Circle from Diameter Calculator
Calculate a circle's area and circumference from its diameter instantly with this free geometry calculator in your browser.
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Shape
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the diameter.
- Press Calculate.
How it works
From the diameter: radius = diameter ÷ 2, circumference = π × diameter, area = π × radius².
Worked example
diameter 2 m. radius 1 m, circumference ≈ 6.28 m.
Common mistakes
- Plugging the diameter into πr² — halve it to the radius first.
- Mixing radius and diameter formulas; circumference is πd or 2πr.
Frequently asked questions
How do I find a circle's area from its diameter?
Halve the diameter to get the radius, then use πr². Or directly: area = π × (diameter ÷ 2)².
How do I get circumference from diameter?
Circumference = π × diameter, since the diameter is twice the radius and circumference = 2πr.
When should I use this instead of the radius?
When you've measured straight across the circle (the diameter) rather than from the centre out. It halves it to the radius for you.
Related tools
- Circumference Calculator
- Circle Area Calculator
- Sector Area Calculator
- Arc Length Calculator
- Hemisphere Volume Calculator
- Annulus Area Calculator
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Cite this tool
Zerdly. (2026). Circle from Diameter Calculator. Retrieved from https://www.zerdly.com/tools/circle-from-diameter-calculator/
Tip: Enter any known values to calculate the remaining results.
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