Area Calculator

Computes the area of a rectangle, a triangle, or a circle in the unit you pick.

1. Choose the shape and the unit.
2. Enter the lengths in that unit.
3. Read the area in square units.

Rectangle, triangle, and circle area

Rectangle: length × width. Triangle: base × height ÷ 2. Circle: π × radius × radius.

Source. A rectangle is length times width. A triangle is base times perpendicular height, divided by 2. A circle is π times the radius squared. The selected unit labels the result and does not convert it.

Updated November 1, 2026

A rectangle 4 ft by 5 ft has an area of 20 square feet. A triangle with base 4 and height 3 has area 6.

• The unit is a label. Changing it does not convert the number. Use the unit converter to change units.
• Lengths cannot be negative.

Which height does the triangle use?

The height perpendicular to the base you entered, not the length of a slanted side.