In general, measurement being subject to the limitations of measuring devices. A ruler may measure a length to the nearest millimetre for example (not that even though the ruler is calibrated in mm, you do not say because the zero point also has the same size error, so a length measured with a rule calibrated in m is written)

There are specific rules for how to consistently express the uncertainty associated with a number. In general, the last significant figure in any result should be of the same order of magnitude (i.e.. in the same decimal position) as the uncertainty. Also, the uncertainty should be rounded to one or two significant figures. Always work out the uncertainty after finding the number of significant figures for the actual measurement.

For example,

The following numbers are all incorrect.

is wrong butis fine

is wrong butis fine

is wrong butis fine

In practice, when doing mathematical calculations, it is a good idea to keep one more digit than is significant to reduce rounding errors. But in the end, the answer must be expressed with only the proper number of significant figures. After addition or subtraction, the result is significant only to the **place ** determined by the largest last significant place in the original numbers. For example,

should be rounded to get 90.4 (the tenths place is the last significant place in 1.1). After multiplication or division, the **number ** of significant figures in the result is determined by the original number with the smallest number of significant figures. For example,

(2.80) (4.5039) = 12.61092

should be rounded off to 12.6 (three significant figures like 2.80).