How to Use the Perimeter Calculator

Pick a shape — Rectangle, Square, Triangle, or Regular Polygon — and enter its dimensions. The perimeter updates immediately.

Rectangle:        perimeter = 2 × (length + width)
Square:           perimeter = 4 × side
Triangle:         perimeter = a + b + c
Regular Polygon:  perimeter = number of sides × side length

Perimeter is the one measurement that never needs an angle, a height, or a square root — every shape here just adds up its boundary, which is what makes it the most forgiving geometry calculation to check by hand. A circle's equivalent isn't listed as a shape here at all — it has its own name, circumference, and its own dedicated calculator.