Circumference Calculator

How to calculate circumference

This circumference calculator multiplies the diameter of a circle by π (pi, about 3.14159) to give the distance around it. The formula is C = π × d. A circle 10 ft across measures 31.416 ft around. That is the length of edging, coping, fencing, or banding you need to wrap it once.

Enter the diameter in feet and the result is in feet, shown to three decimal places. You can also enter inches or meters, and the answer comes back in the same unit.

What π means

Take any round object, such as a bucket lid, a pool, or a tree trunk. Wrap a string around it, then lay the string straight across the middle. The string is always just over three times as long as the distance across, and the exact ratio is 3.14159. That ratio is π. Because it never changes, you only need one measurement, the diameter, to know the distance around.

If you measured the radius (center to edge), double it first: d = 2 × r.

Worked examples

1. Round above-ground pool, 24 ft across

• C = 3.14159 × 24 = 75.398 ft
• If the ledge or trim is sold in 8 ft pieces: 75.398 ÷ 8 = 9.4, so buy 10 pieces.

2. Concrete column form, 18 inches across

• Enter 1.5 (ft): C = 3.14159 × 1.5 = 4.712 ft, or about 56.5 in
• Or enter 18 (in): C = 56.549 in

That is the width of decorative wrap or trim needed to go around the column once. Add a few inches for the overlap at the seam.

Circumference reference table

Diameter	Circumference	Typical object
4 in	12.566 in	Drain or downspout pipe
8 in	25.133 in	Sonotube for deck footing
12 in	37.699 in	Sonotube for porch post
18 in	56.549 in	Round column
4 ft	12.566 ft	Fire pit ring
10 ft	31.416 ft	Round patio or garden bed
12 ft	37.699 ft	Stock tank or small pool
15 ft	47.124 ft	Above-ground pool
18 ft	56.549 ft	Above-ground pool
24 ft	75.398 ft	Above-ground pool
30 ft	94.248 ft	Large round pool or tank

Each step up of 1 ft in diameter adds 3.14 ft to the distance around, whatever size you start from.

How to measure the diameter

• Go through the center. The diameter is the longest straight line across the circle. If you are not sure where the center is, swing the tape slightly. The longest reading is the diameter.
• For big circles, measure twice at right angles. Pools and patios are often slightly oval. Average the two readings.
• For trunks, posts, and pipes, measure around instead. Wrap a flexible tape, then divide by π to get the diameter.
• Decide inside or outside. Edging that sits outside a ring needs the outside diameter. A liner or seat inside it needs the inside diameter. On a pool, the two can differ by a foot or more.

Common mistakes

• Entering the radius as the diameter. Your answer comes out half the real length. If a 24 ft pool shows 37.7 ft, you entered 12.
• Using 3 for π. On a 30 ft circle that short-changes you by 4.25 ft, which is more than half of an 8 ft trim piece.
• Forgetting overlap and joints. Flexible edging, banding, and liner beads need extra at the joint. Rigid sections often need one extra piece to finish the ring.
• Mixing up circumference and area. Coping and edging follow the circumference. Concrete, gravel, or sod for the same circle follow the area. Use the cylinder volume calculator for anything with depth.

Related tools

For round pools, the round pool calculator turns the same diameter into gallons. For straight runs of trim or edging, the linear feet calculator and the perimeter of a rectangle calculator cover square shapes.