The international system for units.
Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.
It is based upon 7 principal units:

From these basic units many other units are derived and named.

(A or amp) The SI base unit of electric current.
Definition: Two parallel conductors, infinitely long and having negligible cross section should be placed 1 meter apart in a perfect vacuum. One ampere is the current that creates between them a force of 0.2 µN (micronewton) per meter of length.
One ampere represents a current flow of 1 coulomb of charge per second.
One ampere of current results from a potential distribution of 1 volt per ohm of resistance, or from a power production rate of 1 watt per volt of potential.
The unit is known informally as the amp, but A is its official symbol and is named for the French physicist André-Marie Ampère.
See also System International.

(Bq) Becquerel is the system international (SI) since 1985 new unit of radioactivity. 1 becquerel is equal to 1 disintegration per second.
1 Bq = 0.027 x10^-9 Ci (Curie).
In medicine and radiation protection the SI measurement units of becquerel, gray and sievert (should) have replaced the conventional units of curie, rad and rem.

To convert:


See also Gray, Sievert, Roentgen Equivalent In Man, Radiation Absorbed Dose, Count and Becquerel Antoine Henri.

(Hz) The standard SI unit of frequency.
Definition: The number of repetitions of a periodic process per unit time. It is equal to the old unit cycles or oscillations each second of a simple harmonic motion. The unit is named for the German physicist Heinrich Rudolf Hertz.
Larger units are:
kilohertz (KHz) = 1 000 Hz = 10³ Hertz
megahertz (MHz) = 1 000 KHz = 10⁶ Hertz
gigahertz (GHz) = 1 000 MHz = 10⁹ Hertz
terahertz (THz) = 1 000 GHz = 10¹² Hertz
petahertz (PHz) = 1 000 THz = 10¹⁵ Hertz
exahertz (EHz) = 1 000 PHz = 10¹⁸ Hertz

See also Oscillation, Coherence, Duty Cycle, Cine Mode, and System International.

(m) The SI base unit of distance.
Definition: 1983 defined as the distance traveled by light in a vacuum during the time interval of 1/299 792 458 of a second.
The speed of light in a vacuum, c, is one of the fundamental constants of nature.

1 meter (m) is equal to approximately 39.370 079 inches (in)
1 meter is equal to approximately 3.280 840 feet (ft)
1 meter is equal to approximately 1.093 613 3 yard (yd)
1 square meter (m²) is equal to approximately 10.763911 square feet (ft²)
1 inch = 2.54 centimeters
Smaller or larger units are, e.g.:
1 meter (m) = 1 000 millimeter (mm)
1 micrometer (µm) = 10^-6 meter (m)
1 nanometer (nm) = 10^-9 meter (m)
1 picometer (pm) = 10^-12 meter (m)
1 femtometer (fm) = 10^-15 meter (m)
1 kilometer (km) = 1 000 meter (m)
1 kilometer (km) = 0.62137 (statute) miles (mi)

See also System International.