Primary Dimensions and SI Units
Quantification is essential to the understanding of both continuous and discontinuous media. The first issue is determining the dimension or dimensions to be measured. Seven “primary” dimensions are recognized: mass [M], length [L], time [T], amount of a substance [N], electric current, temperature, and luminous intensity. Square brackets at this website are used to indicate dimensions of a variable, and only the four dimensions whose abbreviations are shown in such brackets will be used extensively. “Secondary” or “derived” dimensions are made up of primary dimensions. Velocity, for example, is distance (length) per unit of time [L T-1].
An "N" in square brackets [N] is not standard notation and is included here as a primary dimension to deal with some cruder measures than are typically needed in laboratory physics and chemistry. In aquatic environments, e.g., for purposes of estimating encounter rates, one may often be interested in number concentrations of organisms or inanimate particles. In general, this website uses [N] for countable entities, and does not restrict its use to elementary entities that could be properly counted in moles. A chemist would cringe at the thought of a mole of (furry) moles, but this website would sanction the idea of Avogadro's number of bacteria as a legitimate treatment of countable entities, including both the strict chemist's moles and numbers of any countable entity under the primary dimension [N].
Other than this site-specific artifice for countable entitites, this site uses SI units (Système International d’Unités) for all calculations. Serious mischief can occur if systems of units are mixed: NASA lost the 125 million-dollar Mars Climate Orbiter because one development team used SI, whereas another used English, units. If you have obtained measurements in one set of units and have trouble remembering how, a number of helpful websites can help you make the necessary conversions, and Google’s calculator can now help directly. If you have the name of a measurement unit but don’t recognize it, you can probably find it in an online dictionary. Be especially careful not to mix two different subsets of metric conventions, so called mks (for meter, kilogram second; which conform to SI) with cgs (centimeter, gram, second; which in general require conversion to SI). This site does use grams and centimeters when they are convenient, but not in formulas or calculations. If you need a refresher on acceptable prefixes (e.g., to find out how many grams in a petagram), visit the table of SI prefixes at the NIST (U.S. National Institute of Standards and Technology) site. NIST has useful help on all aspects of SI units and physico-chemical constants.
Try to stick with or convert to the following:
If you need more information on dimensions, units or physical constants, the U.S. National Institute of Standards and Technology maintains one of the most useful sites.