This grade 3 perimeter worksheet answer key solves all 23 problems of the perimeter worksheet step by step.
Full worked solutions for all 23 perimeter problems. Every answer shows the sum of the sides or the subtraction used for an unknown side, so students can see where a mistake happened.
Grade 3 Perimeter Worksheet Answer Key: problems 1–5
Problem 1: Perimeter of a triangle
Add the three sides: \(5 + 7 + 6 = 18\).
The perimeter is 18 cm.
Problem 2: Perimeter of a rectangle
\(9 + 4 + 9 + 4 = 26\).
The perimeter is 26 inches.
Problem 3: Perimeter of a square
A square has 4 equal sides: \(8 + 8 + 8 + 8 = 32\), or \(4 \times 8 = 32\).
The perimeter is 32 feet.
Problem 4: Perimeter of a trapezoid
\(5 + 5 + 8 + 4 = 22\).
The perimeter is 22 feet.
Problem 5: Perimeter of a pentagon
\(7 + 5 + 6 + 8 + 4 = 30\).
The perimeter is 30 cm.
Grade 3 Perimeter Worksheet Answer Key: problems 6–10
Problem 6: Regular hexagon
There are 6 equal sides: \(6 \times 4 = 24\).
The perimeter is 24 meters.
Problem 7: Rectangle from numbers
\(12 + 7 = 19\), and \(2 \times 19 = 38\).
The perimeter is 38 feet.
Problem 8: Equal sides
\(3 \times 10 = 30\).
The perimeter is 30 inches.
Problem 9: Count the units
Count the side lengths along the outside, starting at the top: 5, 2, 3, 2, 2, 4.
\(5 + 2 + 3 + 2 + 2 + 4 = 18\).
The perimeter is 18 cm.
Problem 10: Missing side of a triangle
Known sides: \(8 + 9 = 17\). Subtract: \(24 – 17 = 7\).
Check: \(8 + 9 + 7 = 24\).
The missing side is 7 meters.
Grade 3 Perimeter Worksheet Answer Key: problems 11–15
Problem 11: Missing width
Two lengths: \(9 + 9 = 18\). The two widths share \(30 – 18 = 12\), so one width is \(12 : 2 = 6\).
Check: \(9 + 6 + 9 + 6 = 30\).
The width is 6 feet.
Problem 12: Side of a square
Think: \(4 \times ? = 36\). Since \(4 \times 9 = 36\), each side is 9 inches.
Each side is 9 inches.
Problem 13: Missing side of a quadrilateral
\(11 + 9 + 12 = 32\). \(40 – 32 = 8\).
The fourth side is 8 cm.
Problem 14: Missing length
Two widths: \(8 + 8 = 16\). \(44 – 16 = 28\). Half of 28 is 14.
Check: \(14 + 8 + 14 + 8 = 44\).
The length is 14 feet.
Problem 15: Same perimeter
Perimeters: \(6 + 2 + 6 + 2 = 16\), \(5 + 3 + 5 + 3 = 16\), \(4 + 4 + 4 + 4 = 16\).
Areas: \(6 \times 2 = 12\), \(5 \times 3 = 15\), \(4 \times 4 = 16\).
All three have a perimeter of 16 units. The areas are 12, 15 and 16 square units.
Grade 3 Perimeter Worksheet Answer Key: problems 16–20
Problem 16: Compare two rectangles
A: \(8 + 2 + 8 + 2 = 20\). B: \(6 + 4 + 6 + 4 = 20\). The perimeters match.
Areas: \(8 \times 2 = 16\) and \(6 \times 4 = 24\).
Both perimeters are 20 cm. Rectangle B has the greater area (24 square cm compared with 16 square cm).
Problem 17: Same area, different perimeters
\(12 + 1 + 12 + 1 = 26\), \(6 + 2 + 6 + 2 = 16\), \(4 + 3 + 4 + 3 = 14\).
The perimeters are 26, 16 and 14 units. The shape closest to a square has the shortest distance around.
Problem 18: Fence for a yard
\(15 + 10 + 15 + 10 = 50\).
He needs 50 feet of fence.
Problem 19: Walk around the park
\(120 + 150 + 90 = 360\).
She walks 360 yards.
Problem 20: Ribbon for a frame
Perimeter: \(11 + 9 + 11 + 9 = 40\) inches.
She has 36 inches, and \(40 – 36 = 4\).
No, she needs 4 more inches of ribbon.
Grade 3 Perimeter Worksheet Answer Key: problems 21–23
Problem 21: Square pen
Four equal sides: \(48 : 4 = 12\).
Each side is 12 feet. Check: \(4 \times 12 = 48\).
Problem 22: Find the mistake
Liam added only one length and one width: \(8 + 5 = 13\). Perimeter is the distance around all four sides.
\(8 + 5 + 8 + 5 = 26\).
The correct perimeter is 26 inches.
Problem 23: Banner at the market
Two widths: \(7 + 7 = 14\). \(34 – 14 = 20\), and half of 20 is 10, so the length is 10 feet.
Going around twice: \(2 \times 34 = 68\).
The length is 10 feet, and 68 feet of rope go around it twice.
How to use this grade 3 perimeter worksheet answer key
Read this grade 3 perimeter worksheet answer key after trying each problem yourself, and redo any problem you missed.
See every chapter on the Grade 3 math page.