When a number is expressed in Scientific Notation, the number of significant figures is the number of Digits needed to express the number to within the uncertainty of measurement. For example, if a quantity had been measured to be , four figures would be significant. No more figures should be given than are allowed by the uncertainty. For example, a quantity written as is incorrect; it should really be written as .
The number of significant figures of a Multiplication or Division of two or more quantities is equal to the smallest number of significant figures for the quantities involved. For Addition or Subtraction, the number of significant figures is determined with the smallest significant figure of all the quantities involved. For example, the sum is 115.7574, but should be written 115.8 (with rounding), since the quantity 5.2 is significant only to .
See also Nint, Round, Truncate