Before going into methods, you may wonder such things as, "What exactly is a volt?" or "How many electrons are in an Ampere?". You don't need to know these things to understand electronics, but you may be curious about them.

The scientific world uses the International System of Units (SI units) to measure things. The International System of Units starts with three basic units: the kilogram, the meter and the second. Every other unit is derived from these three basic units. For example, to know what a volt is, we have to know what a watt is. To know what a watt is, we have to know what a joule is. To know what a joule is we have to know what a meter and a Newton are. Finally, to know what a Newton is we have to know what a kilogram a meter and a second are.

Below, the units of measure are grouped into base units, derived non-electrical units and derived electrical units. These definitions are taken from the NIST Special Publication 330 The International System of Units (SI) 2019 Edition and are currently used by scientists worldwide.

Kilogram (kg) -The definition of the kilogram is a mathematical relationship of the Planck constant, the speed of light and the meter (for the specifics you can refer to page 7 of the NIST Special Publication 330, 2019 edition). This definition makes the mass of a kilogram equal to the mass of the international prototype of the kilogram as of November 2018. The international prototype of the kilogram is a particular cylinder of platinum-iridium alloy that is preserved in a vault at Sevres, France, by the International Bureau of Weights and Measures. The kilogram was originally defined as the mass of one liter of water. However, several variables that make a volume of water unsuitable for a standard of mass. The kilogram was the last unit to be defined by an actual object.

Second (s) - The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the hyperfine levels of the ground state of the cesium 133 atom. The second was originally defined as ¹/_86,400 of a mean solar day. However, due to the gravitational effects of the Moon, the speed of the Earth’s rotation is slowing down, making it unsuitable for a standard of measurement. In 1954 the second was redefined as ¹/_{31 556 925.9747} of the year 1900, which corresponds to the cesium standard above.

Meter (m) - The meter is the length of the path traveled by light in vacuum during a time interval of ¹/_299,792,458 of a second. The meter was originally defined as ¹/_10,000,000 of the distance from the  Earth's pole to the equator. However, the earth is not uniform, so this measurement is somewhat subjective. Since we are able to measure time with very high accuracy and the speed of light is absolute, the modern standard was established.

Newton (N) - The Newton is that force which gives to a mass of 1 kilogram an acceleration of 1 meter per second per second (^1m/_s²). In other words, if you put a force of 1 newton against a mass of 1 kilogram, 1 second later that mass will be moving 1 meter per second faster than it was before.

Joule (J) - The joule is the work done when the point of application of 1 newton is displaced a distance of 1 meter in the direction of the force. In other words, put a force of 1 newton against any object and apply that force until that object has moved 1 meter and you have done 1 Joule of work.

Ampere (A) - The ampere is flow of 6.241 509 074×10¹⁸ elementary charges per second. Since an electron carries a single elementary negative charge, this essentially defines the ampere as a flow of 6,241,509,480,000,000,000 electrons per second.^[1] Before 2019, the definition of an ampere was that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one meter apart in vacuum, would produce between these conductors a force equal to 2×10⁻⁷ newtons per meter of length.

Coulomb (C) - The coulomb is the quantity of electricity transported in 1 second by a current of 1 ampere.  This essentially defines a coulomb as a quantity of 6,241,509,074,000,000,000 electrons.

Watt (W) - The watt is the power that gives rise to the production of energy at the rate of 1 joule per second. In electrical circuits, power is usually manifested by the production of heat. It may also be manifested by the production of electromagnetic waves or mechanical motion.

Volt (V) - The volt is defined as the difference of electric potential between two points of a conducting wire carrying a constant current of 1 ampere when the power dissipated between these points is equal to 1 watt. In the 1880s, the International Electrical Congress (today the International Electrotechnical Commission [IEC]) essentially defined a volt as the voltage produced by a Daniell cell, the standard at the time for telegraph batteries around the world. From 1893 to 1908, it was redefined as ¹/_1.434 of the voltage produced by a Clark cell at 15°C. From 1911 to 1990, it was changed to ¹/_1.018638 of the voltage produced by a Weston cell, which is more temperature-stable.

Ohm (Ω) - The ohm is defined as the electric resistance between two points of a conductor when a constant difference of potential of 1 volt, applied between these two points, produces in this conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.^[2] In 1860, Werner Siemens proposed defining the ohm as the resistance of a column of pure mercury, one millimeter in cross-section and one meter long. This would have made the value of an ohm equal to 0.9537 of today's ohm. In 1881, the International Electrical Congress defined the ohm as the resistance of a mercury column of "specified weight" and 106cm (1.06 meters) long.^[3] Note that this definition precludes applying Ohm's Law to any part of a circuit where power is originating. For example, if you apply Ohm's Law to the operating secondary winding of a transformer—dividing the voltage across the winding by the known ohmic resistance of the winding—the result is meaningless.

Farad (F) - The farad is the capacitance of a capacitor between the plates of which there appears a difference of potential of 1 volt when it is charged by a quantity of electricity equal to 1 coulomb. In other words, if you force 1 coulomb of electricity into a capacitor, after which that capacitor then has a charge of 1 volt, it is a 1-farad capacitor.

Henry (H) - The henry is the inductance of a closed circuit in which an electromotive force of 1 volt is produced when the electric current in the circuit varies uniformly at a rate of 1 ampere per second. Essentially, an inductor of 1 henry has the capacity to store the same energy as a 1-farad capacitor. The capacitor stores the energy as an electric field. The inductor stores it as a magnetic field.

Pascal (Pa) - This is a measure of pressure in force per unit area. The pascal is defined as 1 newton per square meter. It is of interest because it is used to measure sound level. A sound pressure level of 20 micropascals is barely audible to the average human and is referred to as the threshold of hearing.