How Students Learn about Measurement in Common Core Math
In second grade math, Common Core students use rulers to measure lengths. By doing this, they are connecting and extending their use of nonstandard units such as paper clips and linking cubes, which they learned about in first grade.
A ruler is a complicated object to use correctly, so you can expect your second grader to be working on this skill for a few years, gaining accuracy over time. The basic principle of measuring length is that you compare the length of the object that you’re measuring to a standardized length (called a unit). If you measure a toy car to be 5 inches long, it’s the same length as five 1‐inch‐long blocks end‐to‐end.
The process of repeating (lining up) units is called iterating. A ruler is more precise and convenient than lining up units end‐to‐end because the units are carefully marked and you don’t have to worry about leaving gaps between the units.
This property of rulers also makes them challenging for children to learn to use. Among the rules you need to follow to use a ruler properly are these:

Start at zero (not one).

Count the spaces, not the lines.

The longest tick marks are whole inches, the shorter ones are halves, the next shorter ones are quarters, and so on.
In second grade, students don’t have to worry about fractional measurements. They measure using inches, feet, meters, and centimeters. Nonetheless, they’ll be curious about (and possibly flummoxed by) all those little tick marks as they use rulers to measure lengths.
Additionally, students in second grade estimate lengths. Being able to make good measurement estimates is a tremendously helpful skill in life. Second graders work on this concept by guessing how many inches (or meters or centimeters or feet) long something is, and then checking their estimates by measuring. An important part of learning to estimate lengths is becoming familiar with benchmarks. These are familiar examples of approximate measures, such as that your pinkie nail is about 1 cm wide. This gives a basis for comparison.
Second graders also compare measurements. They compare the lengths of two objects by saying which measurement is greater or less than the other, and by how much. (For example, “My leg is longer than my arm by 3 inches.”) They measure the same thing twice — once in inches and once in centimeters — and discuss the relationship between these results. (For example, “My pencil is about 15 centimeters long and about 6 inches long because centimeters are smaller than inches.”)