Circle Calculator
Calculate a circle's area, circumference and diameter from its radius.
Assumptions used in this calculation
- Precision of π: Uses the built-in floating-point value of π (about 15 significant digits), more precise than the 3.14 or 3.14159 approximations used in manual calculations.
About this calculator
Circle measurements come up everywhere from garden beds to pipe diameters to pizza sizing, and while the formulas are simple, hand calculations usually rely on a rounded π like 3.14, which introduces compounding error on larger radii. This calculator takes a radius and computes area as πr², circumference as 2πr, and diameter as 2 × radius, using JavaScript's built-in Math.PI, accurate to about 15 significant digits, far more precise than the 3.14 or 3.14159 approximations typically used by hand. If you only know the diameter rather than the radius, halving it first gives you the radius input this calculator expects. That removes both the arithmetic of squaring and multiplying by π and the small but real precision loss from a truncated π value, which matters more the larger the circle.
Worked example
Radius 5
Result: Area ≈ 78.54, circumference ≈ 31.42
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