Area of a Circle Calculator
Free online circle area calculator — enter radius or diameter. Area A = πr² (or πd²/4). Circumference C = 2πr is shown too. Units stay consistent: feet in, square feet out.
This area of a circle calculator finds the disk’s area and the circumference from one length. Choose radius to area, or diameter to area (the tool halves the diameter). Surface area of a circle in everyday language means this 2D area — not the surface of a sphere (4πr²).
Area of a circle calculator formula
A = π r². A = π d² / 4. C = 2 π r. r = d ÷ 2. r = √(A ÷ π).
Example: radius 10 → A ≈ 314.16, C ≈ 62.83. Diameter 10 → r = 5, A ≈ 78.54. Without a calculator: 22/7 × r² is a fraction approximation; 3.14 × r² is the usual decimal shortcut.
How to use this area of a circle calculator
- Choose Radius or Diameter.
- Enter the length in any unit (inches, feet, meters).
- Read radius, area (unit²), and circumference (same linear unit).
- For two circles, run the calculator twice and subtract areas for the difference.
About this calculator
Circle area calculator formula: A = π r², with π ≈ 3.1415926535. Radius to area of circle: square the radius, multiply by π. Diameter to area: r = d ÷ 2, or A = π d² ÷ 4. Formula for area of circle calculator and area of circle without calculator: 3.14 × r × r is a hand estimate. Circumference and area of circle: C = 2 π r, shown on the same result.
How many square feet in a circle calculator: enter the radius or diameter in feet; the area is in square feet. A 10 ft radius is about 314.16 ft². The same numbers in meters give square meters. Area to circle calculator (reverse): r = √(A ÷ π), then diameter = 2r — work that by hand from the area you already have.
Area difference between two circles: π(R² − r²), or subtract two results from this page. Concentric rings (annulus) use that difference. A cylinder’s end cap is still πr²; full cylinder surface is a different tool.
Examples
Radius 10
A = π × 10² ≈ 314.16. C = 2π × 10 ≈ 62.83.
Diameter 10
r = 5. A = π × 25 ≈ 78.54. C ≈ 31.42.
Circle in square feet
A 24 ft diameter patio: r = 12 ft, A ≈ 452.39 ft².
Radius, diameter, and circumference
Radius to area and diameter to area are the same geometry. Diameter is twice the radius. The circumference and area of a circle calculator on this page returns both so you do not need a second tool for 2πr.
Two circles, spheres, and mental math
Area difference between two circles is |A₁ − A₂|. For a ring, use the larger radius and smaller radius in π(R² − r²). Surface area of a circle is this 2D result; a sphere’s surface is 4πr² on the sphere volume calculator’s related formulas.
Area of a circle without calculator: 3 × r² is a rough floor; 3.14 × r² is close enough for DIY. For homework, keep π in the answer until you are asked to decimalize.
FAQ
Circle area calculator formula?+
A = π r². If you have diameter, A = π d² / 4. Circumference is C = 2 π r. π is about 3.1416 on this page (JavaScript Math.PI).
Area of circle calculator online?+
This page is the free online tool. Enter radius or diameter; you get area and circumference. No app install required.
Area of circle calculator with radius?+
Set Known value to Radius, type r, read A = π r². Example: r = 10 → area ≈ 314.16 (square units).
How many square feet in a circle calculator?+
Use feet for the input. Radius 10 ft → about 314.16 ft². Diameter 10 ft → about 78.54 ft². The calculator does not convert units; it squares whatever you typed.
What is the area of circle calculator?+
It computes the area of a circular disk from radius or diameter, and also the circumference. It is not a sphere surface-area calculator (that is 4πr²).
How do I get area from diameter?+
Choose Diameter, enter d. Internally r = d/2, then A = π r². Same as A = π d²/4.
How do I find the area difference between two circles?+
Compute each area, then subtract: π(R² − r²) if they share a center. This form does one circle at a time.
How do I find radius from area?+
r = √(A / π). That reverse “area to circle” step is not a second input on the form — use the formula after you know A.