This document discusses the rules for determining significant digits in measurements and calculations. It defines what makes a digit significant and provides examples to illustrate the rules. Significant digits indicate the precision or uncertainty in a measurement. The number of significant digits reported in a calculation must be consistent with and justified by the least precise measurement used. Rounding intermediate calculations can introduce inaccuracies into the final reported value. Practice problems are provided to help apply the rules for addition, subtraction, multiplication and division.