far broader set of units than just SI, encompassing both CGS and MKS systems and their predecessors. I also see think each named SI unit merits its own article. Where we could useful economize is on derived units like metre per second. That could useful be merged with foot per second, mile per hour, knot and so on into units of speed. Dondervogel 2 (talk) 22:09, 23 February 2018 (UTC)

Yep. Just make the old names redirects to sections it. PS: If there's a wikiproject about units, they may want in on these conversations.  — SMcCandlish ☏ ¢ 😼  01:42, 24 February 2018 (UTC)

There's a Wikiproject Measurements. I posted a note on their talk page, pointing out the existence of this discussion. Dondervogel 2 (talk) 21:59, 25 February 2018 (UTC)