Perimeter Calculator - Square, Circle, Triangle

How the Perimeter Calculator Works

Pick a 2D shape and enter its dimensions. The calculator computes the perimeter - the total length of all sides - or the circumference for circles. The formula is shown with every step so you can follow the math.

Quick start: Select "Rectangle", enter a = 5, b = 12, click Calculate. Result: P = 2(5 + 12) = 34 units.

Perimeter Formulas

Each shape has a straightforward formula. The perimeter is the total distance around the boundary of the figure.

Shape	Formula	What to enter	Example
Square	P = 4a	side (a)	a=7 28
Rectangle	P = 2(a + b)	two sides (a, b)	5×12 34
Triangle	P = a + b + c	three sides	3+4+5 12
Circle	C = 2πr	radius (r)	r=10 62.83
Parallelogram	P = 2(a + b)	two sides (a, b)	6×9 30

Perimeter vs Circumference vs Area

These three terms often cause confusion. Here is how they differ:

Term	What it measures	Unit
Perimeter	Total length of all sides (polygons)	cm, m, ft
Circumference	Perimeter of a circle (C = 2πr)	cm, m, ft
Area	Surface enclosed inside the shape	cm², m², ft²

Practical Examples

Example 1: Garden fence Rectangular garden 15 m × 8 m
P = 2(15 + 8) = 46 meters of fencing needed

Example 2: Running track Circular track r = 50 m
C = 2π × 50 = 314.16 meters per lap

Example 3: Picture frame Square 30 × 30 cm
P = 4 × 30 = 120 cm of frame material

Example 4: Triangular plot Sides 20 m, 25 m, 30 m
P = 20 + 25 + 30 = 75 meters

Example 5: Parallelogram tile Sides 10 cm and 15 cm
P = 2(10 + 15) = 50 cm edge length

FAQ

What is the difference between perimeter and circumference?

Perimeter refers to the total boundary length of any polygon (shapes with straight sides). Circumference is the specific term for the perimeter of a circle. The formula C = 2πr applies only to circles.

Can I use this to calculate how much fencing I need?

Yes. Select the shape of your property or garden, enter the dimensions, and the perimeter tells you the total fence length. Add 5-10% extra for posts, gates, and cutting waste.

Is the perimeter of a rectangle the same as a parallelogram?

The formula is the same: P = 2(a + b). Both have two pairs of equal sides. The difference is angles - a rectangle has four right angles (90°), while a parallelogram can have any angle.

How do I find the perimeter if I only know the area?

You need at least one dimension. For a square: side = √area, then P = 4 × side. For a circle: r = √(area/π), then C = 2πr. For rectangles and triangles, area alone is not enough.

Do I enter the radius or diameter for a circle?

Enter the radius (half of the diameter). If you know the diameter is 20, enter 10 as the radius. The calculator uses the formula C = 2πr.

What units does the result use?

The result is in the same units as your input. Enter centimeters and the perimeter is in centimeters. Enter meters and the perimeter is in meters. No conversion is applied.

Related Tools

Area Calculator

Calculate the area of 2D shapes - squares, circles, triangles - Open calculator

Volume Calculator

Volume of 3D solids - cube, sphere, cylinder, cone, pyramid - Open calculator

Pythagorean Theorem Calculator

Find the missing side of a right triangle - Open calculator

Quadratic Equation Calculator

Solve ax² + bx + c = 0 with discriminant and vertex - Open calculator

Fraction Calculator

Add, subtract, multiply, and divide fractions - Open calculator

Ready-made calculations

The most searched variants of this calculator. Each link opens it with the value already filled in, ready to calculate.