Lesson objective: Understand that length can be estimated by using mental images of units (inches in this lesson), comparing to objects of known length, and comparing to benchmark objects.

Students bring prior knowledge of estimating length from exploring standard units of measurement 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.

The concept is developed through work with estimating the length of objects, which give students concrete strategies (benchmarks, mental images, etc.) to estimate.

This work helps students deepen their understanding of equivalence because objects used to estimate can be combined to estimate the length of other objects.

Students engage in Mathematical Practice 3 (Construct viable arguments and critique the reasoning of others) as they present and reflect on the various estimation methods.

**Key vocabulary:**

- benchmark
- estimation