Lesson objective: Determine the volume and dimensions of rectangular prisms built with fractional edge-length cubes with fluency.

This lesson helps to build fluency with volume of prisms built with fractional edge-length cubes. Prisms are presented without labeled measures in this lesson because they highlight the need to be able to fluently determine measured edge lengths from ‘cube edge lengths’. This work develops students' understanding that the formula for volume of a rectangular prism is connected to filling the prism with unit cubes.

Students engage in Mathematical Practice 5 (Use Appropriate Tools Strategically) as they use formulas and unit cubes to help them find volume. Students may opt to use cm cubes to represent ‘half inch’ cubes to build the given prisms in order to help them determine the edge lengths of the prisms; or they may choose to compute the edge lengths mathematically. Then students will choose to use the edge lengths or the number of half-inch cubes to find the volume of the prism.

**Key vocabulary:**

- cubic units
- edge lengths
- half-inch cube
- rectangular prism
- volume

**Special materials needed:**

- Optional: cm cubes or unit cubes from Base Ten Blocks to represent ‘half-inch’ cubes (enough for each student/pair/group to have 165 cubes each)