# What is Units of measurement: Definition and 18 Discussions

A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement.
For example, a length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre".
Measurement is a process of determining how large or small a physical quantity is as compared to a basic reference quantity of the same kind.
The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to the present. A multitude of systems of units used to be very common. Now there is a global standard, the International System of Units (SI), the modern form of the metric system.
In trade, weights and measures is often a subject of governmental regulation, to ensure fairness and transparency. The International Bureau of Weights and Measures (BIPM) is tasked with ensuring worldwide uniformity of measurements and their traceability to the International System of Units (SI).
Metrology is the science of developing nationally and internationally accepted units of measurement.
In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results is central to the scientific method. A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures historically developed for commercial purposes.Science, medicine, and engineering often use larger and smaller units of measurement than those used in everyday life. The judicious selection of the units of measurement can aid researchers in problem solving (see, for example, dimensional analysis).
In the social sciences, there are no standard units of measurement and the theory and practice of measurement is studied in psychometrics and the theory of conjoint measurement.

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1. ### A Convention of units for densities in cosmology

I have a table of densities of galaxies : Expected number density of galaxies for photometric survey per unit area and redshift intervals, ##\mathrm{d} N / \mathrm{d} \Omega \mathrm{d} z\left[\mathrm{sr}^{-1}\right]## and the corresponding density of galaxies per ##\operatorname{arcmin}^2## for...
2. ### A How to convert mW/m^2 to W/m^2/nm?

I have a vizier catalog with H-alpha fluxes as logF(Haplha) in units as mW/m^2. See sample data here: http://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=J/MNRAS/431/2/fluxes How do I convert mW/m^2 to W/m^2/nm or erg/s/cm^2/A? The first entry is logF(Halpha) = -12.03 mW/m^2
3. ### B How to describe complicated dimensions?

Hi, just wondering if the dimension of velocity is m/s that can be described as what distance is passed in a specific time, then how can I describe volt which is kg•m²÷s³÷A? Mass in the area that moved... I can't even imagine. Thanks.
4. ### I Which units is this conversion factor for (molar volume)? 0.023901488

I'm getting the wrong results when using an old, undocumented code and just realized there's a number lurking in it that I can't account for. It's: 0.023901488 and it is multiplied with molar volume and pressure. I have searched for a couple of hours but just can't figure out what the units...
5. ### I Converting density unit ##MeV^4## to SI units

How to transform density unit in natural units $MeV^4$ to SI units $kg/m^3$, Here's my trial: ##MeV^4 = (10^6)^4 ~ eV^4 = 10^{24} ~ eV^4 ##, ## eV = 1.6 * 10^{-19}~ kg~ m^2 / sec^2, ## ##MeV^4 = 10^{24} ~ 1.6^4 * 10^{-40} ~ kg^4 m^8 / sec^8 ## This is not simply ##kg/m^3##! Any help how to...
6. ### I Redefining the Mass Unit: Counting Particles for Precision

Can you give details?
7. ### Multiplying epsilon naught by a length quantity

Homework Statement Note that this formula (##C=4 \pi \epsilon_0 R##) and the others we have derived for capacitance involve the constant multiplied by a quantity that has the dimensions of a length. Homework Equations ##\epsilon_0## has the following units in SI: $$\frac {C^2} {N \cdot m^2}$$...
8. ### I Cgs or SI in quantum field theory?

I have an acquaintance who maintains that in quantum field theory, primarily the cgs system is used. OK, I know it's not really important, but I was under the impression that everyone had switched to SI. (My book on quantum field theory has very few actual quantities with units outside of GeV...
9. ### I Why Planck's Constant Has Dimensions and a Unit?

Despite being a constant, It has both dimensions and a unit, can someone kindly explain why ?
10. ### B Understanding the Equations and Units

I have been struggling through my physics class this summer (it doesn't help that 5 months worth of material is crammed into 2 months in an online classroom). This site has helped me out a lot, but I am still stumbling through understanding when to use certain equations. When I started reading...
11. ### Dimensional Analysis: Subtracting Units & Unitless Numbers

this question is about dimensional analysis involving a number with units and a number with no units, if the question is already answered in another post please redirect me if not here is a simple example, for example, : say i have 2[in]-1. the 1 is dimensionless and the 2 has units of [in]...
12. ### Calculating something wrong with Poiseuille's Law

Homework Statement At resting, can you breath sufficient air through a tube of 100 cm length and 2 cm radius? normal resting respiration rate: 10 - 20 breaths per minute (3 - 6 seconds per breath) normal resting respiration volume: 0.5 L normal pressure difference in respiration = 1 mmHg =...
13. ### A strange inconsistency when calculating area with decimals

I have a question about a seemingly illogical and strange aspect of multiplication and unit conversion that I have never noticed until now. It concerns the issue of finding the area of a square/rectangle when the length and width are expressed as decimals/fractions. Ordinarily, when you find...
14. ### Can you explain this 'Theory of Everything' formula?

I recently come across with an amazing equation of Theory of Everything; I wonder if TOE has been formulated (?) I found this equation on a website, check it out http://www.preposterousuniverse.com/blog/2013/01/04/the-world-of-everyday-experience-in-one-equation/. While seeing briefly this...
15. ### Angular momentum Units a very basic question

I have a very basic questions about units for angular momentum. The measure is in kg m^2/s Angular velocity is in radians/s and therefore radians do not appear in the units. Here is my question, can we leave this in degees/s? Sure its not used but is it wrong? If we are dealing with...
16. ### Measuring Length, Area & Volume: Classical vs Quantum Physics

I am talking about length area and volume. As I reason it out in my own mind area (and volume) is based on length and involves 2 random measurements of length that are combined (with multiplication as the device of convention - could any other function be used to work as well and as...
17. ### TGM units of measurement (note: unfamiliar notation used)

The units of measurement of the SI (System International) are commonly used around the world ·· they form a coherent system usable for a variety of branches of science. But.. there's another coherent metric system that may be a better fit for many applications in chemistry and physics. The...
18. ### Units of Measurement for Railcar Operating Loads

Hi everybody! Would someone please advise on the following matter. I'm making up a table of railcar operating loads. One of the column includes abbreviated units of measurement such as kN, MN, N/mm2, etc. The question is: How should I title the column? I was thinking about titling it...