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  • 3rd Grade
  • Math
  • About 10 minutes

Missing Side Length

Use the perimeter or area to find the missing side length.

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Missing Side LengthName:Date:Use the perimeter or area to find the missing side length.Perimeter = 20 m4 m?? =mArea = 45 square ft5 ft?? =ftPerimeter = 38 in10 in?? =inArea = 70 square cm10 cm?? =cmPerimeter = 36 cm10 cm?? =cmArea = 42 square cm7 cm?? =cmk5printables.com · Sheet #76024601 · Aligned with Common Core 3.MD.C.7

This is sheet #76024601.

How to do it

  1. Read whether the perimeter or the area is given.
  2. Use the side you know.
  3. Find the missing side.

Teaching tip

For area, think multiplication: 5 × ? = 20. For perimeter, the two sides you know add to half. This sheet is aligned with Common Core 3.MD.C.7.

Missing Side Length worksheet for 3rd grade (area and perimeter), free printable
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About this worksheet

Rectangles with the perimeter or area given and one side missing.

What kids practice

Area is the amount of space inside a flat shape, measured in square units. Perimeter is the distance all the way around a shape, measured in units of length. Third graders find area by counting squares and then by multiplying length × width. They find perimeter by adding the side lengths, and learn that two shapes can have the same area but different perimeters.

Watch for

  • Mixing up area and perimeter. Area is inside; perimeter is around.
  • Adding only the two labeled sides of a rectangle instead of all four.
  • Writing area in plain units instead of square units.

It's free to print, with Fewer, Standard or More on the page and an answer key. See the full area and perimeter guide for 3rd grade.

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