3.MD.8: Solve Perimeter Problems
Also written as 3.MD.D.8 and CCSS.MATH.CONTENT.3.MD.D.8 · Cluster: Geometric measurement: recognize perimeter.
The Official Wording
Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.
Common Core State Standards, Grade 3, Measurement & Data · Read it at the source · Checked October 14, 2026
What 3.MD.8 Means in Plain English
Third graders find the distance around a shape by adding its sides: a 7 by 4 garden has a perimeter of 7 + 4 + 7 + 4 = 22 feet. They work backward from a perimeter to a missing side, and build rectangles to compare perimeter with area.
The contrast is the point: a 1 by 6 and a 3 by 4 rectangle both have a perimeter of 14, but areas of 6 and 12. Second grade added lengths along a path (2.MD.5). In 4th grade perimeter becomes a formula used with bigger numbers (4.MD.3).
3.MD.8 I Can Statements
- I can find the perimeter of a shape by adding its sides.
- I can find a missing side when I know the perimeter.
- I can draw rectangles with the same perimeter and different areas.
3.MD.8 Worked Example
A rabbit pen with 16 feet of fence, built every whole-number way to see how much room each rectangle gives.
Nora has 16 feet of fence for a rectangular rabbit pen, with sides in whole feet. Which rectangle gives the rabbit the most room?
- The fence goes all the way around, so the perimeter is 16 feet. One length and one width make half the trip around.length + width = 8 feet
- List every pair of whole numbers that adds to 8.1 and 7, 2 and 6, 3 and 5, 4 and 4
- Check one pen walking all four sides, so no side is forgotten.1 + 7 + 1 + 7 = 16 feet
- Room for the rabbit is area, so multiply the sides of each pen.1 × 7 = 7, 2 × 6 = 12, 3 × 5 = 15, 4 × 4 = 16
- Same fence every time, but the room inside changes. The square pen holds the most.4 by 4: 16 square feet
Check it: Walk the 4 by 4 pen: 4 + 4 + 4 + 4 = 16 feet, all of the fence used. The 1 by 7 pen uses the same 16 feet but holds only 7 square feet.
Common Mistakes with 3.MD.8
Adding only the labeled sides
A 7 by 4 rectangle with two sides labeled, perimeter given as 11
Try this: Have students trace the shape with a finger and say a length for every side, then write all four lengths in the sum.
Mixing up perimeter and area
A 3 by 4 rectangle given a perimeter of 12, found by multiplying
Try this: Ask whether the question is about the fence or the grass. Fence means walking the edge and adding, grass means covering and multiplying.
Subtracting only one side
A rectangle with a perimeter of 20 and a length of 6, width given as 14 because 20 − 6 = 14
Try this: Sketch the rectangle and label all four sides. Take away both lengths, 20 − 12 = 8, then split 8 between the two widths.
Where 3.MD.8 Fits
Practice 3.MD.8
Quick Check
- A triangle has sides of 5 cm, 7 cm and 8 cm. What is its perimeter?
- A rectangle has a perimeter of 24 inches and a length of 8 inches. How wide is it?
- Two rectangles each have an area of 12 square units: one is 2 by 6, the other 3 by 4. Do they have the same perimeter?