Perimeter calculator results show the full boundary length of a common shape or a custom polygon, with mixed-unit dimensions and a check for impossible side lists.
Find perimeter from boundary measurements
Trace the outside edge before entering measurements. Choose a shape when its defining dimensions are known, or choose Custom polygon for an irregular straight-sided shape and enter every straight outside side in one unit. Rectangles use length and width; parallelograms use adjacent sides, not perpendicular height. A circle uses its full diameter.
Do not add interior edges shared by adjacent rooms or sections.
Use Regular polygon only when all sides and angles are equal.
Each ordinary dimension has a connected unit selector; a custom list uses one common unit.
Perimeter Calculator example
- Enter a length of 12 and width of 10 .
- The tool adds those adjacent sides, doubles the sum and returns 44 ft, equivalent to 13.4112 m.
- More options lets you choose another result unit without changing the boundary.
Check whether the sides can close
The longest straight side must be shorter than the sum of the others for a nondegenerate closed boundary. A triangle with sides 3 , 4 and 5 passes and gives 12 m. The five-side preset—8 , 5 , 4 , 6 and 7 —gives 30 ft.
The calculator rejects a triangle or polygon list that fails that length condition. Passing it does not prove that a drawn lot closes, its corners have the intended angles, a trapezoid has parallel sides, or a path avoids self-intersection. Use surveyed coordinates or angles when those facts matter. This page adds measured lengths only.
For a circle sector, the boundary includes the arc and both radius edges. A 3 radius with a 90° angle gives 10.712389 m. Choose Circle for a full turn, because a complete circle’s boundary has no radius edges. OpenStax’s angle chapter explains the arc-length relationship.
Extremely near-degenerate sides are rejected at the supported floating-point precision as well. This prevents a mathematically flat boundary from appearing valid after unit conversion; use higher-precision geometry if a genuine shape is that close to flat.
Area does not determine every perimeter
Two rectangles can enclose the same area while having different boundary lengths. A 10 square and a 20 by 5 rectangle each enclose 100 sq ft, but their perimeters are 40 ft and 50 ft. The tool therefore asks for boundary measurements rather than inferring perimeter from an area total.
Use Square Footage Calculator for the area inside a measured shape. For a floor or wall order, the Flooring Calculator, Tile Calculator and Paint Calculator handle their separate coverage and package inputs. A perimeter number by itself does not determine a material order.
A boundary measurement can also inform edging or trim lengths. The Linear Feet Calculator handles straight runs and their separate board inputs.
A known surface area can be expressed in different units with Area Converter. That conversion keeps the area unchanged and cannot supply a missing boundary length.