# Guide to SI Units

The International System of Units (SI) has been adopted in the publications of several scientific and technical societies in the United States and other countries. It is expected that in due course that these units will come into general use. The SI units and conversion factors applicable to this book are given below. For further information, see “The International System of Units (SI),” National Bureau of Standards Special Publication 330, U.S. Government Printing Office, Washington, D.C. 20402.

Base Units
Quanity SI Unit Symbol
Length meter m
Mass kilogram kg
Time second s
Electric current ampere A
Temperature* kelvin K

* (Temperatures may also be expressed in °C)

Derived Units
Quantity Unit Symbol Formula
Force newton N kg·m/s2
Pressure pascal Pa N/m2
Energy, heat, etc. joule J N·m
Power watt W J/s
Frequency hertz Hz I (cycle)/s
Radioactivity becquerel Bq I (decay)/s
Absorbed dose gray Gy J/kg
Conversion Factors
To convert from: to: multiply by:
Length, Area, Volume
inch meter (m) 2.540 × 10-2
foot meter (m) 0.3048
yard meter (m) 0.9144
mile kilometer (km) 1.609
centimeter meter (m) 10-2
angstrom meter (m) 10-10
square inch meter2 (m2) 6.452 × 10-4
square foot meter2 9.290 × 10-2
square mile kilometer2 (km2) 2.590
cubic foot meter3 (m3) 2.832 × 10-2
Mass
pound kilogram (kg) 0.4536
ounce kilogram (kg) 2.835 × 10-2
Energy
calorie joule (J) 4.187
erg joule (J) 1.00 × 10-7
MeV joule (J) 1.602 × 10-13
ton (TNT equivalent) joule (J) 4.2 × 109
Miscellaneous
density (lb/ft3) kg/m3 1.602 × 10
pressure (psi) pascal (Pa) 6.895 × 103
radiant exposure (cal/cm2) J/m2 4.187 × 104
speed (ft/sec) m/s 0.3048
speed (miles/hour) m/s 0.4470
dose (rads) gray (Gy) 1.00 × 10-2
dose rate (rads/hour) Gy/s 2.778 × 10-6
curie becquerel (Bq) 3.700 × 1010

The only multiples or submultiples of SI to which appropriate prefixes may be applied are those represented by factors of 10n or 10-n where n is divisible by 3. Thus, kilometer (103m or 1 km), millimeter (10-3m or 1 mm), and micrometer (10-6m or 1 μm). The centimeter and gram are not used in the SI system, but they are included in the metric system proposed for adoption in the United States