SME
Grade 3 Math · Measurement & Data
Perimeter of polygons
Use this for “A rectangle is 6 cm long and 4 cm wide. What is its perimeter?”
Diagram
This is an example. Type your own values to start a new problem.
Length (cm)
+
Width (cm)
+
Length (cm)
+
Width (cm)
=
Perimeter (cm)
Formulas
6 + 4 + 6 + 4 = 20
Length + width + length + width = perimeter
Assumptions
•
Perimeter is the distance all the way around a shape.
•
A rectangle’s opposite sides are equal: add length + width + length + width.
•
Perimeter is a length, not square units.
Step-by-step
WE KNOW
Length: 6 cm
Width: 4 cm
FIND
perimeter
Step 1 · Find perimeter
6 + 4 + 6 + 4 = ?
Length + width + length + width = perimeter
Add all four sides.
6 + 4 = 10
10 + 6 = 16
16 + 4 = 20
Perimeter = 20 cm
Check
Put the numbers back in. Both sides should match.
6 + 4 + 6 + 4 = 20 ✓
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Area of a shape with a corner cut out
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Perimeter and unknown side lengths
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