Lesson objective: Measure length following the rules for measuring with length units.

This lesson helps to build fluency with measuring with length units. The "Rules for Measuring with Length Units" are used here because these rules support how to measure accurately with length units. An additional rule is added to the original three (no gaps, no overlaps, length units all need to be the same size). The additional rule is that the length units must be aligned with the object being measured. This work develops students' understanding that length measurements are only accurate if the length units and the object being measured are aligned.

Students engage in Mathematical Practice #2 (Reason abstractly and quantitatively) as they measure with length units and make approximations when the length units and the object they are measuring do not exactly align; Mathematical Practice #5 (Use appropriate tools strategically) as they use length units to measure length; and Mathematical Practice #6 (Attend to precision) as they work with the non-standard length units to determine the length of the object being measured. Students determine the length measurement of strips of paper using the rules for measuring with length units as a guide to measure accurately.

**Key vocabulary:**

- accuracy
- alignment
- gaps
- length measurement
- length units
- overlaps
- same-size
- span

**Special materials needed:**

- non-standard length units (blocks, building cubes, paper clips)
- items to measure (e.g. strips of paper)