Circle Measurements Calculator — Area, Circumference & Diameter
Enter the radius to instantly find the area, circumference, diameter, and quarter-circle sector values of any circle. Results scale to any unit — centimetres, metres, inches, feet.
units
A = π × r²
- 1
Square the radius
r² = 5² = 25 - 2
Multiply by π
π × 25 = 78.5398
How does this calculator work?
From radius r: Area = πr² ≈ 3.14159 r² and Circumference = 2πr ≈ 6.2832 r. Diameter = 2r. A quarter-circle has area πr²/4 and arc πr/2. Enter the radius — results work in any unit (area in unit²).
Formula
How this is calculated
A circle is fully defined by one number — its radius r (the distance from the centre to any edge point). Every measurement follows directly. The diameter d = 2r crosses the full width. The circumference (perimeter) C = 2πr ≈ 6.2832 r gives the total boundary length. The enclosed area A = πr² ≈ 3.14159 r² — area grows with the square of the radius, so doubling r quadruples the area.
The quarter-circle values split the full circle into four equal 90° sectors: each sector has area A/4 and arc length C/4 = πr/2. These are convenient reference points because many real-world shapes (rounded corners, quarter pipes, cable management) are based on quarter circles.
The formulas assume a perfect flat Euclidean circle. For circles on a curved surface (e.g. great circles on a sphere) the relationships differ. Use any consistent unit — the area result is always in that unit squared.
Frequently asked questions
Halve the diameter to get the radius, then apply A = π × r². Equivalently, A = π × (d/2)². For example, diameter 10 gives A = π × 25 ≈ 78.54 square units.
Circumference is proportional to r (linear), so it doubles when r doubles. Area is proportional to r² (quadratic), so it quadruples when r doubles. This r² relationship is why a circle twice as wide encloses four times the area.
No — enter any unit you like (cm, m, inches, feet). The circumference and diameter results are in the same unit; the area is in that unit squared. Just keep all inputs in one unit.
Also known as
TG we-Calculate Editorial Team. (2026). Circle Measurements Calculator — Area, Circumference & Diameter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/circle-measurements-calculator
TG we-Calculate Editorial Team. "Circle Measurements Calculator — Area, Circumference & Diameter." TG we-Calculate. 2026. https://we-calculate.com/calculator/circle-measurements-calculator.
TG we-Calculate Editorial Team, "Circle Measurements Calculator — Area, Circumference & Diameter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/circle-measurements-calculator
@misc{wecalculate_circle_measurements_calculator, title = {Circle Measurements Calculator — Area, Circumference & Diameter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/circle-measurements-calculator}}, year = {2026}, note = {TG we-Calculate} }
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