Area of Rectangles & Triangles (Grades 5–6)
Area is the space inside a shape, measured in square units. A rectangle's area is length × width.
Understanding area of rectangles & triangles
Two copies of any triangle fit together to make a parallelogram with the same base and height, so a triangle's area is ½ × base × height. (For a right triangle, that parallelogram is simply the rectangle around it.) The height is the perpendicular distance from the base to the opposite corner, whichever way the triangle is turned.
Seeing it in action
Worked example
Area of a triangle with base 6 and height 4?
½ × base × height = ½ × 6 × 4 = 12 square units.
The base is 6 and the top corner is 4 squares up. The two unshaded corners add up to the same area as the shaded triangle, so the triangle is half of the 6 by 4 rectangle: ½ × 6 × 4 = 12. The height is the straight-up distance (dashed), not a slanted side.
A common mistake, and the fix
Forgetting the ½ for a triangle, or using a slanted side as the height.
For base 6 and height 4, 6 × 4 = 24 is the rectangle around the triangle; the triangle is half of it, ½ × 6 × 4 = 12 square units. The height meets the base at a right angle, and a slanted side is longer than the height.
Try a few
Rectangle 5 by 3 — area?
Triangle base 10, height 4 — area?
½ × 10 × 4.
Rectangle 7 by 7 — area?
Perimeter Patrol
A calm tile-grid game for building and reading area and perimeter.
Try it in Numeris.
A short lesson, then practice with feedback on every answer.
Put the reasoning on paper.
Try six fraction, angle, perimeter and area questions with explained answers, for grades 4–6.
Free printable · 3 pagesText version (screen-reader friendly): Draw it. Explain it. Math sample
Want a printable set too?
Practise this skill in Numeris, browse more free math pages, or join our optional resource updates.
All free math pages →