Key Terms
- Perimeter: Total distance around the outside of a shape
- Circumference: The perimeter of a circle
- Compound shape: Shape made from two or more simple shapes
- Linear units: cm, m, mm, km — NOT squared (that's area)
Must-Know Facts
- Perimeter = sum of ALL sides
- Rectangle: P = 2(l + w) or 2l + 2w
- Square: P = 4s
- Circle: C = πd = 2πr
- For compound shapes, find missing sides first using given information
- Perimeter uses linear units (cm, m) — never cm² or m²
Key Formulas
- Rectangle: P = 2(l + w)
- Square: P = 4s
- Triangle: P = a + b + c
- Circle (circumference): C = πd = 2πr
Common Mistakes
- Perimeter of compound shapes: Only count the OUTER boundary — don't include internal lines
- Semicircle perimeter: C = πr (the curved part) + 2r (the diameter) — don't forget the diameter
- Units: Perimeter is always a length (cm, m) — never cm² or m²
- Missing sides: For irregular shapes, use given information (e.g. opposite sides equal) to find unknown sides before adding
This topic summary covers Knowledge Organiser: Perimeter within Perimeter for GCSE Mathematics. Revise Perimeter in Geometry & Measures for GCSE Mathematics with 10 exam-style questions and 2 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 5 of 5 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Perimeter
What is the perimeter of a shape?
Two similar rectangles have lengths in the ratio 3:5. The perimeter of the smaller rectangle is 24 cm. Explain how to find the perimeter of the larger rectangle without knowing its individual dimensions.
Quick recall flashcards
Perimeter
Total distance around a shape, add all sides
Perimeter vs Area
Perimeter = distance AROUND (1D - cm, m). Area = space INSIDE (2D - cm², m²).