Lesson objective: Understand that length (in centimeters) can be estimated by comparing to benchmark objects - an index finger width.

Students bring prior knowledge of estimating length from exploring standard units of measure in Grade 2. This prior knowledge is extended to estimation as students estimate the length of several objects. A conceptual challenge students may encounter is not understanding that estimation and measuring exactly are different.

This lesson helps to build fluency with estimating length in centimeters. Finger widths is used here because they are about the length of a centimeter. This work develops students' understanding that benchmarks can be used to estimate length.

This work helps students deepen their understanding of numbers because benchmarks are repeated.

Students engage in Mathematical Practice 5 (Use appropriate tools strategically) as they use their finger widths to estimate.

**Key vocabulary:**

- benchmark
- estimate
- finger width