4th Grade
Math
About 10 minutes
See it all: a chart to keep

How to Find a Missing Side

Work backward from the area or the perimeter to find a missing side.

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How to do it

Write the formula with the numbers you know.
For area, divide by the side you know.
For perimeter, subtract the two sides you know and divide by 2.

Teaching tip

Think of the area as a multiplication fact with a blank. This sheet is aligned with Common Core 4.MD.A.3.

How to Find a Missing Side chart for 4th grade (area and perimeter), free printable — Printable preview

About this chart

Worked examples finding a missing length from an area of 48 sq ft and from a perimeter of 30 m.

What kids practice

Perimeter is the distance around a shape, and area is the space inside it. Fourth graders use formulas for rectangles. Perimeter is P = 2 × l + 2 × w, or 4 × s for a square. Area is A = l × w, measured in square units. They also work backward to find a missing side, like a rug with an area of 48 square feet and a width of 6 feet, which is 8 feet long.

Watch for

Mixing up area and perimeter, especially in word problems.
Adding only two sides for perimeter, like 8 + 5 = 13 instead of 26.
Writing perimeter in square units or area in plain units.
Dividing the perimeter by 4 to find the length of a rectangle that is not a square.

It's free to print, as many copies as you need. See the full area and perimeter guide for 4th grade.