Perimeter Calculator
Calculate the perimeter of a square, rectangle, triangle or the circumference of a circle.
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How Perimeter Is Calculated
Perimeter is the total length of a shape's boundary — a one-dimensional measurement in linear units (m, ft, etc.), never square units. The formulas: rectangle = 2 × (length + width); square = 4 × side; circle circumference = 2 × π × radius (or π × diameter); triangle = side a + side b + side c; regular polygon = number of sides × one side length. Worked example: a rectangular garden measuring 20 m by 30 m has a perimeter of 2 × (20 + 30) = 100 m of fencing needed (plus a small margin for gates and overlaps in practice). A circular flower bed with a 5 m radius has a circumference of 2 × π × 5 ≈ 31.42 m.
What to Know
- Perimeter is not area. This is the single most common confusion in geometry — perimeter measures the boundary length (linear, e.g. meters), while area measures the surface a shape covers (two-dimensional, e.g. square meters). They answer completely different questions: perimeter tells you how much fencing, trim, or edging you need; area tells you how much paint, flooring, or turf you need.
- The same perimeter can enclose very different areas. A 100 m perimeter forms a 25×25 m square with 625 m² of area, but the same 100 m perimeter forms a 10×40 m rectangle with only 400 m² of area — same boundary length, 36% less usable space. Among all rectangles with a fixed perimeter, the square always encloses the most area.
- Real-world jobs need a margin. Fencing, skirting boards, and picture-frame calculations should add a small percentage (often 3-10%) on top of the raw formula result to cover overlaps, corner joins, gates, and cutting waste.
- Units must match before adding. All sides must be converted to the same unit (all in meters, or all in feet) before summing — mixing meters and centimeters in one calculation is a common source of error.
Frequently Asked Questions
What's the difference between perimeter and area?
Perimeter is the total length of a shape's outer boundary, measured in linear units like meters — think of it as how far you'd walk to trace the outline. Area is the amount of surface the shape covers, measured in square units like square meters — think of it as how much floor space or land is inside. A shape's perimeter tells you how much fencing you need; its area tells you how much grass seed or flooring you need. They're unrelated in magnitude — a long, thin shape can have a large perimeter but tiny area.
How much fencing do I need for a 20×30 m garden?
Perimeter = 2 × (20 + 30) = 100 meters of fencing material. In practice, add roughly 3-10% extra to account for gate openings, corner posts, and overlap at joins, so budget for around 103-110 m of material.
What's the circumference of a circle with a 5 m radius?
Circumference = 2 × π × radius = 2 × π × 5 ≈ 31.42 meters. If you only know the diameter, use circumference = π × diameter instead — both formulas give the same result since diameter is twice the radius.
Can two shapes have the same perimeter but different areas?
Yes, dramatically different. A square with a 100 m perimeter (25 m per side) encloses 625 m² of area. A rectangle with the same 100 m perimeter but sides of 10 m and 40 m encloses only 400 m² — 225 m² less, despite using the exact same amount of boundary material. This is why perimeter alone never tells you how much space a shape actually contains.
How do I calculate the perimeter of an irregular shape?
For any polygon, however irregular, simply add the lengths of every side together. The rectangle, square, and regular-polygon formulas are just shortcuts for shapes where multiple sides share the same length — the underlying rule (sum of all boundary segments) always applies.
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