Circle Area Calculator
Enter the radius. The calculator returns A = πr², the area of the flat disc.
Calculate the area of a circle from its radius. A = πr² gives the flat area enclosed by the circle.
Mix and match length units per input (mm, cm, m, km, in, ft, yd, mi) and pick the area unit you want for the result (mm², cm², m², km², in², ft², yd², mi², acres or hectares). The calculator shows the formula being used and a clear breakdown so you can sanity-check the maths and not just trust the number.
Looking for other shapes? See our Area Calculator hub for the full list of 2D shapes we cover.
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What this calculator does
Computes the area of a circle from its dimensions, using the formula A = πr². Supports all common length units on input and any area unit on output, with a worked breakdown.
Who it is for
Students, teachers, DIYers, builders, fabricators, packaging designers and anyone who needs the area of a circle quickly and accurately.
How to use it
Enter Radius (r). Choose your preferred unit for each input — you can mix units freely (e.g. some in inches and others in feet). Pick the area unit for the answer (m², ft², in², cm², acres, hectares and more). The result, the formula and the breakdown appear instantly.
How the calculation works
A = πr². A circle with r = 1 m has area π ≈ 3.142 m².
Worked example
A circle with r = 1 m has area π ≈ 3.142 m².
Assumptions and limitations
- Length unit conversions use exact factors (1 in = 0.0254 m, 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 mi = 1609.344 m).
- Area outputs convert from m² using exact factors squared.
Frequently asked questions
What is the area of a circle?
A = πr², where r is the radius. For r = 1, A = π ≈ 3.14159.
How do I find the area from the diameter?
Halve the diameter to get the radius: A = π·(d/2)² = πd²/4.
What units can I use in this circle calculator?
You can enter dimensions in millimetres (mm), centimetres (cm), metres (m), kilometres (km), inches (in), feet (ft), yards (yd) or miles (mi) — and you can mix units freely between fields. The result can be shown in any area unit: mm², cm², m², km², in², ft², yd², mi², acres or hectares.
How accurate is this circle area calculator?
Calculations use the exact closed-form formula (A = πr²) with full double-precision arithmetic, so the answer is as accurate as your inputs. Length conversions use exact SI/Imperial factors (1 in = 0.0254 m, 1 ft = 0.3048 m).
Is this circle area calculator free to use?
Yes — every calculator on Korvaix is completely free, with no sign-up, no ads in the result, and no usage limit. You can run as many calculations as you need.
What is the difference between area and perimeter?
Area is the amount of flat space inside a 2D shape (square units like m² or ft²). Perimeter is the total length of the boundary around it (linear units like m or ft). Use area for paint, flooring, grass seed and tiling; use perimeter for skirting, fencing and edging.
Where else can I find area calculators on Korvaix?
Visit our Area Calculator hub for the full list of 12 common 2D shapes. Each shape has its own dedicated page with the formula, worked example and unit-by-unit breakdown.
Related calculators
- Annulus Area Calculator — Enter the outer and inner radii. The calculator returns A = π·(R² − r²).
- Area Calculator — Pick a 2D shape — circle, square, rectangle, triangle, trapezium, parallelogram, ellipse, regular polygon, rhombus, sector, annulus or kite
- Capsule Surface Area Calculator — Enter the radius and the straight middle length. The calculator returns 4πr² for the two rounded ends plus 2πrh for the cylindrical side.
- Circular Sector Area Calculator — Enter the radius and the angle in degrees. The calculator returns the sector's area: A = ½·r²·θ (with θ converted to radians).
- Cone Surface Area Calculator — Enter the base radius and height. The calculator splits the answer into the base (πr²) and the lateral surface (πr·s), where s = √(r²+h²) is
- Conical Frustum Surface Area Calculator — Enter the top radius, bottom radius and height. The calculator returns lateral area π(r₁+r₂)·s plus the two end discs.
Related guides
- Surface Area Formulas for Common 3D Shapes — A clear, plain-English guide to surface area formulas for sphere, cube, cone, cylinder, rectangular tank, capsule, spherical cap, conical frustum, ellipsoid and
Sources and references
Standard geometric area formula. SI and Imperial unit conversions per the International System of Units (SI) and NIST.
Last updated
Last reviewed: 2026-04-21.