How to use it
Enter the diameter in feet and inches. You get the circumference as a feet-inch-fraction length, the area in square feet and square inches, and the radius. Add an arc angle in degrees to get the arc length, the straight chord across it and the area of the sector.
Worked example
A circle 4' 6" across has a circumference of about 14' 1 5/8", an area of 15.9 sq ft and a radius of 2' 3". A 90° arc of that circle is about 3' 6 7/16" long, with a chord of about 3' 2 3/16".
The math
- Circumference = π × diameter.
- Area = π × diameter² ÷ 4.
- Arc length = circumference × angle ÷ 360.
- Chord = diameter × sin(angle ÷ 2).
Useful for round columns and footings, curved walls, arched openings, patios, fire pits and tank or duct wrap.
Typing measurements
Every box takes feet, inches and fractions the way you would write them: 9' 1 1/8", 7 1/2", 2.5' or 300mm. Put a space between whole inches and a fraction. A box also takes a whole calculation, so 14' 6" / 2 works anywhere.
Questions
How do I find the circumference of a circle in feet and inches?
Multiply the diameter by pi (3.14159). A 4' 6" circle is 54 inches across, so the circumference is 169.65 inches, or about 14' 1 5/8".
How do I find the length of an arc?
Multiply the circumference by the arc angle divided by 360. A 90° arc is a quarter of the circumference.
What is a chord?
The straight-line distance between the two ends of an arc. It is the measurement you can pull with a tape, which makes it useful for laying out curves.
How precise is the answer?
The math is done at full precision and only the displayed fraction is rounded, to the nearest 1/16 inch by default. You can change that from 1/2 down to 1/64. When rounding changes the value, the answer is marked with ≈.
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