CONVERSION FACTORS
The following table gives conversion factors from various
units of measure to SI units. It is reproduced from NIST Special
Publication 811
, Guide for the Use of the International System of
Units (SI)
. The table gives the factor by which a quantity expressed
in a nonSI unit should be multiplied in order to calculate its value
in the SI. The SI values are expressed in terms of the base, supple
mentary, and derived units of SI in order to provide a coherent
presentation of the conversion factors and facilitate computations
(see the table “International System of Units” in this section).
If
desired, powers of ten can be avoided by using SI prefixes and
shifting the decimal point if necessary.
Conversion from a nonSI unit to a different nonSI unit may be
carried out by using this table in two stages, e.g.,
1 cal
th
= 4.184 J
1 Btu
IT
= 1.055056 E+03 J
Thus,
1 Btu
IT
= (1.055056 E+03 ÷ 4.184) cal
th
= 252.164 cal
th
Conversion factors are presented for ready adaptation to com
puter readout and electronic data transmission.
The factors are
written as a number equal to or greater than one and less than ten
with six or fewer decimal places. This number is followed by the
letter E (for exponent), a plus or a minus sign, and two digits that
indicate the power of 10 by which the number must be multiplied
to obtain the correct value.
For example:
3.523 907 E02 is 3.523 907
×
10
–2
or
0.035 239 07
Similarly:
3.386 389 E+03 is 3.386 389
×
10
3
or
3 386.389
A factor in boldface is exact; i.e., all subsequent digits are zero.
All other conversion factors have been rounded to the figures
given in accordance with accepted practice.
Where less than six
digits after the decimal point are shown, more precision is not
warranted.
It is often desirable to round a number obtained from a conver
sion of units in order to retain information on the precision of the
value. The following rounding rules may be followed:
1. If the digits to be discarded begin with a digit less than 5, the
digit preceding the first discarded digit is not changed.
Example:
6.974 951 5 rounded to 3 digits is 6.97
2. If the digits to be discarded begin with a digit greater than
5, the digit preceding the first discarded digit is increased
by one.
Example:
6.974 951 5 rounded to 4 digits is 6.975
3. If the digits to be discarded begin with a 5 and at least one
of the following digits is greater than 0, the digit preceding
the 5 is increased by 1.
Example:
6.974 851 rounded to 5 digits is 6.974 9
4. If the digits to be discarded begin with a 5 and all of the fol
lowing digits are 0, the digit preceding the 5 is unchanged
if it is even and increased by one if it is odd. (Note that this
means that the final digit is always even.)
Examples:
6.974 951 5 rounded to 7 digits is 6.974 952
6.974 950 5 rounded to 7 digits is 6.974 950
Reference
Taylor, B. N.,
Guide for the Use of the International System of Units (SI)
,
NIST Special Publication 811, 1995 Edition, Superintendent of Documents,
U.S. Government Printing Office, Washington, DC 20402, 1995.
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 Spring '10
 Dr.N
 ........., Imperial units, Conversion of units, SI derived units

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