Perimeter Calculator

Walk the boundary once before typing measurements

An 18 ft by 9 ft garden bed needs 54 ft of edging before gates or overlaps are considered. Perimeter is the distance travelled around that closed outline. A useful field check is to start at one corner, follow every outside edge in order and finish at the same corner. This prevents a recess from being skipped or one side from being counted twice.

For straight edges, convert every measurement to one unit, then add only the exposed outside lengths. The answer stays in feet, metres, inches or another length unit. Square feet and square metres belong to area, so they should not appear in this result.

• Square: multiply one side by four.
• Rectangle: double the sum of length and width.
• Triangle: add the three sides after checking that they can form a triangle.
• Regular polygon: multiply its number of equal sides by one side length.

The sum 4a also works for a rhombus, and 2(a+b) for a parallelogram. Equal sides alone do not make these shapes squares or rectangles; those names require right angles. An irregular quadrilateral needs four separate measurements, while a triangle perimeter calculator must reject 2, 3 and 6 because the two shorter sides do not reach the longest one.

A semicircle includes a straight edge

The circumference of a circle is C = 2πr, or C = πd when the diameter is known. A circumference calculator needs the radius in the radius field; entering a 14 cm diameter as r would double the correct answer. Use r = 7 cm instead to obtain about 43.98 cm.

For a closed semicircle, half the curved distance is not the whole boundary. The diameter must be added, giving P = πr + 2r. If a drawing asks only for the arc, use πr. For an ellipse, enter semiaxes: a 10 cm by 6 cm oval has a = 5 and b = 3. This option uses Ramanujan’s first approximation, P ≈ π[3(a+b) − √((3a+b)(a+3b))], giving about 25.527 cm. A very flat ellipse needs a more accurate method for precision fabrication.

Convert the result into an order quantity

Fencing, baseboard, cable tray and picture-frame jobs rarely end with the geometric number. Subtract doors, gates or other deliberate openings. Add cutting loss, corner joints and overlap afterward, using a rate that fits the material rather than a universal percentage. The calculator does not add that allowance on its own.

Perimeter tells you how much material follows the edge; area tells you how much surface lies inside. A 5 by 5 square and an 8 by 2 rectangle each have an outside total of 20 units, yet enclose 25 and 16 square units. Knowing one value therefore cannot identify the other without more dimensions.

Measured plans also need a quick reality check. A regular-polygon result is valid only when every side is equal. For a floor plan with alcoves, trace each turn and add the actual segments. If the project mixes inches and feet, convert first and keep the original measurements beside the final calculation so a purchasing error can be found later.

Enter 1.5 for a decimal or 1,500 for fifteen hundred, without spaces or unit labels. A polygon needs an integer side count from 3 to 9,007,199,254,740,991. Calculations use JavaScript numbers with roughly 15–17 significant digits. The main result rounds to at most six decimal places; the working and copied value use up to ten. Very small or large values switch to scientific notation. Extra displayed digits do not make the original measurements more accurate.