2D shape area and perimeter calculator

Formula

How it works

A rectangle's area is length times width. A triangle's area is half its base times its height (its perimeter needs all three side lengths, which base and height alone do not give). A trapezoid's area is half the sum of its two parallel sides times its height.

Worked example

The same calculation with the default values:

• Shape Rectangle
• Rectangle: length · Triangle: base · Trapezoid: side a 8
• Rectangle: width · Triangle: height · Trapezoid: side b 5
• Triangle: third side (for perimeter) · Trapezoid: height 6
• Trapezoid: fourth side (for perimeter) 7

1. A = 8 × 5 = 40
2. P = 2 × (8 + 5) = 26

Result: Area: 40 · Perimeter: 26

Common mistakes

• Entering side lengths that can't actually form the chosen shape (e.g. a triangle where one side is longer than the sum of the other two) - the formula will still compute a number, but it won't correspond to any real figure.
• Mixing up height (the perpendicular distance used in the area formula) with a slanted side length - for a triangle or trapezoid, the height must be measured perpendicular to the base, not along a slanted edge.
• Using inconsistent units across the different sides entered (e.g. one side in meters, another in centimeters) - every length must be in the same unit before calculating.