Circle Area & Circumference Calculator

How to calculate a circle

1. Choose Radius or Diameter, select the input unit and enter your measurement. Radius is center to edge; diameter is edge to edge through the center.
2. Read the area and circumference. Open More options to change result units. Area uses the square of the chosen unit, while circumference uses its length unit.
3. Changing measurement type or input unit keeps your number and changes its meaning; it does not convert the entry. Copy saves the current interpretation and answers together.

Radius, area and circumference

Diameter is twice the radius: d = 2r. If you measure across the entire circle through its center, choose Diameter. Treating that value as a radius makes the calculated area four times too large.

Area is the space inside the circle: A = πr², or A = πd² ÷ 4. It is measured in square units such as cm², m², in² or ft².

Circumference is the distance around the edge: C = 2πr = πd. It is a length, so it uses cm, m, in or ft rather than squared units.

The calculator uses Math.PI and rounds only the displayed calculated values to up to eight significant digits. About marks positive pi-based answers. Zero gives zero area and circumference without an approximation qualifier.

Worked circle examples

Radius 5 m gives area about 78.539816 m² and circumference about 31.415927 m. Diameter 10 m describes the same circle and gives the same answers.

Diameter 40 cm, with result units set to meters, gives area about 0.12566371 m² and circumference about 1.2566371 m. Area conversion uses a squared factor.

Radius 12 in is radius 1 ft. With result units set to feet, the area is about 3.1415927 ft² and circumference about 6.2831853 ft.

A two-dimensional circle

Enter a nonnegative decimal measurement up to 1,000,000,000 in its selected unit, with up to six decimal places. A point or comma can mark the decimal, but grouping marks, fractions and compound feet-and-inches entries are not supported.

This tool starts from radius or diameter. It does not solve backward from area or circumference, calculate sectors, rings, ellipses or volumes, or estimate material quantities and costs. Display digits are not a guarantee of measurement accuracy.

Your entries

The arithmetic runs in your browser. This tool adds no input upload, saved record or input-bearing URL. Existing website consent, analytics and network services remain separate.