Diameter of a Circle Calculator
The diameter is the longest straight line across a circle. Enter whichever measurement you have — radius, circumference, or area — and get the diameter with the math shown.
Example: with I know the Radius (r) · Value 5 → Diameter (d): 10.00.
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Check it outHow to find the diameter of a circle
The diameter is the longest chord of a circle — a straight line edge to edge through the center, exactly twice the radius. Which diameter formula you need depends on what you already know: d = 2r from the radius, d = C ÷ π from the circumference, and d = 2√(A ÷ π) from the area.
Worked through: a radius of 5 gives d = 2 × 5 = 10. A trunk measuring 62.83 in around gives d = 62.83 ÷ π ≈ 20 in. A circle covering 50 sq ft has d = 2√(50 ÷ π) ≈ 7.98 ft. Whichever you start from, the calculator also returns the matching circumference and area, so you can cross-check: the circumference should always be about 3.14× the diameter you get.
How it’s calculated
d = 2r. From a circumference: d = C ÷ π. From an area: d = 2√(A ÷ π), which is A = πr² solved for r and doubled. π ≈ 3.14159 via JavaScript’s full-precision Math.PI; rounding happens only at display (2 decimals).
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Forgetting to double after solving from area — √(A ÷ π) is the radius, not the diameter.
- Dividing the circumference by 2π — that gives the radius; divide by π alone for the diameter.
- Measuring across off-center — any chord that misses the center reads shorter than the true diameter.
Frequently asked questions
What is the diameter of a circle?
The straight-line distance across the circle through its center — the longest possible chord. It is exactly twice the radius.
What is the diameter formula?
Three forms, depending on the input: d = 2r from the radius, d = C ÷ π from the circumference, and d = 2√(A ÷ π) from the area.
How do I convert circumference to diameter?
Divide by π: a 62.83 in circumference is 62.83 ÷ 3.14159 ≈ 20 in across.
How do I find the diameter from the area?
Solve A = πr² for r, then double it: with A = 50, r = √(50 ÷ π) ≈ 3.99, so d ≈ 7.98.
Is the diameter always twice the radius?
Yes, by definition — the radius runs from center to edge, and the diameter runs edge to edge through the center.