Perimeter Calculator. Calculate the perimeter of common 2D shapes in seconds. Select your shape (square, rectangle, triangle, circle, parallelogram, trapezoid, or ellipse), enter the required side lengths or radius, and get the perimeter back immediately. Each shape shows only the fields you need — no clutter, no confusion. Also try the calculate Missing Side, Side a & Side b — Pythagorean Theorem.
Perimeter is the total length of the boundary of a closed 2D shape. It is calculated by summing the lengths of all sides (or, for curved shapes, the arc length around the boundary). The result is always expressed in a unit of length such as meters, feet, or centimeters. See also our calculate Area.
The perimeter of a rectangle is P = 2(a + b), where a is the length and b is the width. For example, a rectangle that is 10 m × 6 m has a perimeter of 2 × (10 + 6) = 32 m.
The circumference of a circle is C = 2πr, where r is the radius. You can also write it as C = πd, where d is the diameter. For a circle with radius 5 m, the circumference is approximately 31.416 m.
Add the lengths of all three sides: P = a + b + c. For an equilateral triangle, this simplifies to P = 3a. Make sure all side lengths are in the same unit before adding. You might also find our calculate Rhombus useful.
Perimeter measures the total length around the outside of a shape (a 1D measurement expressed in units like meters), while area measures the space enclosed within the shape (a 2D measurement expressed in square units like m²). They are related but independent — two shapes can have the same perimeter but different areas.
There is no exact simple formula for the perimeter of an ellipse. A common approximation is P ≈ π × [3(a + b) − √((3a + b)(a + 3b))], known as Ramanujan's approximation, where a is the semi-major axis and b is the semi-minor axis. This calculator uses that approximation for ellipses.
For irregular polygons, measure every individual side and add them all together. If the shape has curved sections, you would need to measure or calculate each arc length separately and include those in the total.
Perimeter calculations are used in fencing a yard or field, framing a picture or room, laying borders around a garden, calculating the length of trim or molding needed for a room, and many construction and landscaping tasks. Check out our Triangle Calculator as well.