Education
Perimeter Calculator
Last updated: June 19, 2026
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.
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).
Perimeter
26
2 × (8 + 5) = 26
Hub view. For area, diagonals, angles, and special cases, open the shape-specific calculator linked below.
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
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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.
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