Perimeter — Grade 3 Mathematics
In Grade 3, we extend perimeter to irregular shapes (L-shapes, rectilinear shapes) and learn to find missing side lengths using known total dimensions. We also apply perimeter to real-world problems with decimal measurements.
Perimeter of Irregular Shapes
For rectilinear shapes (right-angled only, like L-shapes), use the outer dimensions to calculate missing sides before adding.
L-Shape with Missing Sides
Total width = 10 cm → missing bottom-right width = 10 − (110−20)/10 → solve using outer dimensions.
Worked Examples
A shape has sides: 5 cm, 8 cm, 4 cm, 6 cm, 7 cm. Find the perimeter.
- P = 5 + 8 + 4 + 6 + 7
- P = 30 cm
An L-shape has total width 12 cm, total height 9 cm. The inner cut-out is 5 cm wide and 4 cm tall. Find the perimeter.
- Label all 6 sides: known are 12 cm (top), 9 cm (left), then we work out the rest.
- Bottom = 12 − 5 = 7 cm (bottom-left portion)
- Right-side segments: 9 − 4 = 5 cm (bottom-right) and 4 cm (inner right)
- Inner top = 5 cm
- P = 12 + 9 + 7 + 4 + 5 + 5 = 42 cm
A rectangular park is 85.5 m long and 46.2 m wide. Fence posts are placed every 3 m. How many posts are needed?
- Perimeter = 2(85.5 + 46.2) = 2 × 131.7 = 263.4 m
- Posts = 263.4 ÷ 3 = 87.8 → 88 posts
Practice Questions
1. A hexagon has all sides equal to 6.5 cm. What is its perimeter?
2. A rectangle has perimeter 36 cm and width 7 cm. Find the length.
3. An L-shape: outer width 15 cm, outer height 10 cm, cut-out 6 cm × 4 cm. Find perimeter.
4. A square field has a perimeter of 96 m. Find the length of one side.
5. A shape has 8 equal sides, each 4.5 cm. What is its perimeter?
Key Points to Remember
- Perimeter = sum of all sides — count every side.
- For L-shapes: opposite parallel sides sum to the outer total dimension.
- Rectangle: P = 2(l + w). Square: P = 4s. Regular n-gon: P = n × s.
- To find a missing side: total perimeter − sum of known sides.
- Perimeter is a length, measured in cm, m, km etc. (not squared).