en.wikipedia.org/wiki/International_System_of_Units

At the end of the Second World War, a number of different systems of measurement were in use throughout the world. Some of these systems were metric system variations; others were based on customary systems of measure, like the U.S customary system and Imperial system of the UK and British Empire. The Practical system of units

en.wikipedia.org/wiki/Mass

the atomic mass unit (u) is 1/12 of the mass of a carbon-12 atom, approximately 1.66 × 10 −27 kg. The atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units of mass include: the slug (sl) is an Imperial unit of mass (about 14.6 kg).

en.wikipedia.org/wiki/Hartree_atomic_units

Use and notation. Atomic units, like SI units, have a unit of mass, a unit of length, and so on.However, the use and notation is somewhat different from SI. Suppose a particle with a mass of m has 3.4 times the mass of electron. The value of m can be written in three ways: " = ".This is the clearest notation (but least common), where the atomic unit is included explicitly as a symbol.

chemed.chem.purdue.edu/genchem/topicreview/bp/ch1/index.php

The English System of Units. There are several systems of units, each containing units for properties such as length, volume, ... Mass is a measure of the amount of matter in an object, ... a number of units that are not strictly acceptable under the SI convention are still in use. Some of these non-SI units are given below.

en.wikipedia.org/wiki/System_of_measurement

A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce.Systems of measurement in use include the International System of Units (SI), the modern form of the metric system, the imperial system, and United States customary units

www.quora.com/What-are-the-different-types-of-system-units

OK, everyone is providing one or two conversion factors between two specific units of volume but you can perform conversions between 300 different pairs of units using vCalc’s (vCalc ) vVolume conversion calculator. Try actually using the vCalc ca...

www.mathsisfun.com/metric-numbers.html

Metric Numbers (See also Metric/Imperial Conversion Charts and Unit Converter). What is kilo, mega, giga, tera ... ? In the Metric System there are standard ways to talk about big and small numbers: "kilo" for a thousand, "mega" for a million, and more ...

en.wikipedia.org/wiki/Units_of_measurement

Different systems of units are based on different choices of a set of related units including fundamental and derived units. Calculations with units of measurement Units as dimensions. Any value of a physical quantity is expressed as a comparison to a unit of that quantity.

www.youtube.com/watch?v=6h0G-7WxSDk

These three units are the most common measurements of mass in the metric system. The SI base unit of mass is the kilogram A gram, which is written with a g, weighs about the same as a paper clip.

www.britannica.com/science/kilogram

Kilogram, basic unit of mass in the metric system. A kilogram is very nearly equal (it was originally intended to be exactly equal) to the mass of 1,000 cubic cm of water. It was once defined in terms of a platinum-iridium cylinder, but in 2018 the CGPM agreed to define it in terms of Planck’s constant.