Talk:SI derived unit

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what about the byte like gigabyte and megabyte —Preceding unsigned comment added by (talk) 06:13, 29 September 2008 (UTC)

Those are technically dimensionless, and only really signify a count of something, similar to the radian. --nmk


Hmmm. Are I'm infringing ones copyright by taking commonly used definitions??? Tobias Hoevekamp

Ah, here we go, perhaps Lee Daniel Crocker's comments in United States/President might reassure you. If not, perhaps this is a question for talk:Copyright?

-- dja

Yes, I don't see anything copyrightable here. Even if you took whole sentences out of the ISO specs, they're not copyrighted either--deliberately. That's the whole purpose of the ISO. --LDC

(Internation Standards (published by ISO) are copyright, the copyright is owned by ISO, they reserve all rights. There is no right to copy anything from an International Standard. Obviously this is often a problem if you want to implement, use, or descrribe one. Providing copyright free specifications is not the purpose of ISO. Alas. --drj).

Are you sure that ISO waives their rights? ISO charges big bucks for most standards as a way to finance themselves. If these were really public domain, we would have seen a lot of them printed cheaply, or floating around the net, no? --Robbe

Yes, some (OK, most) ISO specs are copyrighted--especially industrial standards. But they do grant explicit rights to the metric weights and measures stuff. I'll see if I can dig up a citation somewhere. --LDC


Woulnd't it be better to try to make this chart more free form, using indentation to get pre formatting, as with periodic table, so that it is easier to edit?

Although they are derived units, there are standard base units for derived quantities such as area (the are, as in "hectare") and the liter, these should be included, just as have been units like webers etc.

I'd do it, but frankly, I'm a bit put off by the table markup. Maybe I'll make an alternative table to demonstrate how I think it should be done.

And no, I don't think it's a violation of the copyright to transcribe facts, so long as you don't make an exact copy of the layout from the original. I saw someone address this issue recently in another context, I'll see if I can dig up the discussion and then post a link to it here.

-- dja

Something like this: SI derived units freeform table

I started by reading the HTML-formatted table with the character-based web browser "links" in a sufficiently wide terminal window, saving the formatted output to a file. I then passed it through the following perl one-liner to preserve the free formatted links, and to restore the superscripting:

perl -ne 's/|/ /g; s/[(S+s*S*s*S*)]?/$1/g; s/(S+)?/$1/g; s/(w)(-*d+)/$1$2/g; print' si_derived_units.txt > si_derived_units5.txt

Finally, I did some manual cleaning up in a text editor before copying and posting it.

FWIW, YMMV, and all that.

-- dja

Neither SI derived units freeform table nor SI derived unit look good to me. What I don't like is, that we are restricted to very basic mathematical formulations... -- th

Do you have an example of what would be more appropriate? Seems to me that one is going to run into such limitations with any tabular format. The freeform table is not intended so much for its prettiness (although I don't think it is downright *ugly* even so), but rather to make the layout more accessible and editable by people who are more interested in the content than in the minutiae of HTML markup. Such concern is fairly central to Wikis, in general.

-- dja

Problems with the freeform table: (a) It is difficult to get the alignment right, because the link brackets (4 chars) can turn into zero, one (the question mark), or three chars (? plus brackets). If somebody creates one of the dangling links on your page, the alignment changes and must be fixed. (b) Browsers may word-wrap PRE content, which makes the table almost unreadable. Mozilla does this. (c) SUP is not allowed inside PRE, and therefore most of the units are broken (m^2, m^-1, etc.). --Robbe

(a) Granted, though perfect alignment isn't always necessary (b) OK (c) I see that this is so from

Good thing I didn't change the main page too early.  :-)


Non-SI metric units[edit]


what is a liter?

Hmmm, seems to be a Cubic Decimeter. I think it would fit here.
"The litre (or liter in US) is a metric unit of volume. The litre is not an SI unit, but (along with units such as hours and days) is listed as one of the "units outside the SI that are accepted for use with the SI." The SI unit of volume is the cubic metre (m³)." So it seems like it's Metric, although not SI derived...


As_I_Understand_It, degree Celsius is NOT an SI unit, the Kelvin is. Anyone know how to confirm / refute this? SGBailey 10:34 Dec 12, 2002 (UTC)

The table suggests the "Celsius" is an unofficial unit of temperature. Shouldn't that be the "degree Celsius"? — Preceding unsigned comment added by (talkcontribs) 00:03, 29 April 2010 (UTC)

I'm puzzled by its inclusion here, but BIPM claims that it's a derived SI unit. But its definition is certainly not "K - 273.15". You can't subtract a dimensionless number from a unit. Rather, its definition is simply "K", and the quantity it measures is "temperature relative to 273.15 K". — Smjg (talk) 11:30, 13 April 2011 (UTC)
Thinking about it, it seems to me there is this definition of an SI derived unit: a measurement unit that is a multiplicative combination of zero or more SI base units. Zero is the case of the radian and steradian; one is the case of the hertz, lumen and becquerel. And the degree Celsius is an SI derived unit purely because it's another example of the same. Does this seem right? — Smjg (talk) 00:12, 25 April 2011 (UTC)


I dont see the dyne (symbol: dyn) mentioned here or on the force page. It appears to have a defined relationship with the newton. Is it a bastard unit of measure?

Quinobi 21:16, 2 Oct 2004 (UTC)
Follow and you will find that the dyne is 'not accepted' for use with SI (by omission). Contrast this with the deg Celsius which also has a defined relationship with an SI unit - kelvin, but this one is 'accepted for use' with SI. Dyne appears in the CGS system of units, which is very much less widely used than SI. Ian Cairns 21:36, 2 Oct 2004 (UTC)

Maths notation[edit]

I have started changing some of the math formulas to use <math> constructs. I think this looks much better, and is not much more difficult to edit (see Wikipedia:TeX markup). If nobody objects, I will finish this off sometime soon. I agree that the freeform table looks pretty bad, and the html table isn't much better. I don't think the html table is too difficult to edit, but maybe I will format it in a way that makes it easier for people to understand how to edit. Or maybe I'll make my own attempt at a freeform table. We'll see... --HN 21 May 2003

Why not use Wikipedia table markup instead? (Random832 01:05, 14 Jun 2004 (UTC))
Does anyone else find the page extremely s..l..o..w.. to load - principally because of the huge amount of maths involved in simple text fields? I would like to replace the maths by text, with a suitable appearance / emphasis. Does anyone have any objections, please? Ian Cairns 00:16, 8 Nov 2004 (UTC)


There are templates available e.g. [[Template:SI_derived_units]] and [[Template:SI_accepted_units]] which could be used to start this article. Would they be useful in this article? Ian Cairns 00:31, 8 Nov 2004 (UTC)

I've put them in. I've also removed the old tables. However, I'll go through these to see whether anything has been lost. If so, I'll put it back.
Okay, I've found what had been lost & put it back. It was very clear-cut: Other quantities and units had been lost whilst Units with special names and symbols had become redundant. I replace the former to its rightful place. The latter I'm leaving here on the talk page.
Well, almost clear-cut: there is no Expression in terms
of other units
column now. However, if we're to put this back, then it's the template which should be editted. I'm putting the old Units with special names and symbols table at the template's talk page.
Jimp 14Oct05

Conversion between kelvins and degrees Celsius[edit]

I propose that this be merged to Degree Celsius#Conversions. Jimp 14Oct05

Keep it here. (It is already there, isn't it?) It belongs here, because unlike any of the other conversions, this is a conversion within the International System of Units between two different SI units (or two different names for the same SI unit, depending how you look at it). It is quite relevant to the structure and usage of SI. Gene Nygaard 08:34, 14 October 2005 (UTC)

It's already there, yes, but not in as concise a form. If we don't remove it from here it still may be worth duplicating it over there. You have a point, Gene, "this is a conversion within the International System of Units" however I wonder whether this is enough to warrant keeping the section here. The conversion is also covered in the SI derived units with special names table (as a mathematical formula). Is this not enough? There are a great number of things that might be called "quite relevant to the structure and usage of SI" (many of which may have just as good a reason to be included here) do we have room for them all? I say keep the details of particular units on their own pages, kelivins to Celcius being no exception. Jimp 21Oct05

I still think there is a huge difference between giving the relationship of one SI unit to another SI unit, as compared to giving a conversion factor between an SI unit and a non-SI unit. There aren't going to be many of these; even the newton metre and the joule, which are both listed here, while dimensionally equivalent are used to measure different quantities. Degrees Celsius and kelvins measure the same quantity. That N·m to J relationship, of course, is pretty trivial in any case, not warranting the slightly more detailed explanation for the temperature readings. Gene Nygaard 00:50, 21 October 2005 (UTC)

Is there a reason why the SI base unit page says "...the fraction 1⁄273.16 (exactly)..." while the SI derived unit page says "t°C = tK − 273.15". I realize that 0.01 is a small amount, but they can't both be correct.TheAmigo42 (talk) 22:06, 27 February 2008 (UTC)

Sure they can. The triple point of water is at 0.01 °C. --DrTorstenHenning (talk) 08:16, 18 July 2008 (UTC)

All this chatter is irrelevant, as the Kelvin should not even appear in this list, as it is one of the seven basic SI units, NOT a "derived unit". Dumbhick (talk) 13:09, 17 July 2008 (UTC)

But the degree Celsius is a derived unit. --DrTorstenHenning (talk) 08:17, 18 July 2008 (UTC)

Possible error in lumen?[edit]

Shouldn't the "Expression in terms of SI base units" be cd ^ -1 as it is 1/cd? --I hate to register 12:43, 31 July 2006 (UTC)

Units for Newton and Pascal query[edit]

Would it not be better and clearer to quote the Newton as being kg.m/s^2 rather than^2? After all Force is usually quoted as F=ma (mass x acceleration). Similar with Pascals. —Preceding unsigned comment added by (talk) 15:05, 6 November 2007 (UTC)

No, the units used in definitions are standardised in the order m, kg, s, A, K, mol, cd (not sure why). Hence odd-looking things like m-2∙cd. (talk) 11:18, 6 August 2008 (UTC)
They're in that order for the first table "Derived units" but the second table "Other units" puts kg before m. I think the second table should be corrected. —Preceding unsigned comment added by (talk) 23:16, 9 February 2010 (UTC)

What the heck is kelvin doing here!?[edit] (talk) 11:19, 6 August 2008 (UTC)

Indeed, the Kelvin is a base unit, not derived. Could anyone comment on why it appears on this page ? Update: I went ahead and changed it to the definition of the degree Celcius, since that is most likely what was intended. [1] (talk) 04:07, 7 August 2008 (UTC)


Correct me if I'm wrong, but I do not believe that E should be here. It's the symbol for the electromagnetic field, but not a unit of magnitude. It's comparable to T(time) and L(length) as opposed to being comparable to s(second) or kg(kilogram). I will go ahead and remove it on the grounds that it doesn't belong here. Feel free to revert back if I am wrong. I am a student and not a scientist. - (talk) 04:44, 22 November 2008 (UTC)

Specific Energy Density[edit]

Specific Energy Density is the product of the specific energy of a material times its energy density. The higher the number, the more potential energy is contained in that substance. As the typical units of Specific Energy are m2/s2 or J/kg, and of Energy Density are kg/m·s2 or J/m3, so the product is equivalent to kg·m/s4 or J·J/kg·m3. Unfortunately there is no SI unit for this, but it is the best way of classifying a material by its energy content, since lower mass or less volume will increase the overall number. It's a pity there's no SI derived unit for this concept. May I name it? Surely I'm not the first to discuss it? But can we call it Jacobs?  :)

--TimeHorse (talk) 19:18, 4 January 2011 (UTC)

Sort by power of SI unit[edit]

I suggest adding 7 columns to the table: m, kg, s, A, K, Cd and mol, the values in each of these columns will be the exponent of the SI unit or 0 if that unit does not form part of the definition. For instance, Hertz would be 0, 0, -1, 0, 0, 0, 0; Newtons would be 1, 1, -2, 0, 0, 0, 0; Lumen would be 0, 0, 0, 0, 0, 1, 0; Volt would be 2, 1, -3, -1, 0, 0, 0; etc.

--TimeHorse (talk) 19:24, 4 January 2011 (UTC)

I'm not sure whether such a table would be useful. But if we do have it, I think it would be clearer if we represented 0 by a blank cell. — Smjg (talk) 09:14, 29 April 2011 (UTC)

Rearranging this article[edit]

I propose to swap the order of the tables (to match the SI Brocure), so that Table 1 gives examples of the unlimited number of derived units and Table 2 is a list of specially-named derived units (so that the article discusses the general before the particular) Adamtester (talk) 00:38, 14 January 2011 (UTC)

what does dimensionless mean?[edit]

radians somehow cannot be expressed wouldn't that make it a base unit? —Preceding unsigned comment added by (talk) 22:56, 2 April 2011 (UTC)

The radian is considered to be equal to 1, because an angle is essentially just a ratio of arc length to radius. — Smjg (talk) 11:08, 28 April 2011 (UTC)
For what it's worth, some older books give the radian and steradian as base units. My guess is that BIPM/CIPM/CGPM at the time didn't consider them to be fundamentally different from the other base units, even though each is defined in the most natural way possible. Then they were reclassified as supplementary units as an acknowledgement of this subtle difference. It was later again they decided that, since a radian is really just a metre (of arc length) per metre (of radius), it is really a derived unit, even though all the units that make it up cancel each other out. Similarly with the steradian, which is a square metre (of surface) per metre (of radius) squared. But it does lead to such oddities as that the lumen is, when expressed in terms of SI base units, just a candela. — Smjg (talk) 20:26, 29 April 2011 (UTC)
I understand that Radians are ratios and aren't dependent on radius but arc length, but an example is good. — Preceding unsigned comment added by (talk) 13:16, 11 March 2013 (UTC)

corrected error in degree Celsius equivalence[edit]

The table stated incorrectly that 1 °C = 1 K - 273.15. What is meant is that 1 °C = 1 K, but that the degree Celsius is used for reporting temperatures relative to 273.15 K. Dondervogel 2 (talk) 07:31, 5 January 2015 (UTC)

@User:Dondervogel 2 1 °C = 1 K - 273.15 it is obvious. but 1 °C = 1 K is definitely wrong! °C can never be equal to K.


~"aGastya" ✉ let’s talk about it :) 14:50, 18 February 2015 (UTC) Dondervogel 2 (talk) 15:29, 18 February 2015 (UTC) I think what you mean is that the numerical value of temperature expressed in deg Celsius is never equal to the numerical value of temperature expressed in kelvin. I agree with this statement, but it is not what I said. What I said, in equation form, (and meant) is that the unit degree Celcius is identical the unit kelvin. Dondervogel 2 (talk) 15:29, 18 February 2015 (UTC)

Yeah, they just depend in +273.15 but deg C isn't SI unit ~"aGastya" ✉ let’s talk about it :) 11:22, 19 February 2015 (UTC)

Newton Metre (or meter as per US)[edit]

N-m is for Torque as well as Energy too. Should not that too be added in the table? thanks! ~"aGastya" ✉ let’s talk about it :) 11:50, 18 February 2015 (UTC)

Meter inverse vs Second inverse[edit]

second inverse is there in table of "Named units derived from SI base units" metre inverse is names in table of "Some SI derived units" Why so? both of them should be in a same table, isn't it? And then also "Angular Frequency" is also missing in the table (second inverse) Thanks ~"aGastya" ✉ let’s talk about it :) 11:58, 18 February 2015 (UTC)


Whoever wrote this misspelled the word "meter" like 30 times... — Preceding unsigned comment added by 2601:5CA:4100:D800:9D52:630D:7537:6140 (talk) 04:40, 14 May 2016 (UTC)

What about Nautical Mile?[edit]

According to [[2]], nautical miles (and by extension, knots) are acceptable non-SI units. Why aren't they on the page or the template? Is this an example of systemic bias against imperial units? — Preceding unsigned comment added by (talk) 06:20, 8 July 2016 (UTC)

Neither of the 2 units you mention are SI derived units so I see no compelling reason to include them in an article entitled 'SI derived units'. If you disagree there's nothing to stop you editing the page. Dondervogel 2 (talk) 23:06, 8 July 2016 (UTC)

lumen per watt[edit]

The 'expression in terms of SI base units' for lumen per watt 'm−2⋅kg−1⋅s3⋅lm' should be changed into 'm−2⋅kg−1⋅s3⋅cd' in order to be consistent with the SI base unit table. — Preceding unsigned comment added by (talk) 08:47, 10 September 2016 (UTC)