Area vs. Perimeter: Never Mix Them Up
Perimeter is a walk around the edge — how much fence you need. Area is the carpet inside — how much paint, tile, or grass seed you need. The classic exam trap is giving one when the question asks for the other, so always re-read what is being asked.
Formula Cheat Sheet
- Rectangle: A = l×w, P = 2(l+w)
- Square: A = s², P = 4s
- Triangle: A = ½bh, P = sum of the three sides
- Circle: A = πr², P (circumference) = 2πr
- Trapezoid: A = ½(a+b)h, P = sum of all four sides
- Parallelogram: A = b×h (height, not slant side!), P = 2(b+s)
The #1 Parallelogram Mistake
For a parallelogram's area, use the perpendicular height — not the slanted side length. The slant side is only for the perimeter. Sketch the altitude if it helps.
Circles deserve their own spotlight — try our circle calculator for radius, diameter, and circumference work.