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Perimeter is the total length of a shape's outer boundary — the sum of its sides, or for a circle, the circumference. Fencing a garden, adding edge banding to a table, measuring a plot's border, or solving a geometry question all need the perimeter. The key is not to confuse it with area: perimeter is a length (m), area is a surface (m²).
Pick a shape, enter the side measurements, and the perimeter is calculated instantly.
Perimeter Calculator: step-by-step worked example
Fencing needed around a round pool with a 3.5 m radius
- Shape
- Circle (circumference)
- Radius
- 3.5
- 1
Diameter
d = 2 × r
2 × 3.5 = 7 m
- 2
The number π
π = Circumference ÷ Diameter (the same for every circle)
π ≈ 3.14159265 = 3.1416
- 3
Circumference
C = 2 × π × r = π × d
2 × 3.14159265 × 3.5 = 21.9911 m
- Circle (π × d)21.9911 m
- Square (4 × d)28 m
- Equilateral triangle (3 × d)21 m
Good to know: The most common mistake is entering the diameter as the radius — the perimeter doubles. When buying fencing or edging, add about 5% for joins and gates.
Perimeter Calculator — how it works
Formulas
| Shape | Perimeter formula |
|---|---|
| Square | 4 × side |
| Rectangle | 2 × (length + width) |
| Triangle | sum of the three sides |
| Circle (circumference) | 2 × π × radius |
Perimeter or area?
This is the most common mix-up: boundary jobs — fencing, edging, skirting, edge banding — are perimeter; surface jobs — paint, sod, tiles — are area. If you're fencing a garden look at the perimeter; if you're laying grass, look at the area.
Radius and diameter in a circle
The circumference is 2πr; if you have the diameter (d), the formula can also be written πd (since d = 2r). A circle with radius 5 has a circumference of 2 × 3.14159 × 5 = 31.42.
Practical uses
- Fencing: find the plot's perimeter and multiply by the price per metre; subtract the gate opening.
- Edge banding/skirting: take the room's perimeter and subtract the doorway width.
- Running/walking: one lap of a circular track is found from the radius with 2πr.
For a surface calculation use an area calculator; for volume use a volume calculator.
Worked example
You're laying edging around a circular pool with a radius of 5 m: the perimeter = 2 × π × 5 = 31.42 m of edging needed. To fence a square garden with a 7 m side, you need 4 × 7 = 28 m of fence. Careful: turfing that same square garden is an AREA job, not perimeter (7 × 7 = 49 m²) — fence is measured in metres, grass in square metres, and confusing the two ruins your material estimate.
Frequently asked questions
How is perimeter calculated?+
By summing the sides: square is 4 × side, rectangle is 2 × (length + width), triangle is the sum of the three sides, circle is 2 × π × radius. Pick the shape and enter the dimensions.
How do I find the circumference of a circle?+
With 2 × π × radius (2πr). A circle with radius 5 has a circumference of 31.42. If you have the diameter, you can also use π × diameter.
What is the difference between perimeter and area?+
Perimeter is the length of the outer boundary (metres); area is the surface it covers (square metres). Fencing and edging are perimeter; paint and grass are area.
How do I estimate fencing material?+
Calculate the plot perimeter and multiply by the price per metre; remember to subtract gate and entrance openings. For irregular land, measure and add each side.
Why is the rectangle perimeter 2 × (length + width)?+
A rectangle has two equal lengths and two equal widths. Adding length and width then doubling gives the total of all four sides.
What unit does perimeter come out in?+
The same length unit you enter (m, cm…). Unlike area it is not squared — perimeter is a length. All sides must be in the same unit.
About this page
- Publisher:
- Kalkoro — Teknolojig Bilişim Teknolojileri A.Ş., İstanbul
- Written by:
- Ahmet Tur — Editor (Math, date & time, unit conversion, physics & chemistry, everyday tools)
- Reviewed by:
- Kübra Yıldız — Editor
Parameters come from official sources; the last update date is shown at the bottom of the page.
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