Square Perimeter Calculator

P = 4 × s

Four equal sides, so the perimeter is four times any one side. A 5 cm square has a 20 cm perimeter. A 10 ft square has 40 ft of perimeter.

Where this matters in practice:

• Fencing a square garden. A 20 ft × 20 ft vegetable bed needs 80 ft of fence panels, plus four corner posts and a gate. Adding 5% for waste and cut errors: order at least 84 ft.
• Trim around a square room or rug. A 9 ft × 9 ft area rug needs 36 ft of trim or rope edging. Border tape sold in 50 ft rolls covers one rug with room to spare.
• Picture-frame molding around a square painting. A 24-inch-square canvas needs 96 in (8 ft) of molding plus mitre cuts. Buy 10 ft to allow for the cut waste at each 45° corner.
• Concrete curbing around a square patio. A 12 ft × 12 ft patio needs 48 linear ft of curb stones.

Quick relations from the side s:

• Perimeter: P = 4s
• Diagonal: d = s√2 ≈ 1.414 × s
• Area: A = s²

A square with perimeter P has side s = P/4 and area (P/4)² = P²/16. That’s the largest area you can enclose with that perimeter using a four-sided shape.

Worked example: backyard fence

You want a square dog run of 144 sq ft. The square root of 144 is 12, so the run is 12 ft × 12 ft and the perimeter is 48 ft. With six-foot panels that is 48 / 6 = 8 panels exactly, no cuts.

Now count the posts, and count them properly, because this is where the estimate usually goes wrong. Every panel needs a post at each end, and each side takes two panels, so there is a post at all four corners AND one halfway along every side. That is 8 posts, not 4. Swap one panel for a gate and you still need its two posts, so the order is 7 panels, 1 gate, 8 posts and hinge hardware.

The rule for any straight run: posts = panels + 1. Around a closed loop the last post is shared with the first, so posts = panels exactly.

Worked example: pool tile border

A 10 ft × 10 ft square pool gets a tile border. Perimeter is 40 ft = 480 in. Border tile is sold in 6-inch lengths, so 480 / 6 = 80 tiles. Order 88 for a 10% spare, because the corners need mitre cuts and some breakage is inevitable.

Why perimeter is not the same as the diagonal. A square’s diagonal is one straight line across, length s√2. The perimeter is the path around all four sides, length 4s. For a 1 m square the diagonal is 1.414 m and the perimeter is 4 m. Related numbers, completely different measurements, and a supplier quoting one when you meant the other is off by a factor of nearly three.