3rd Grade Math · Perimeter
Find a missing side Worksheets
Fresh practice, ready to print. Answer key included.
The perimeter is given. Find the side marked ?.
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Or start with a ready-made version.
How it works: a quick refresher
The rules this worksheet practices, then one problem worked from start to finish.
| Strategy | How it works |
|---|---|
| Perimeter is the distance around | Add the lengths of all the sides. The answer is in plain units, such as cm. |
| Rectangles | Opposite sides are equal, so a 7 cm by 3 cm rectangle has perimeter 7 + 3 + 7 + 3 = 20 cm. |
| A shortcut | For a rectangle, add the length and width, then double: (7 + 3) × 2 = 20 cm. |
| Missing side | Add the sides you know, then subtract from the perimeter: 20 − 14 = 6 cm. |
| Mark as you go | Tick each side as you add it, so none is counted twice or missed. |
Worked example
A triangle has sides 6 cm and 8 cm and a side marked ?. Its perimeter is 24 cm. Find the missing side.
- 1Add the sides you know: 6 + 8 = 14 cm.
- 2The perimeter is 24 cm, so the missing side is 24 − 14 = 10 cm.
- 3Check: 6 + 8 + 10 = 24 cm.
Ready-made versions
Three fixed versions you can link to or share. Each is a PDF with its own answer key.
- Find a missing side Worksheets, version 1Worksheet PDFAnswer key PDF
- Find a missing side Worksheets, version 2Worksheet PDFAnswer key PDF
- Find a missing side Worksheets, version 3Worksheet PDFAnswer key PDF
Grade level and standard
Written for 3rd grade math (measurement) and aligned to CCSS 3.MD.D.8. It also works as review in later grades.
Common questions
- Is there an answer key?
- Yes. Every worksheet has a matching answer key with each answer worked on the same grid or layout as the question. The worksheet code printed at the bottom of each page pairs a sheet with its key.
- Why is perimeter in cm and area in square cm?
- Perimeter measures a length around the outside, so it uses plain units. Area counts the squares inside, so it uses square units.
- My child keeps missing a side on the L-shaped figures. Any tips?
- Start at one corner and tick each side as it is added, all the way around until you are back where you started. L-shaped figures have six sides.
- How many different versions are there?
- About a million per skill. Each time you press the button the problems are generated fresh, so you can print a different version for every student, a retake, or another night of practice.
- Are the worksheets free?
- Yes. They are free to print and copy for your home or classroom, with no sign-up.
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