Guide for the Use of the International System of Units (SI)

Guide for the Use of the International System of Units (SI) 
 
40 
Appendix B. Conversion Factors
6
 
 
B.1 Introduction 
 
Sections B.8 and B.9 give factors for converting values of quantities expressed in various units—
predominantly units outside the SI that are unacceptable for use with it—to values expressed either in (a) SI 
units, (b) units that are accepted for use with the SI (especially units that better reflect the nature of the 
unconverted units), (c) units formed from such accepted units and SI units, or (d) decimal multiples or 
submultiples of the units of (a) to (c) that yield numerical values of convenient magnitudes. 
An example of (d) is the following: the values of quantities expressed in ångströms, such as the 
wavelengths of visible laser radiations, are usually converted to values expressed in nanometers, not 
meters. More generally, if desired, one can eliminate powers of 10 that appear in converted values as a 
result of using the conversion factors (or simply factors for brevity) of Secs. B.8 and B.9 by selecting an 
appropriate SI prefix (see Sec. B.3). 
B.2 Notation 
 
The factors given in Secs. B.8 and B.9 are written as a number equal to or greater than 1 and less 
than 10, with 6 or fewer decimal places. The number is followed by the letter E, which stands for exponent, 
a plus (+) or minus (–) sign, and two digits that indicate the power of 10 by which the number is multiplied. 
Examples: 3.523 907 E−02 means 3.523 907 × 10
−2
= 0.035 239 07 
3.386 389 E+03 means 3.386 389 × 10
3
= 3386.389 
A factor in boldface is exact. All other factors have been rounded to the significant digits given in 
accordance with accepted practice (see Secs. 7.9, B.7.2, and Refs. [4: ISO 31-0] and [6]). Where less than 
six digits after the decimal place are given, the unit does not warrant a greater number of digits in its 
conversion. However, for the convenience of the user, this practice is not followed for all such units, 
including the cord, cup, quad, and teaspoon.