Pick a shape, enter its measurements and get the perimeter (the total distance around it) with the formula worked out, a labeled diagram and the area as a bonus. Results are converted to metric and imperial units.
Last updated:
Half of the longest width
Half of the shortest width
Polygon type
All sides in order, separated by commas or spaces
°
Enter your values to see the result.
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How to use the Perimeter Calculator
Choose the shape from the list.
Choose the unit your measurements are in.
Enter the measurements asked for. The fields change with the shape.
Press Calculate to see the perimeter, the formula steps and the diagram.
Check the unit table to read the perimeter and area in other units, or copy the results.
Perimeter formulas
Shape
Perimeter formula
Example
Square
P = 4s
s = 5 → P = 20
Rectangle
P = 2(l + w)
12 × 7 → P = 38
Triangle
P = a + b + c
7, 8, 10 → P = 25
Right triangle
P = a + b + √(a² + b²)
legs 3, 4 → P = 12
Circle
C = 2πr = πd
r = 5 → C ≈ 31.416
Semicircle
P = πr + 2r
r = 5 → P ≈ 25.708
Ellipse
P ≈ π(a + b)(1 + 3h ÷ (10 + √(4 − 3h)))
a = 8, b = 5 → P ≈ 41.38
Trapezoid
P = a + b + c + d
14, 8, 5, 5 → P = 32
Parallelogram
P = 2(a + b)
10, 6 → P = 32
Regular polygon
P = n × s
hexagon, s = 4 → P = 24
Circle sector
P = 2r + rθ (θ in radians)
r = 6, 60° → P ≈ 18.283
Practical tips
Fencing and edging: measure the perimeter, then subtract the width of any gates or doorways and add 5–10% for waste and overlaps.
Baseboards and trim: use the room's perimeter minus door openings. Irregular rooms are easiest as a polygon: list every wall length in order.
Keep units consistent: convert all sides to the same unit before entering them. 1 ft = 12 in = 30.48 cm, and 1 m ≈ 3.281 ft.
Same perimeter, different area: a 10 × 10 square and a 19 × 1 rectangle both have a perimeter of 40, but areas of 100 and 19. For a fixed perimeter, a circle encloses the most area.
Sides must fit: a triangle only exists if each side is shorter than the other two combined, and a polygon only closes if its longest side is shorter than the sum of the rest.
Frequently asked questions
What is perimeter?
Perimeter is the total length of the boundary of a flat shape: the distance you would walk going once all the way around it. It is measured in length units such as cm, m, inches or feet. For a circle the perimeter is called the circumference.
How do I find the perimeter of a rectangle?
Add the length and the width and double the result: P = 2(l + w). A 12 ft by 15 ft room has a perimeter of 2 × (12 + 15) = 54 ft.
How do I find the perimeter of a triangle?
Add the three sides: P = a + b + c. For a right triangle where you only know the two legs, first find the hypotenuse with c = √(a² + b²). A 3-4-5 triangle has a perimeter of 12.
What is the formula for the circumference of a circle?
C = 2πr or C = πd, where r is the radius and d the diameter, and π ≈ 3.14159. A circle with a 3 m diameter has a circumference of about 9.42 m.
What is the perimeter of a semicircle or a sector?
A semicircle's perimeter is the curved half plus the straight diameter: P = πr + 2r. A sector (pie slice) has two radii plus the arc: P = 2r + rθ, with θ in radians (degrees × π ÷ 180).
Is there an exact formula for the perimeter of an ellipse?
No simple exact formula exists; it requires an elliptic integral. This calculator uses Ramanujan's second approximation, P ≈ π(a + b)(1 + 3h ÷ (10 + √(4 − 3h))) with h = (a − b)² ÷ (a + b)², which is accurate to a tiny fraction of a percent for almost any ellipse.
What is the difference between perimeter and area?
Perimeter measures the length around a shape (units like ft), while area measures the surface inside it (square units like ft²). Use perimeter for fencing, trim, edging or baseboards, and area for flooring, paint or turf.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.