Common Core State Standard: 8.NS.2 

Grade  8 
Subject  MATH 
Domain  The Number System 
Theme  Know that there are numbers that are not rational, and approximate them by rational numbers 
Standard  Use rational approximations of irrational numbers to compare the size of irrational numbers, locate them approximately on a number line diagram, and estimate the value of expressions (e.g., ?^2). For example, by truncating the decimal expansion of ?2 (square root of 2), show that ?2 is between 1 and 2, then between 1.4 and 1.5, and explain how to continue on to get better approximations. 
Reference URL  http://www.corestandards.org/Math/Content/8/NS 