Accuracy refers to how closely a measurement value agrees with the correct value. Precision refers to how closely individual measurements agree with each other. The result of a calculation based on experimental measurements cannot be more precise than the measurement that has the greatest uncertainty. A measurement can only be reported to the extent or degree of certainty, or uncertainty, for that matter. The quality of a measurement depends on the quality of the measuring device or instrument .
Some simple rules govern the use of significant figures in calculations.
Zeroes used to position the decimal point are not significant figures. In multiplication and division, an answer contains no more significant figures that the least number of significant figures in the operation. The answer can only be expressed to 3 significant figures. In addition and subtraction, the last digit retained in the sum or difference is determined by the position of the first doubtful digit. Sometimes to express a number in the proper number of significant figures it is necessary to round it off first.