How do you calculate the perimeter?

The perimeter is the total length around a 2D shape. For a rectangle it is P = 2 × (L + W), for a square P = 4 × a, for a triangle P = a + b + c, for a circle P = 2 × π × r, and for a polygon it is the sum of all sides.

Can I calculate the perimeter of an irregular polygon?

Yes. Enter each side length and the calculator will sum them. This is useful for real-life plots, rooms and non-standard shapes.

Which units does this perimeter calculator support?

You can enter dimensions in millimeters, centimeters, meters or inches. The tool returns the perimeter in the same base unit and converts to meters/feet when useful.

Perimeter Calculator (Rectangle, Square, Triangle, Circle, Polygon)

Calculator

Shape

Unit

Length

Width

Results

Perimeter in selected unit, plus automatic conversions.

Perimeter (selected unit)

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Perimeter (meters)

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Perimeter (centimeters)

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Perimeter (feet)

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Common perimeter formulas

Shape	Formula	Notes
Square	P = 4a	a = side
Rectangle	P = 2(L + W)	L = length, W = width
Triangle	P = a + b + c	Any triangle, any sides
Circle	C = 2πr = πd	r = radius, d = diameter
Regular n-gon	P = n × s	n = number of sides, s = side length
Irregular polygon	P = Σ s_i	Sum all sides

How this perimeter calculator works

Perimeter is the total distance around a 2D shape. Most shapes have very simple perimeter formulas, so the real challenge is supporting many shapes and many units in a single tool — exactly what this calculator does.

Supported shapes

Rectangle: \( P = 2(L + W) \)
Square: \( P = 4a \)
Triangle: \( P = a + b + c \)
Circle (circumference): \( C = 2 \pi r \)
Regular polygon: \( P = n \times s \)
Irregular polygon: sum of all side lengths

Units and conversions

You can enter your dimensions in mm, cm, m or inches. The calculator converts everything internally to meters so that it can output:

perimeter in your selected unit (for example “3.5 m”)
perimeter in meters
perimeter in centimeters
perimeter in feet (useful for construction or landscaping)

FAQ

How do I find the perimeter of a room?

Measure each wall and sum the lengths. If the room is rectangular, use 2 × (length + width). If it has odd corners, use the “Irregular polygon” option and list all sides.

What if I only know the area?

Perimeter cannot be uniquely determined from area alone, unless you know the shape (e.g. square). For mixed or unknown shapes you must measure the sides.

Why is circle included in a perimeter calculator?

Because the circumference is the perimeter of a circle, and it is one of the most common perimeter problems.