Geometry

Perimeter Calculator

Find the perimeter (or circumference) of any 2D shape with full step-by-step working. Supports circles, squares, rectangles, triangles and trapeziums. Enter the side lengths and the calculator shows how the formula is applied.

Shape

Length (l)

Width (w)

Perimeter

26 units

How it works

Write the formula
$P = 2(l + w)$
Perimeter of a rectangle = 2 × (length + width).
Calculate
$P = 2(8 + 5) = 2 \times 13 = 26$
P = 2 × (8 + 5) = 26 units.
Result:26 units

Perimeter formulas

P_{\text{rect}} = 2(l+w) \quad P_{\text{tri}} = a+b+c \quad C_{\text{circle}} = 2\pi r

l, w

Length and width of a rectangle

a, b, c

Side lengths of a triangle

r

Radius of a circle

Worked examples

Find the perimeter of a rectangle 12 cm × 7 cm.

1
Formula: P = 2(l + w)
2
Substitute: P = 2(12 + 7) = 2 × 19
3
Calculate: 38 cm

Answer: 38 cm

Find the circumference of a circle with radius 5 cm.

1
Formula: C = 2πr
2
Substitute: C = 2 × π × 5
3
Calculate: ≈ 31.42 cm

Answer: ≈ 31.42 cm

Find the perimeter of a triangle with sides 5, 7 and 9 cm.

1
Formula: P = a + b + c
2
Substitute: P = 5 + 7 + 9
3
Calculate: 21 cm

Answer: 21 cm

Frequently asked questions

What is perimeter?+

Perimeter is the total distance around the outside of a 2D shape.

How do you find the perimeter of a rectangle?+

Perimeter = 2(length + width).

What is the perimeter of a circle called?+

The circumference. C = 2πr = πd.

How do you find the perimeter of a triangle?+

Add all three side lengths together.

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