Calculate the perimeter or circumference of a circle, triangle, rectangle, square, trapezoid, parallelogram or regular polygon — instantly, with the formula shown.
Choose a shape, enter its dimensions, and calculate to see the perimeter
This free perimeter calculator finds the perimeter or circumference of seven common shapes — circle, triangle, rectangle, square, trapezoid, parallelogram, and regular polygon — showing the exact formula behind every result. Perimeter measures the total distance around a shape's boundary, always expressed in linear units like meters, feet, or inches — never squared, unlike area. Whether you need a circumference calculator for a round tank, a perimeter of a rectangle calculator for a room, or a regular polygon perimeter calculator for a hex-shaped deck, this single tool covers every common case with the working shown.
Perimeter is a one-dimensional measurement of a shape's boundary — the total length you would walk if you traced every edge exactly once. Because it only sums linear distances, perimeter (and its circular special case, circumference) is always reported in the same unit you entered, unlike area, which multiplies two dimensions together and produces a squared unit. Two shapes can share an identical perimeter while enclosing very different amounts of area, which is why perimeter and area are always calculated separately.
This perimeter calculator is built for students checking geometry homework, homeowners planning fencing or edging for a yard, framers and crafters sizing trim or ribbon, contractors estimating baseboard or coping length, and athletics or landscaping professionals measuring the distance around a track or field. Anyone who needs a fast, verified perimeter or circumference calculation without manually rearranging a formula can use it.
Perimeter translates directly into the amount of linear material a project needs — fencing, trim, edging, wire, or piping are all priced and purchased by length, not by area. Underestimating a perimeter means running short mid-project; overestimating wastes material and budget. Because perimeter scales linearly with a shape's size (unlike area, which scales with the square), it's also a simpler, more forgiving measurement to estimate by hand as a sanity check.
Landscapers use rectangle and polygon perimeter to size fencing and edging. Framers use rectangle perimeter for picture-frame trim and molding. Track-and-field coaches use circle circumference to measure lap distances. Contractors use trapezoid and parallelogram perimeter for baseboard on angled rooms. Students use the triangle inequality check built into this calculator to understand why not every set of three lengths forms a real triangle.
How this perimeter calculator turns your dimensions into a linear-unit result, shape by shape
Every perimeter result stays in the unit you entered (m, ft, in) — no squaring, unlike area.
Circumference uses JavaScript's full-precision Math.PI (≈3.14159265358979), more accurate than 3.14 or 22/7 shortcuts.
π itself is defined as a circle's circumference divided by its diameter — a constant ratio for every circle, regardless of size.
From picking a shape to reading a verified result
Open the Shape dropdown and select circle (circumference), triangle, rectangle, square, trapezoid, parallelogram, or regular polygon.
The input fields update automatically for your chosen shape, all measured in the same unit.
The calculator applies that shape's perimeter formula — for a triangle, it first checks the triangle inequality before adding the sides.
The result panel shows the numeric perimeter (or circumference) in linear units alongside the exact formula and substitution used.
Pick a different shape from the dropdown to reset the input fields and calculate another perimeter without reloading the page.
Add the side lengths by hand, or estimate circumference as roughly 6.28 times the radius, to sanity-check the answer.
Finding the circumference of a circle with radius 5 — the calculator's default shape
A circular garden bed has a radius of 5 units. How much edging is needed to go around it?
Explanation: The result stays in linear units because perimeter and circumference only sum or scale straight-line distances — no squaring involved. If the radius were measured in feet, 31.4159 would be reported in feet of edging needed. Doubling the radius to 10 exactly doubles the circumference to ≈62.8319, unlike area, which would quadruple.
Unit conventions and sanity checks for each shape
| Shape | Unit Convention | Sanity Check |
|---|---|---|
| Circle (circumference) | Same linear unit as the radius entered | Should be roughly 6.28 × r — always more than 6 times the radius |
| Triangle | Same linear unit as the three sides | Must be greater than twice the longest side (triangle inequality) |
| Rectangle | Same linear unit as length/width | Should equal 2 × (length + width), always an even multiple of that sum |
| Square | Same linear unit as the side length | Always exactly 4 × side — a quick multiplication check |
| Trapezoid | Same linear unit as all four sides | Must be greater than the sum of just the two parallel sides |
| Parallelogram | Same linear unit as the two adjacent sides | Equal to 2 × (side a + side b), same form as a rectangle |
| Regular polygon | Same linear unit as the side length | Always exactly n × s — approaches a circle's circumference as n grows |
Reading the result: the number shown is always in whatever linear unit you entered — meters stay meters, feet stay feet, with no squaring involved.
Typical ranges: perimeter should always be a positive number strictly greater than any single side (or, for a triangle, greater than twice the longest side); an error message signals an invalid or impossible input.
Manual verification: for polygons, simply add the side lengths on a calculator or by hand; for a circle, multiply the radius by roughly 6.28 (2π) as a fast mental check.
Where a fast, accurate perimeter calculation genuinely helps
Check perimeter and circumference homework, including triangle-inequality problems.
Calculate the linear footage of fencing or edging needed to enclose a rectangular or polygonal yard.
Determine trim, ribbon, or molding length for a rectangular or square frame border.
Estimate baseboard, coping, or trim length for a room, including trapezoidal or angled walls.
Calculate the circumference of a circular or oval track for lap-distance planning.
Measure the perimeter of a rectangular or polygonal field for boundary fencing or irrigation piping.
Calculate edge banding or trim length for rectangular or polygonal panels and boxes.
Estimate boundary length for a rectangular, trapezoidal, or triangular land parcel.
Plan rug binding, curtain trim, or wall molding length relative to a room's perimeter.
Size edging or border material for a circular, rectangular, or polygonal garden bed.
Estimate the outline length of a printed part's cross-section for toolpath or material planning.
Calculate binding or piping length for a rectangular or polygonal fabric panel.
Quickly size trim, wire, or edging for any home project shaped like a common polygon.
What this perimeter calculator does well, and where manual judgment is still needed
Formula and typical use, side by side
| Shape | Formula | Typical Use |
|---|---|---|
| Circle (circumference) | C = 2πr | Round tanks, tracks, wheels, pipe wrap |
| Triangle | P = a + b + c | Triangular lots, roof trusses, gussets |
| Rectangle | P = 2(l + w) | Rooms, yards, picture frames |
| Square | P = 4s | Square plots, tiles, patios |
| Trapezoid | P = a + b + c + d | Angled walls, ramps, non-rectangular rooms |
| Parallelogram | P = 2(a + b) | Slanted panels, some land shapes |
| Regular polygon | P = n × s | Hex decks, gazebos, octagonal tables |
Summary: This perimeter calculator gives you an instant, free way to find the perimeter or circumference of a circle, triangle, rectangle, square, trapezoid, parallelogram, or regular polygon — with the formula and substitution shown for every result. Pair it with the Area Calculator and Volume Calculator for a complete picture of a shape's edges, surface, and solid capacity.
Common questions about perimeter formulas and this perimeter calculator
Trusted educational references to go deeper on the math behind perimeter and circumference
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