What units of measurement do scientists use?

Base Units

The SI is a system based around seven base units, each with their own symbols:

metre (m): length
kilogram (kg): mass
second (s): time
ampere (A): electric current
candela (cd): luminous intensity
mole (mol): amount of substance
kelvin (K): temperature

Derived units

Derived units are formed by various combinations of the base units.

For example, velocity is defined as distance per unit of time, which in SI has the dimensions of metres per second (m/s).

Some of these derived units have special names and symbols:

hertz (Hz) — frequency (#"s"^"-1"#)
newton (N) — force (#"kg⋅m⋅s"^"-2"#)
pascal (Pa) — pressure (#"N·m"^"-2"# or #"kg⋅m"^"-1""s"^"-2"#)
joule (J) — energy (#"N⋅m"# or #"kg⋅m"^2⋅"s"^"-2"#)
watt (W) — power (#"J·s"^"-1"# or #"kg⋅m"^2⋅"s"^"-3"#)

Multiplying Prefixes

SI multiplying prefixes are used to form decimal multiples and submultiples of SI units.

There are prefixes for multipliers ranging from #10^24# to #10^"-24"# in steps of #10^3#.

The prefixes for the most common powers of ten are:

#"Prefix"color(white)(l) "Symbol" color(white)(l)"Power"#
#stackrel(—————————)("tera"color(white)(mml) "T" color(white)(mml)10^(12))#
#"giga" color(white)(mml)"G" color(white)(mml)10^9#
#"mega" color(white)(mll)"M" color(white)(mml) 10^6#
#"kilo" color(white)(mmll)"k" color(white)(mmll)10^3#
#color(white)(mmmmmmmll)10^0#
#"milli"color(white)(mml) "m" color(white)(mm) 10^"-3"#
#"micro" color(white)(mm)µ color(white)(mm)10^"-6"#
#"nano" color(white)(mml)"n" color(white)(mm)10^"-9"#
#"pico" color(white)(mmll)"p" color(white)(mm)10^"-12"#

Thus, instead of writing a measurement as #"0.000 000 000 001 m"# or #1 × 10^"-12" color(white)(l)"m"#, we would write it as #"1 pm"#.