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Calculator Answers

Perimeter Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A perimeter calculator is a geometry utility that computes the boundary length of various two-dimensional shapes. The calculator supports rectangles, circles (circumference), triangles, trapezoids, parallelograms, and regular polygons. It applies the specific geometric formula for each shape, summing the side lengths or calculating boundaries using radii and pi. Homeowners, landscapers, and students use this tool to estimate fencing requirements, determine material borders, and verify academic geometry calculations.

Pick a shape, enter its dimensions, and the calculator returns the perimeter (or circumference for a circle) along with the formula used and a one-line step.

See also: Circle Calculator – Area & Circumference, Area Calculator – Find the Area of a Shape, and Circumference Calculator.

Quick Answer

Find the boundary length of 2D shapes. Select your shape and enter its dimensions to compute the total perimeter.

Shape

e.g. 8

e.g. 5

This is a hub. Each shape's dedicated calculator covers more (area, diagonals, angles, special cases).

Rectangle perimeter

Perimeter

26

2 × (8 + 5) = 26

ShapeRectangle
FormulaP = 2 × (length + width)
Perimeter26

Hub view. For area, diagonals, angles, and special cases, open the shape-specific calculator linked below.

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Examples

Rectangle 8 × 5

perimeter 26

Triangle 5, 6, 7

perimeter 18

Circle r = 5

circumference ≈ 31.416

Hexagon side 5

perimeter 30

How it works

Perimeter sums all sides of a polygon. For shapes with curved boundaries, the perimeter has a different name. The hub picks the right formula based on the shape you select.

Rectangle · P = 2 × (length + width)

Triangle · P = a + b + c

Circle · C = 2 × π × r

Trapezoid · P = base 1 + base 2 + side 1 + side 2

Parallelogram · P = 2 × (base + slant side)

Regular polygon · P = n × s

Shape-specific calculators

Related Calculators

More tools from Education

Circle Calculator – Area & CircumferenceCalculate the area of a circle from the radius, diameter, circumference, or area itself. Shows the circle area formula A = πr² with your own numbers substituted, and names the exact multiple of π whenever the coefficient is a clean one.Area Calculator – Find the Area of a ShapeFind the area of a shape from one tool. Supports circle, triangle, rectangle, and other common 2D geometric shapes.Circumference CalculatorCalculate the circumference of a circle from the radius or diameter. Includes a visual diagram and bonus area output.Fence CalculatorEstimate fence posts, sections, panels or pickets, rails, gate openings, and optional concrete bags from fence length, post spacing, and fence type.

Frequently asked questions

Perimeter is the total distance around the outside of a 2D shape, measured in linear units. For a rectangle 8 by 5, the perimeter is 26 units. For a circle, the perimeter has its own name: circumference. A circle of radius 5 has a circumference of about 31.416 units.

Perimeter is the distance around a polygon (a shape with straight sides). Circumference is the distance around a circle. Both measure the boundary of a 2D shape; they are just named differently for historical reasons.

Rectangle, triangle (three sides), circle (circumference), trapezoid (all four sides), parallelogram (base and slant side), and regular polygon (number of sides and side length). For variations like irregular polygons or shapes with curved sides, use the shape-specific calculators.

Rectangle: P = 2 × (length + width). Triangle: P = a + b + c. Circle: C = 2 × π × r. Trapezoid: P = base 1 + base 2 + side 1 + side 2. Parallelogram: P = 2 × (base + slant side). Regular polygon: P = n × s.

Whatever unit you used for the inputs. If you entered feet, the perimeter is in feet. The calculator does not convert units; keep all inputs consistent.

Not directly. For an irregular polygon, sum the side lengths yourself; the perimeter is just the total. For curved boundaries that are not a circle (ellipse, parabola, irregular curve), the formula is more involved and is not handled by this hub.