Basics of Spaceflight: Units of Measure

Units of Measure

If you don't find the term you're looking for here, look in the Glossary.

In the Basics of Space Flight, most abbreviations for units of measure are not spelled out the first time they are used, unlike the way other abbreviations are. They are all listed on this page instead.

The first list contains abbreviations you'll find most frequently in the Basics of Space Flight. Below it you'll find some of the International System of Units, SI, and a link to the The National Institute of Standards for complete details. Next is a selection of formulas for converting SI to English measures, a link to a conversions engine, and a link to a comprehensive set of conversion tables.

1. Unit Abbreviations Frequently Found in the Text

au
Astronomical Unit, a measure of distance, based on the mean Sun-Earth distance. The International Astronomical Union defines the AU as the distance from the Sun at which a particle of negligible mass, in an unperturbed orbit, would have an orbital period of 365.2568983 days (a Gaussian year). The AU is thus defined as 1.4959787066E+11m (149,597,870.66 km). [More]
bps
Bits per second, a measure of data rate
c
Speed of light in a vacuum, 299,792,458 m/sec
G
Giga, a multiplier,* x109 from the Latin "gigas" (giant). In the U.S., 109 is a billion, while in other countries using SI, 1012 is a billion. Giga means 109 everywhere.
g
Gram, a unit of mass (see SI units below)
Hz
Hertz, the number of cycles per second
k
Kilo, a multiplier,* x103 from the Greek "khilioi" (thousand)
LY
Light Year, a measure of distance, the distance light travels in one year; about 63,240 au
M
Mega, a multiplier,* x106 from the Greek "megas" (great)
m
Meter, a unit of length (USA spelling; elsewhere, metre) (see SI units below)
N
Newton, a unit of force (see SI units with special names, below)
s
Second, the SI unit of time (see this extensive definition)
W
Watt, a unit of power (see SI units with special names, below)

* The remaining multipliers are listed in the GLOSSARY.

2. International System of Units, SI

SI has long been the notation universally used in science and technology. It has also become the dominant language of international commerce and trade, except in the U.S.

Base quantity
Name
Symbol
SI base unit
length
meter
m
mass
kilogram
kg
time
second
s
electric current
ampere
A
thermodynamic temperature
kelvin
K
amount of substance
mole
mol
luminous intensity
candela
cd
SI Unit

For a comprehensive and definitive reference on all aspects of SI, as well as many other quantities and standards, please visit the National Institute of Standards (NIST) website. Some of the information on this page has been obtained from there.

See also the Solar System Temperature Reference for examples and temperature comparisons of objects and conditions in space, from absolute zero through planet temperatures, to those of stars.

Some Derived SI Quantities

Selected from NIST website

Derived quantity
Name
Symbol
SI derived unit
area
square meter
m2
volume
cubic meter
m3
speed, velocity
meter per second
m/s
acceleration
meter per second squared
(meter per second per second)
m/s2
wave number
reciprocal meter
m-1
mass density
kilogram per cubic meter
kg/m3
specific volume
cubic meter per kilogram
m3/kg
current density
ampere per square meter
A/m2
magnetic field strength
ampere per meter
A/m

SI Units with Special Names

Selected from NIST website

Derived quantity
Name
Symbol
Expression
in terms of
other SI units
Expression
in terms of
SI base units
SI derived unit
plane angle
radian
rad
-
m·m-1 = 1
solid angle
steradian
sr
-
m2·m-2 = 1
frequency
hertz
Hz
-
s-1
force
newton
N
-
m·kg·s-2
pressure, stress
pascal
Pa
N/m2
m-1·kg·s-2
energy, work, quantity of heat
joule
J
N·m
m2·kg·s-2
power, radiant flux
watt
W
J/s
m2·kg·s-3
electric charge, quantity of electricity
coulomb
C
-
s·A
electric potential difference,
electromotive force
volt
V
W/A
m2·kg·s-3·A-1
capacitance
farad
F
C/V
m-2·kg-1·s4·A2
electric resistance
ohm
Omega
V/A
m2·kg·s-3·A-2
electric conductance
siemens
S
A/V
m-2·kg-1·s3·A2
magnetic flux
weber
Wb
V·s
m2·kg·s-2·A-1
magnetic flux density
tesla
T
Wb/m2
kg·s-2·A-1
inductance
henry
H
Wb/A
m2·kg·s-2·A-2
Celsius temperature
degree Celsius
°C
-
K
luminous flux
lumen
lm
cd·sr
m2·m-2·cd = cd
illuminance
lux
lx
lm/m2
m2·m-4·cd = m-2·cd

3. A Few Handy SI-to-English Conversions

Take the number of SI units and apply the conversion to get the number of English units. For example, 2 meters equals about 6.56 feet.

Millimeters to inches
mm
x
0.0393700787401575
=
in
Centimeters to inches
cm
x
0.393700787401575
=
in
Meters to feet
m
x
3.28083989501312
=
ft
Meters to yards
m
x
1.09361329833771
=
yds
Kilometers to miles
km
x
0.621371192237334
=
mi
Grams to ounces
g
x
0.0352739907229404
=
oz
Kilograms to pounds
kg
x
2.20462262184878
=
lbs
Celsius to Fahrenheit
(°C
x
9/5) + 32
=
°F
Newtons to Pounds Force:
N
x
0.224809024733489
=
lbf
SI to English Conversions


Keep Exploring

Discover More Topics From NASA