Circle Circumference & Area Calculator

Enter the radius to get area, circumference, and diameter in one calculation.

Result

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Area
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Circumference
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Diameter
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Formulas Applied

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What This Calculator Returns

The Circle Calculator finds the area, circumference, and diameter of any circle from a single input: the radius. Enter the radius in any unit and all three measurements are returned simultaneously with the formula used for each.

Circles appear in construction, gardening, furniture, and engineering far more often than they get credit for. A swimming pool, a round dining table, a terrace garden bed, a water storage tank cross-section, or a circular driveway turning radius — each requires at least two of these three measurements to work with practically. This tool returns all three so you are not switching between formulas or running partial calculations.

The Three Formulas

Area = π × r²
Circumference = 2 × π × r
Diameter = 2 × r

Where r is the radius (distance from the centre to the edge) and π (pi) is approximately 3.14159. The calculator uses the full precision value of pi built into JavaScript, accurate to 15 decimal places.

Example: A circular garden bed with a 1.8 m radius

Area = π × 1.8² = 3.14159 × 3.24 = 10.18 m² (soil or mulch coverage needed)

Circumference = 2 × π × 1.8 = 11.31 m (edging or border material needed)

Diameter = 2 × 1.8 = 3.6 m (total width of the bed)

The calculator handles all three automatically. The formulas are shown here for transparency.

Circular Spaces and Objects

Garden beds and planters — A circular raised garden bed with a 1.5 m radius has an area of 7.07 m². If you are filling it with potting mix to a depth of 30 cm (0.3 m), multiply the area by the depth to get the volume: 7.07 × 0.3 = approximately 2.12 m³ of soil. For cylindrical raised beds where the depth matters, our Cylinder Volume Calculator handles the full volume calculation.

Round dining tables — The most common round dining tables measure 90 cm to 130 cm in diameter, meaning radii of 45 cm to 65 cm. A 120 cm diameter table (60 cm radius) has a surface area of π × 0.6² = 1.13 m². The circumference tells you how much edging trim or tablecloth overhang to account for at the boundary.

Swimming pools and ponds — A circular pool with a 3 m radius holds water over an area of 28.27 m² at the surface. This figure is used to calculate chemical dosing (typically per 1,000 litres of water volume), solar cover size, and pool heat requirements. The circumference (18.85 m) gives the length of pool fencing or poolside coping needed.

Circular driveways and turning areas — A circular turning area for a car typically needs a radius of at least 5 to 6 metres for a standard sedan. A 5.5 m radius gives an area of 95.03 m² and a circumference of 34.56 m, which determines the total length of kerbing or surface material needed around the turning circle.

How to Measure the Radius

If you only have the diameter (the full width straight across), divide it by 2 to get the radius. A circular table that measures 120 cm across has a radius of 60 cm.

If you only have the circumference (the distance around the edge), divide it by 2π (approximately 6.2832) to get the radius. A circular pool with a circumference of 25 m has a radius of 25 ÷ 6.2832 = approximately 3.98 m.

For a physical object where the centre is hard to identify, measure the full width at the widest point (this is the diameter) and halve it.

Frequently Asked Questions

Multiply pi by the radius squared. Area = π × r². For a circle with radius 3 m, the area is 3.14159 × 9 = 28.27 m². This is the figure used for ordering materials like soil, turf, paint, or any covering sold per square metre.
Multiply 2 by pi by the radius. Circumference = 2 × π × r. For a radius of 3 m, the circumference is 2 × 3.14159 × 3 = 18.85 m. The circumference is the total distance around the edge — the figure used for edging, borders, fencing, or trim.
The radius is the distance from the centre of the circle to any point on its edge. The diameter is the full width straight across the circle through the centre, which equals exactly twice the radius. If a circular pool is 6 m across (diameter), its radius is 3 m — and that is the figure to enter into this calculator.
Divide the diameter by 2. A circular dining table that is 1.2 m in diameter has a radius of 0.6 m. Enter 0.6 into the calculator to get the surface area (1.13 m²) and circumference (3.77 m).
Measure from the centre of the bed to its outer edge to get the radius. Enter that value into the calculator. The area result tells you how much soil, compost, or mulch you need. For a bed with a 1.5 m radius, the area is π × 1.5² = 7.07 m². At 20 cm depth, you need 7.07 × 0.20 = approximately 1.41 m³ of material.
The formula Area = π × r² is derived using the radius because a circle is defined as the set of all points at a fixed distance r from a centre point. The radius is the natural measure of the circle. While the formula can be rewritten as π × (d/2)² using diameter, using radius directly is simpler and is the universal standard.
The calculator uses the full precision value of pi (Math.PI, accurate to 15 decimal places) and displays results to two decimal places. For garden beds, pools, and furniture measurements, this is more than sufficient. For engineering or manufacturing applications, use the full-precision value from a scientific calculator or CAD software.
This calculator is for full circles. For a semi-circle, divide the area by 2 and divide the circumference by 2, then add the diameter for the straight edge. For a quarter circle, divide area by 4 and circumference by 4, then add two radii for the two straight edges. For a cylindrical volume (like a water tank), use our Cylinder Volume Calculator directly.

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