Should I use the radius or the diameter?

Whichever you measured. The radius is the distance from the centre to the edge; the diameter is the full width across, and it is what a tape measure gives you. The switch beside the field takes either, halves a diameter before it goes into the formula, and says so on the formula line: π r², r = d/2. A radius of 5 and a diameter of 10 both give 78.5398. Switching the measure doubles or halves the current number to preserve the same circle.

What is the formula for the area of a circle?

Area = π r², where r is the radius. With a diameter, r = d / 2, so the same formula reads π (d / 2)². The working row shows the numbers in place, for example π × 5² = 78.5398163397, so you can check the arithmetic yourself.

Can I get the area in square feet if I measured in centimetres?

Yes. Choose the unit you measured in, and the rows under the answer give the same area in six more units. Measured in centimetres, that list includes square metres, square inches, square feet and square yards, and the conversions use exact definitions: an inch is exactly 2.54 cm, so a square inch is exactly 6.4516 square centimetres.

What about part of a circle, or a ring?

For a slice from the centre, switch the shape to Sector and give the angle in degrees: 90 degrees is a quarter of the circle, 360 is all of it. For a ring, work out the outer circle and the inner circle and subtract; the page does not do the subtraction for you.

How precise is the answer?

Browser arithmetic rounds the formula and conversions. The stage shows up to six significant figures and the rows up to twelve, including scientific notation. Measured dimensions carry their own error. Extremely small or large derived values are refused.

What does the drawing show?

The labelled drawing identifies the dimensions used by the formula; it is schematic rather than to scale. The working and conversion rows contain the numeric result. Saved inputs restore locally without overwriting their original record on load.

Circle Area Calculator

Type the radius or the diameter of a circle and read its area at once, with the formula π r² shown and the working written out. Sides in millimetres, centimetres, metres, inches or feet, and the answer listed in six more units underneath, from square inches to acres.

Rectangle · l × w96cm²9,600 mm² · 0.0096 m² · 14.88 in²

Flat

Solid, surface area

Length (cm)

Width (cm)

Unit

Length times width. Any two sides that meet at a corner. The drawing is schematic. Choose the measured unit before typing; changing it sets the meaning of the numbers.

Result summary

Rectangle, length 12 cm, width 8 cm
96 cm²
12 × 8 = 96
9600 mm²
0.0096 m²
14.8800297601 in²
0.10333354 ft²
0.0114815044445 yd²
0.00000237221166208 ac

The workingarea, in cm²
12 × 8 = 96
Square millimetresmm²
9,600
Square metresm²
0.0096
Square inchesin²
14.8800297601
Square feetft²
0.10333354
Square yardsyd²
0.0114815044445
Acresac
0.00000237221166208

The formula is on the stage, and the working is the first row. Every answer here is one textbook formula with your numbers put in, printed so you can check it: a circle is π r², a three-sided triangle is Heron's formula, a regular polygon is n s² / (4 tan(π/n)). The solids give surface area for a closed shape, both ends of a cylinder and the base of a cone included. Conversions use exact definitions, so a square foot is 0.09290304 m² and 43,560 of them make an acre.

Common questions

Should I use the radius or the diameter?: Whichever you measured. The radius is the distance from the centre to the edge; the diameter is the full width across, and it is what a tape measure gives you. The switch beside the field takes either, halves a diameter before it goes into the formula, and says so on the formula line: π r², r = d/2. A radius of 5 and a diameter of 10 both give 78.5398. Switching the measure doubles or halves the current number to preserve the same circle.
What is the formula for the area of a circle?: Area = π r², where r is the radius. With a diameter, r = d / 2, so the same formula reads π (d / 2)². The working row shows the numbers in place, for example π × 5² = 78.5398163397, so you can check the arithmetic yourself.
Can I get the area in square feet if I measured in centimetres?: Yes. Choose the unit you measured in, and the rows under the answer give the same area in six more units. Measured in centimetres, that list includes square metres, square inches, square feet and square yards, and the conversions use exact definitions: an inch is exactly 2.54 cm, so a square inch is exactly 6.4516 square centimetres.
What about part of a circle, or a ring?: For a slice from the centre, switch the shape to Sector and give the angle in degrees: 90 degrees is a quarter of the circle, 360 is all of it. For a ring, work out the outer circle and the inner circle and subtract; the page does not do the subtraction for you.
How precise is the answer?: Browser arithmetic rounds the formula and conversions. The stage shows up to six significant figures and the rows up to twelve, including scientific notation. Measured dimensions carry their own error. Extremely small or large derived values are refused.
What does the drawing show?: The labelled drawing identifies the dimensions used by the formula; it is schematic rather than to scale. The working and conversion rows contain the numeric result. Saved inputs restore locally without overwriting their original record on load.

Supported formulas use rounded browser numbers and exact unit definitions. Results show up to twelve significant figures; measurements carry their own error.