Talk:International System of Units

Why o why are the colours in such a weird order ? Why not just stick with the order of the colours in the rainbow. The use of the colour yellow for seconds screams off the page as an anomaly. Who in their right mind would put yellow next to red and then orange next to green. Please, please can someone recolour this graphic so it is not so offensive. Thank you ! 46.208.209.96 (talk) 08:30, 2 April 2025 (UTC)

"Who in their right mind" is the International Bureau of Weights and Measures, from whom the image was obtained.

You have not explained why you consider having certain colours adjacent is so "offensive". The only improvement might be to invert the text colour for any segment whose colouring currently contravenes MOS:CONTRAST. Bazza 7 (talk) 09:02, 2 April 2025 (UTC)

This seems to be an old version of the SI circle. The correct circle (with colours in rainbow order) can be found at https://www.bipm.org/en/measurement-units/si-promotion. I suggest changing it. /Tobias 192.71.100.250 (talk) 09:20, 19 May 2025 (UTC)

Please provide your opinion at Litre. Dondervogel 2 (talk) 16:40, 11 May 2025 (UTC)

No, it isn't. DOI:10.59161/AUEZ1291 chapter 4. -- Wassermaus (talk) 00:52, 30 December 2025 (UTC)

@Wassermaus: Please reply on the Litre talk page. Dondervogel 2 (talk) 08:45, 30 December 2025 (UTC)

I don't get the formula for 1 Kg to work, computing the expression on the right side of the equal sign gives you 1/Kg, so how that can equal 1 Kg beats me. Planck's constant is Js and when you multiply that with the hyperfine transision frequency which is Hz you get J which is kg times (m/s) squared and if you then divide that by the speed of light squared you get Kg. The equation has numerator and denominator switched I think. 178.174.125.168 (talk) 19:06, 30 May 2025 (UTC)

Most of what you say is correct, up to "you get kg". And kg is what you want, so the numerator and denominator are the right way round. However the formula would be much easier to follow if it were developed as a product of its three constituent ratios.---Ehrenkater (talk) 19:16, 30 May 2025 (UTC)

Quantum logic clocks lead the candidates for a future redefinition of the second. The NIST ²⁷Al⁺ quantum logic clock's transition frequency, as of April 2025, is 1 121 015 393 207 859.16(18) Hz relative to the current SI second.EdPeggJr (talk) 03:00, 15 July 2025 (UTC)

in my last edit, you'll see that most of its changes are (symbol: (blank), (what it measures)), i think they could be shortened into efn as they dont seem essential Misterpotatoman (talk) 08:38, 26 December 2025 (UTC)

I don't think this sentence needs to be shorter: the seven units are central to the topic. Please don't use efn in any introduction. Details don't belong in introductions at all. Johnjbarton (talk) 16:20, 26 December 2025 (UTC)

Per MOS:RETAIN "When an English variety's consistent usage has been established in an article, maintain it in the absence of consensus to the contrary." This article was marked as having a specific spelling selected by consensus. In my view this rule exists to avoid frivolous changes back and forth, which is what we have now. @Dondervogel 2 @Misterpotatoman. Johnjbarton (talk) 18:43, 26 December 2025 (UTC)

Fair point about spelling, though I still maintain there were other improvements involved. I would add that there is one reason to prefer "-ize" in this article, which is that international standards (at least by ISO, IEC and BIPM) use that spelling. Dondervogel 2 (talk) 19:49, 26 December 2025 (UTC)

If the official documentation uses a spelling I think that makes a strong case for us to use that spelling if we could create a consensus to do so. Like I said, in my opinion the purpose of the rule is to avoid back and forth. Shall we try?

The template was added in 2012. It seems like the spelling was established in 2008 during this discussion: Talk:International_System_of_Units/Archives/12/2008#WP:ENGVAR. As far as I can tell the official documentation was not consulted. Johnjbarton (talk) 20:12, 26 December 2025 (UTC)

Yes, let's try to establish a consensus for one form of spelling or the other (-ise vs -ize) and then implement that spelling. I would not object to you (or anyone else) restoring "-ise" while the matter is discussed. I'd offer to do that myself if it were not such a pain separating those changes from the others made by Misterpotatoman with the same edit. (To be clear, I am not arguing for US spelling, but for UK spelling with the OED-sanctioned "-ize" variant) Dondervogel 2 (talk) 21:51, 26 December 2025 (UTC)

[] this graphic is quite appropriate but is old and could use improvement, and perhaps someone could add MOMENTUM to the diagram? Lee De Cola (talk) 22:28, 29 December 2025 (UTC)

The section "SI defining constants" has a {{clarify}} tag which constant belongs to which unit?. I don't understand this question. Why should a unit own a constant? The constants have units, not the other way around. @Dondervogel 2 Johnjbarton (talk) 18:11, 5 March 2026 (UTC)

Thank you for starting this discussion. I added the tag because that question (posed by the anonymous editor) seemed a legitimate one. At the time I was thinking each SI unit can be expressed in terms of one or more of these constants. I still think that is possible but I no longer see the value in doing so. I'll remove the tag now.

I don't think adding "and units" to the table adds any useful information and would be happy to see it go, but I'll leave that to others. Dondervogel 2 (talk) 18:31, 5 March 2026 (UTC)

Re "I'll remove the tag now", it looks like you beat me to it. LoL. Dondervogel 2 (talk) 18:34, 5 March 2026 (UTC)

You can remove the "and units" I was just trying to address your concern.

Yes, each SI unit can expressed in terms of one or more constants. But SI first defines "base units" in terms of the constants then defines units in terms of base units.

I think the table in the section on base units should explicitly call out the defining constants involved, as the SI brochure does. Johnjbarton (talk) 19:00, 5 March 2026 (UTC)

The system doesn't depend on base units in the way it used to. As the SI Brochure puts it, "The choice of the base units was never unique, but grew historically and became familiar to users of the SI. This description in terms of base and derived units is maintained in the present definition of the SI, but has been reformulated as a consequence of adoption of the defining constants." It would be possible to describe every unit in terms of the constants, but the base/derived expression is convenient and familiar. Even so, the definitions of the seven constants - as is made explicit on page 127 of the brochure - use the "derived" units hertz, joule, coulomb, lumen and watt. NebY (talk) 19:13, 5 March 2026 (UTC)

Derived units are defined as products of powers of the base units. Page 137. Johnjbarton (talk) 19:19, 5 March 2026 (UTC)

And two paragraphs below that, It is important to note that any of the seven base units and 22 SI units with special names can be constructed directly from the seven defining constants. In fact, the units of the seven defining constants include both base and derived units. NebY (talk) 19:26, 5 March 2026 (UTC)

I agree that would be the wrong point in the article to specify which constants relate to which units. It's not a simple one-to-one relationship, and we do cover it in detail in the table below. However, the new text "Each defining constant consists of an exact numerical value and an SI unit name. These constants could logically be used as the SI base unit" looks problematic. Firstly, each defining constant has a fixed value when expressed in SI units; the constant doesn't consist of a value and a unit name, and the constants are generally expressed in a combination of SI units (only the second and the mole don't involve other units). Secondly, the idea that the constants could be used as units is startling and not to be found in our source; it's both WP:OR and a claim that won't generally help our readers.

The old text "The magnitudes of all SI units are defined by declaring that seven constants have certain exact numerical values when expressed in terms of their SI units" might perhaps have been improved by omitting "their", which rather implied that their was a one-to-one correspondence. NebY (talk) 19:04, 5 March 2026 (UTC)

Here is the SI text I used:

• As for any quantity, the value of a fundamental constant can be expressed as the product of a number and a unit.
• The seven constants are chosen in such a way that any unit of the SI can be written either through a defining constant itself or through products or quotients of defining constants.
• Defining the SI by fixing the numerical values of seven defining constants has the effect that this distinction is, in principle, not needed, since all units, base as well as derived units, may be constructed directly from the defining constants.

Johnjbarton (talk) 19:17, 5 March 2026 (UTC)

Well, the second and third bullets are a long way from saying that we could use the constants as units, and we're not entitled to make that deduction as a WP:CALC "routine calculation". A literal reading of the first is difficult, as that section (2.2, p127) goes on to express the speed of light, the Planck and Boltzmannn constants, and a particular luminous efficacy in terms of two units each. NebY (talk) 19:38, 5 March 2026 (UTC)

The second bullet point is equivalent to "we could use the constants as units".

In section 2.2 it reads

• the Planck constant, h, is 6.626 070 15 × 10−34 J s,

so a product of a number, 6.626 070 15 × 10−34, and a unit, "J s". The singular/plural for unit is somewhat arbitrary.

Any I removed the sentence you complain about since it is not core to the section anyway. Johnjbarton (talk) 23:05, 5 March 2026 (UTC)

I don't understand

• The magnitudes of all SI units are defined by declaring that seven constants have certain exact numerical values when expressed in terms of SI units.

What is a "magnitude" here? The derived SI units are defined in terms of base units. As far as I can tell this sentence is expressing the "These constants could logically be used as the SI base unit" without calling it out explicitly. Johnjbarton (talk) 19:23, 5 March 2026 (UTC)

Well, if we gave that impression then it certainly does need rewording. I'm not fond of "magnitude" either. NebY (talk) 19:29, 5 March 2026 (UTC)