Perimeter of a Square Calculator

Perimeter of a square formula

A square has four sides of equal length, so the distance around it is four times one side. Writing a for the side:

P = 4 × a

Two variations are handy when the side is not what you measured:

• From the area A: the side is √A, so P = 4 × √A.
• From the diagonal d: the side is d ÷ √2, so P = 2√2 × d ≈ 2.828427 × d.

How to use the calculator

1. Type the side length in Square Side. The field starts at 3.
2. Choose the unit next to it: mm, cm, dm, m, km, in, ft, yd or mile.
3. Press Calculate to read the Square perimeter in the same unit.

The answer keeps the unit you picked, so there is nothing to convert. It is rounded to six decimals and trailing zeros are dropped in pairs: 12 stays 12, 10 stays 10, but 1.332 is shown as 1.3320. Large results get comma separators.

Examples: find the perimeter of a square

Example 1: a tile. A square tile has a side of 3 cm. P = 4 × 3 = 12 cm.

Example 2: a fence from the area. A square plot covers 625 m². Its side is √625 = 25 m, so the fence around it is 4 × 25 = 100 m.

Example 3: from the diagonal. The diagonal of a square table top is 10 dm. The side is 10 ÷ √2 ≈ 7.0710678 dm and the perimeter is 4 × 7.0710678 ≈ 28.284271 dm. You get the same figure from 2.828427 × 10.

Example 4: skirting board. A square room is 4.2 m on each side, so the walls add up to 4 × 4.2 = 16.8 m. Take away a 0.9 m doorway and you need 15.9 m of skirting board.

Perimeter and area side by side

The perimeter grows in step with the side, while the area grows with its square. Doubling a side doubles the perimeter but quadruples the area. For a side of 4 the two numbers happen to match (16 and 16), which is a coincidence of the units, not a rule: they measure different things.

Why a square is the efficient rectangle

Among all rectangles with the same perimeter, the square encloses the most area. With 24 units of fence, a 6 × 6 square covers 36 square units, while a 5 × 7 rectangle covers 35 and a 2 × 10 rectangle only 20. If you do need a rectangle, the perimeter of a rectangle calculator handles two different sides, and the area of a square calculator gives the enclosed area.

Where the perimeter of a square is used

Squares are everywhere in building and crafts: square floor tiles, window panes, garden beds, picture mats and patio slabs. When something has to run along the outside, such as a border, a frame, edging or a fence, the perimeter gives the length of material. For a bed made of four equal timber boards, the perimeter is simply the sum of the four boards, and 4 times the side is the quick way to check your cut list. Add a small allowance if the boards overlap at the corners.

Remember that the unit you choose beside the side also names the answer. A side typed in feet returns a perimeter in feet, so convert first if you need the result in another unit.

Common mistakes

• Squaring the side when you wanted the perimeter. Squaring gives the area; the perimeter multiplies by 4.
• Quoting a perimeter in square units. A perimeter is a length, so cm, m or ft, never cm² or m².
• Using the diagonal as if it were a side. The diagonal is about 1.4142 times longer than a side.

For other shapes, including triangles, circles and trapezoids, use the general perimeter calculator.