Talk:SI base unit

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Template:WikiProject Measurement

Removed table

here's the table I took out in case eveyone wants it back:

Physical quantity Symbol Name of SI base unit Symbol for SI unit Remarks
length metre m One metre is defined as the distance light travels in a vacuum in 1/299792458 second. This standard was adopted in 1983, when the speed of light in vacuum was defined to be precisely 299792458 m/s.
mass kilogram kg One kilogram is defined to be the mass of a specific cylinder of platinum-iridium alloy, kept at the International Bureau of Weights and Measures (near Paris).
time second s One second is defined as the time required for 9192631770 cycles of a hyperfine transition in cesium 133. This definition was adopted in 1967.
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

Amount of substance?

Is 'amount of substance' the technical name for what a mole is a unit of? It's not really an amount; it's a number. That is, you apply it to things you can have fewer of, not stuff you can have less of. It's definately the oddball of the bunch, being a 'dimentionless unit'. I don't really understand why it needs to be a base unit at all; it seems lie a better base unit would be 1. --Spikey 01:10, 19 Feb 2004 (UTC)

I agree that it might not be entirely accurate or completely sensible, but since the BIPM calls it "amount of substance" [1], and the NIST agrees [2], it's probably the Right Thing. - Plutor 18:58, 13 May 2004 (UTC)
Yeah, seems to me that the mole is as dimensionless as the radian. I wonder what these guys were thinging. --Doradus 21:50, Sep 18, 2004 (UTC)


Ampere, a base unit, is defined in terms of a Newton, which is not a base unit?

Yes. I imagine that it is for convenience. If you like, you can mentally substitute "amount of force required to accelerate a one-kilogram mass at a rate of one meter per second per second" for "newton". It's also "equal to a flow of one coulomb per second" (ampere). I am not a physicist, but I don't really see any problem with defining base units in terms of non-base units — as long as there are no circular references — since those non-base units are in turn defined in terms of base units. See also: --Timc 19:43, 15 Jun 2004 (UTC)

kilograms vs. grams

Why list the fundamental unit as 1000 grams instead of 1 gram? 02:53, 27 Jul 2004 (UTC)

Because the kilogram is the base unit. Timc 19:07, 27 Jul 2004 (UTC)
Pascals are small enough as it is. Pascal = Force/Area = (kg m/s^2)/(m^2) = kg/m/s^2. An atmosphere (standard ground level pressure) is already 1.01325 x 10^5 Pa. -ub3rm4th 17:42, 29 Jul 2004 (UTC)
There's a couple of anonymous posters who keep changing from kilograms to grams. Unless it changed since I was at school it's definitely kg. I'm changing it back again, but there's a danger of an edit war with well-meaning people who aren't fully up to date with the facts. Does anyone want to write an explanation as to why it's a kg and not a g? What's the usual procedure here? --Matt Westwood 05:18, 1 September 2011 (UTC)


"Yes. I imagine that it is for convenience. If you like, you can mentally substitute "amount of force required to accelerate a one-kilogram mass at a rate of one meter per second per second" for "newton""

Please edit this into the article 02:55, 27 Jul 2004 (UTC) what it means the MASS?


This article says that 1s is the amount of time it takes for a caesium-133 to shift however many times at 0K, but there's no way of reaching 0K, I thought? I don't know for sure, but might it mean 0C, or 273K, or even 25C or something? 18:45, 25 Mar 2005

No, 0 K is correct. Although there's no way to reach absolute zero, what this simply means is that the transitions should be taken as occurring on an atom with no temperature. To relate the abstract number of transitions to an actual measurement, the temperature has to be taken into account. This is just to simplify the definition. 22:55, 13 December 2005 (UTC)
So the length of a second varies with temperature?--Eddwilson 10:52, 21 December 2006 (UTC)
The time it takes for a cesium atom to shift 9,192,631,770 times varies with temperature, and how long it would take if it were at absolute zero is defined as 1 second. Someguy1221 12:14, 24 February 2007 (UTC)

circular references

I do not understand the section titled circular references. It says there are two base units above which do not appear to be defined purely in terms of other base units. Surely that should be do appear instead of do not appear. Can anyone explain? Bobblewik 01:03, 28 September 2005 (UTC)


From Candela talk page: It is base unit, because it is connected with waveleight, and sensitivity of human eye at that waveleight. Corect formula is where V(l) is the relative sensitivity of the eye at wavelength l. Values of V(l), defined by the International Commission on Illumination (CIE)[3], are available online from the Color and Vision Research Laboratories of the University of California [4] at San Diego and the University of Tübingen, Germany. read [5] Stijak 18:59, 13 March 2006 (UTC)

I think you mean [DELETED]. --Srleffler 23:36, 13 March 2006 (UTC)
I deleted the formula I had put here, because it was confusing and perhaps incorrect. A better statement would be that a 1 cd source of wavelength must have a radiant intensity given by --Srleffler 17:15, 14 March 2006 (UTC)
The candela is a base unit by definition, but is in fact completely defined in terms of other units and an arbitrary mathematical curve (the luminosity function). One can construct an experiment to measure a light source in candelas purely using instruments calibrated to other SI base units, with no reference to a standard SI luminous intensity reference. This makes the candela definitely not a fundamental unit.--Srleffler 03:50, 14 March 2006 (UTC)

Further discussion of this issue can be found at Talk:Candela.--Srleffler 04:04, 14 March 2006 (UTC)


It seems like Annum should be included as well. Although only some variations are used, Kilo-annum and Mega-annum, they do seem to be SI units with Annum as the base unit 02:48, 16 September 2006 (UTC)

Where is volume in the chart and why is the chart hidden

the chart does not contain The cubic metre (symbol ) which is the SI derived unit of volume. Why? hey could you put Volum on the Chart

and why is the chart hidden --Antiedman 13:19, 14 October 2007 (UTC)

Most units are derived from other units (cubic meter, millitre, volt, newton,…). A few units must have arbitrary definitions so that all the other units can be derived from them. That is what the base units are. Once the metre is defined, the cubic metre can be defined from it, so cubic metre is not a base unit. --Gerry Ashton 15:36, 14 October 2007 (UTC)
As for why the chart is "hidden", it is a template, so that the same chart can easily be presented in more than one article. If you enter Template:SI_base_units in the search box you will be taken to the template, and you can even edit it. You can also click the toolbox command "What links here" (on the left edge of the screen) and discover that this is the only article using the template, although it is linked in a few things that are not articles. --Gerry Ashton 15:41, 14 October 2007 (UTC)

Different information in different places

--non registered user 12:34 16 December 2007 (GMT) "The fundamental units of the SI system are kilogram, second, ampere, kelvin, and mole. The metre and candela were formerly fundamental units but have been redefined in terms of the other units." Fundamental units I don't know which is correct but it should say the same thing on both places.

Fundamental units and base units are different concepts. You will notice, for example, that the SI base unit metre is defined in terms of the second, by making measurements that depend on the speed of light. The candella is defined in terms of the watt and the inverse of the second (Hz). The watt is further defined in terms of the kg, m, and second, or, in terms of the kg, second, and the speed of light.
While the metre and candella are not fundamental units, references to the speed of light in the definition of the metre would make dimensional analysis difficult. I've never worked with the candella, but working directly with its definition might be difficult as well. Thus, the metre and candella are retained for the purpose of defining other units and dimensional analysis. --Gerry Ashton (talk) 18:52, 16 December 2007 (UTC)


Where does the unit 'Degree' fall? I mean Degree as in angle, not as in temperature. For instance, "An equilateral triangle has equal sides and angles, i.e., each angle is 60 degrees". Kerina yin (talk) 04:49, 16 April 2009 (UTC)

The degree of arc is one of the non-SI units accepted for use with SI. The SI unit of plane angle is the radian. Physchim62 (talk) 09:59, 16 April 2009 (UTC)
So there it is! Thanx for the concise answer. Kerina yin (talk) 08:34, 17 April 2009 (UTC)
There's not a lot more to say! One degree of arc is exactly equal to 2π/360 radians. You can't put an exact figure on the conversion factor, but you can calculate it to as many decimal places as you like. In physics, it's more common to use radians rather than degrees simply because you can (sometimes) avoid the factor of 2π by doing it that way! Physchim62 (talk) 15:03, 17 April 2009 (UTC)

dimensionally independent

I rewrote the lede to remove the phrase 'dimensionally independent'. It is not clear that the intended meaning was properly represented by this term. Dimensional analysis doesn't seem to define what such a term might mean. However, the theory states, that the set of dimensions fundamental to any system of units must be 'linearly independent'. Furthermore, the realization of these units indeed often defines them in terms of each other, making them perhaps 'dimensionally *dependent*' in a given framework. Kbrose (talk) 16:32, 12 June 2009 (UTC)


For physicists, Base units are those not derived from others. The article is INCORRECT, as there are only three, yes, three. THe others can all be derived from them DrP. —Preceding unsigned comment added by (talkcontribs) 19:14, 15 January 2010 (UTC)

Not true. You can find the international metrological definition of a "base quantity" at Template:VIM3rd. You might like to look at our article on natural units, which demonstrates that the choice of base quantities is arbitrary. Indeed, it is frame-dependent in SR, let alone GR! Physchim62 (talk) 15:50, 9 February 2010 (UTC)

No Angle measurement

I see no units of angles measurement on the list and radians only appear on derived units. —Preceding unsigned comment added by (talk) 20:56, 8 September 2010 (UTC)

That's because it is a derived unit. Dauto (talk) 18:47, 21 February 2011 (UTC)

Charge, coulomb

Proposed addition, coulomb, isnt it neccessary? — Preceding unsigned comment added by (talk) 03:50, 12 September 2011 (UTC)

The authors of the SI Handbook have seen fit to define the ampere and the second as base unit and the coulomb to be dI/dt. Wikipedia reports what they have written. Martinvl (talk) 06:57, 12 September 2011 (UTC)

I thought the number of base units had been increased to 9

I've been under the impression for some time now that the number of base units has been increased to 9 to include the radian and steradian. Does anybody know for certain? Anybody seen evidence for this change? LL — Preceding unsigned comment added by Lindy Louise (talkcontribs) 15:26, 27 November 2012 (UTC)

A few years ago the radian and steradian were recognised as dimensionless derived units, not base units. Prior to that they were "units that could be used alongside SI". Martinvl (talk) 20:48, 27 November 2012 (UTC)

The main article diagram, "SI base unit.svg", is confusing

The diagram for SI base unit.svg is labeled "The seven SI base units and the interdependency of their definitions". The article text says that these units are "mutually independent dimensions", which seems to contradict the diagram. The arrows are unlabeled and their meaning is unclear. The arrows seem to show some sort of interdependency relationship, but given that these dimensions are mutually independent what relationship do these arrows imply? Do the arrows imply SI derived units? If so, I can't find any derived unit for the relationship shown by "kilogram -> candela". I know a Luminous intensity in candelas has dimensions in J, which is not Joules, but the Joule is the only obvious unit that would imply some sort of relationship to kilograms (a Joule being kg*m^2/s^2)... At any rate, my point is that the diagram is confusing. What interdependency relationships is it trying to describe? --NoahSpurrier (talk) 01:45, 7 December 2012 (UTC)

The relationships are given in the table below. In the case of the candela, and ampere, they are defined in terms of three other units, so have three arrows pointing at them. The definition of the candela is in terms of frequency and power, but frequency is measured in seconds (or Hz == 1/sec), while power is energy/sec and energy can be measured in kg × m (potential energy).--JohnBlackburnewordsdeeds 02:28, 7 December 2012 (UTC)
I have extended the caption to explain the diagram. I hope that this helps.

Revocation of 26 May 2013

I undid the changes by User:IvanderClarent for the following reasons:

  • The original definition of the kilogram (1799) specified the freezing point of water, not 4 C. In addition the definition of the litre has changed over the years. To keep matters simple, it is best to avoid mentioning temperature
  • The text allegedly added by the CIPM is incorrect.
  • The statement "A second is 1 ⁄ (24 × 60 × 60) of the day or 1.1574074074074074074074074074074 x 10s−5 of a day" is already implied by the previous sentence. There is no need to repeat it.

Martinvl (talk) 05:36, 26 May 2013 (UTC)