SI Prefixes

Multiplication Factor Prefix † Symbol
tera T
giga G
mega M
kilo k
hecto‡ h
deka‡ da
deci‡ d
centi‡ c
milli m
micro µ
nano n
pico p
femto f
atto a

† The first syllable of every prefix is accented so that the prefix will retain its identity. Thus, the preferred pronunciation of kilometer places the accent on the first syllable, not the second.

‡ The use of these prefixes should be avoided, except for the measurement of areas and volumes and for the nontechnical use of centimeter, as for body and clothing measurements.


Principal SI Units Used in Mechanics

Quantity Unit Symbol Formula
Acceleration Meter per second squared ... m/s2
Angle Radian radian
Angular acceleration Radian per second squared ... rad/s2
Angular velocity Radian per second ... rad/s
Area Square meter ... m2
Density Kilogram per cubic meter ... kg/m3
Energy Joule J N.m
Force Newton N Kg.m/s2
Frequency Hertz Hz
Impulse Newton-second ... Kg.m/s
Length Meter m
Mass Kilogram kg
Moment of a force Newton-meter ... N . m
Power Watt W J/s
Pressure Pascal Pa N/m2
Stress Pascal Pa N/m2
Time Second s
Velocity Meter per second ... m/s
Volume, solids Cubic meter ...
Liquids Liter L
Work Joule J N.m

t Supplementary unit (1 revolution =2 rad = 360º)

‡ Base unit.


U.S. Customary Units and Their SI Equivalents

Quantity U.S. Customary Units I Equivalent
Acceleration ft/s2 0.3048 m/s2
in./s2 0.0254 m/s2
Area 0.0929 m2
in2 645.2 mm2
Energy ft.Ib 1.356J
Force kip 4.448 kN
Ib 4.448 N
oz 0.2780 N
Impulse Lb.s 4.448 N.s
Length ft 0.3048 m
in. 25.40 mm
mi 1 .609 km
Mass oz mass 28.35 g
lb mass 0.4536 kg
slug 14.59kg
ton 907.2 kg
Moment of a force lb.ft 1.356N.m 0.1130 N.m
Moment of inertia
Of an area in4 0.4162 X 106 mm4
Of a mass lb.ft.s2 1 .356 kg.m2
Power 1 .356 W
hp 745.7 W
Pressure or stress lb/ft2 47.88 Pa
lb/in2 (psi) 6.895 kPa
Velocity ft/s 0.3048 m/s
in./s 0.0254 m/s
mi/h (mph) 0.4470 m/s
mi/h (mph) 1.609km/h
Volume, solids ft3 0.02832 m3
Liquids Gal 3.785 L
qt 0.9464 L
Work 1.356 J


U.S. Customary Units and Their SI Equivalents

Quantity U.S. Customary Unit SI Equivalent
Length Ft 0.3048 m
in. 25.40 mm
mi 1.609km
Mass slug 14.59 kg
Force lb 4.448 N
kip 4.448 kN
Area ft2 0.0929 m2
in.2 0.6452 x
Volume ft3 0.02832 m3
in.3 16.39 X 10-6m3
Velocity ft/s) 0.3048 m/s
mi/h (mph 0.4470 m/s
mi/h (mph) 1.609 km/h
Acceleration ft/s2 0.3048 m/s2
Moment of a force lb.ft 1.356N.m 0.1130 N.m
Pressure or stress lb/ft2(psf) 47.88 Pa (pascal or N/)
lb/in.2(psi) 6.895 kPa (kN/)
Spring constant lb/ft 14.59 N/m
lb/in. 175.1 N/m
Load intensity lb/ft 14.59N/m
kip/ft 14.59kN/m
Area moment of inertia in.4 0.4162 x lO-6m4
Work or energy lb.ft 1.356 J (joule or N.m)
Power Ib.ft/s 1.356(walt or N.m/s)
745.7 W (watt or N.m/s)


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