Plane Figures Calculator
A collection of calculators for the area and perimeter of plane figures such as circles and trapezoids.
Circle Area & Circumference Calculator
Enter the radius to calculate the area and circumference of a circle.
Read MoreTrapezoid Area Calculator
Enter the top base, bottom base, and height to calculate the area of a trapezoid.
Read MoreRectangle Area Calculator
Enter the width and height to calculate the area and perimeter of a rectangle.
Read MoreAbout This Category
Area and perimeter for the shapes that appear in plans, materials and schoolwork: circle, rectangle and trapezoid. Each page takes the dimensions in one unit and returns area in square units, perimeter in the same unit.
The two results answer different questions. Area tells you how much paint, flooring or seed you need; perimeter tells you how much edge, fence or trim. Buying on the wrong one is a common and expensive mistake.
Frequently Asked Questions
Why is the circle area π times the radius squared?
Because a circle of radius r fits inside a square of side 2r with π/4 of the area. Using the diameter instead of the radius gives an answer four times too large.
How do I find the area of a trapezoid?
Average the two parallel sides and multiply by the height: ((a + b) / 2) × h. The slanted sides play no part in the area.
Should I add 10 % for waste?
For tiling, flooring and paint, yes — 5–10 % covers cuts and offcuts. For a single cut piece, measure twice instead.
Calculators in This Category
Sources & last update
2026-10-19