Perimeter & Area
Grades 3–5 Geometry Measurement
Perimeter is the fence: the distance around the outside. Area is the floor: how many unit squares fit inside. Same shape, two different questions.
The formulas
Rectangle P = 2(l + w) A = l × w
Square P = 4s A = s²
Triangle P = a + b + c A = ½ × base × height
Parallelogram A = base × height
Why ½ for the triangle? Any triangle is half of a rectangle with the same base and height — the area is half as big.
Room 8 m × 5 m:
P = 2(8 + 5) = 26 m of trim
A = 8 × 5 = 40 m² of carpet
Units tell the truth
Perimeter answers are in plain units (m, cm) — it's a length.
Area answers are in square units (m², cm²) — it's a count of
squares. If your "area" isn't squared, something's off.
Practice
Rectangle 7 × 5 — perimeter = ___
Same rectangle — area = ___
Triangle: base 10, height 3 — area = ___
Square with perimeter 24 → side 6 → area = ___
Next: surface area & volume · Back: circles