The Perimeter Calculator finds the distance around six common shapes: square, rectangle, circle (circumference), triangle, parallelogram, and trapezoid. Enter the shape's measurements in metres, centimetres, feet, inches, or yards, and the result arrives with conversions into metres, feet, centimetres, and inches, so a fencing estimate sketched in feet can be ordered in metres without a second tool.
Formulas used
| Shape | Formula | Inputs | |---|---|---| | Square | P = 4 x side | One side | | Rectangle | P = 2 x (length + width) | Length and width | | Circle | C = 2 x pi x radius | Radius | | Triangle | P = a + b + c | Three sides | | Parallelogram | P = 2 x (a + b) | Two adjacent sides | | Trapezoid | P = a + b + c + d | All four sides |
Worked examples
A rectangular garden 6 m by 4 m needs 2 x (6 + 4) = 20 m of edging, which the conversions give as about 65.6 feet. A square plot with 9 m sides needs 36 m. A circular patio of radius 3 m has a circumference of 2 x pi x 3, about 18.85 m (roughly 61.8 feet). A triangle with sides 5, 7, and 9 m totals 21 m; a parallelogram with adjacent sides 8 and 5 m totals 26 m; a trapezoid with sides 6, 4, 5, and 7 m totals 22 m.
Perimeter versus area in real decisions
Perimeter is the buying quantity for anything sold by length: fencing, edging, skirting board, trim, string lights, and the tape on a sports court. Area is the quantity for coverage: turf, paint, paving. The two do not move together: doubling a square's side doubles its perimeter but quadruples its area, which is why a modest garden extension can need a startling amount of extra soil but little extra fence. Order by the measure that matches the product, and add a waste margin of 5 to 10% for cuts and joins on length goods.
Units and conversions
Inputs can be entered in any of the five supported units; the computation happens in the unit you select, and conversions are exact by definition where it matters: 1 foot = 0.3048 m, 1 inch = 0.0254 m, 1 yard = 0.9144 m. Mixed-unit projects (a plan drawn in feet, materials priced per metre) are the classic source of ordering errors; let the conversion table carry the arithmetic and sanity-check one figure by hand: a 20 m perimeter is about 65.6 feet, so if a conversion ever reports a smaller number in feet than in metres, something has gone backwards.
Shape notes
For triangles, any three lengths you enter should satisfy the triangle inequality (each pair must sum to more than the third side); lengths that violate it cannot form a triangle at all, and the sum, while computable, describes an impossible figure. For the circle, entering diameter instead of radius doubles the reported circumference, the single most common slip with this formula, so confirm which one your measurement is. Trapezoid inputs are the four side lengths in any order, since the perimeter does not care which side is a base.
Limitations
The calculator handles the six named shapes in two dimensions with straight or circular edges. Compound shapes (an L-shaped garden), ellipses, and regular polygons with more sides need decomposition into these primitives or a dedicated tool. Measurements are used as entered; real-world perimeters over uneven ground or with posts, gates, and overlaps need field allowances the arithmetic cannot know about.
Frequently Asked Questions
Is circumference just perimeter for a circle?
Yes: circumference is the special name for a circle's perimeter, C = 2 pi r (equivalently pi x diameter). The calculator asks for the radius.
How much extra material should I add?
Five to ten percent for straight runs with few cuts; more for patterns, diagonal layouts, or inexperienced installation. It is cheaper to return an unopened bundle than to stop a job for one length.
Why does the answer look different in feet and metres?
Only the unit changes, not the length: feet are smaller units, so the number grows (metres x about 3.281), while inches grow it further (x 39.37). A smaller number in a smaller unit is the tell-tale of a conversion error.