What is the area of a circle with diameter 18 mm?

Here is the answer to questions like: how to find the area of a circle with diameter 18 mm?

Circle Calculator

Inputs:

Radius:		or
Diameter		or
Circumference:
Unit:

The area of a circle with diameter 18 is 254.5
Formulae:
	A = πr² = π(d2)² A = C²4π π = 3.1415 A = area C = circumference or perimeter r = radius, d = diameter

Solution Steps:

Area of a circle in terms of radius:

Area = π·r² = 3.14·9² = 254.5 square mm^(*)

Area of a circle in terms of diameter:

Area = π·(d2)² = 3.14·(182)² = 3.14·(9)² = 254.5 square mm^(*)

Area of a circle in terms of circumference:

Area = C²4π = 56.55²4π = 3197.9(4·3.14) = 3197.912.56 = 254.5 square mm^(*)

^(*) 254.46900494077 mm, exactly or limited to the precision of this calculator (13 decimal places).

Note: for simplicity, the operations above were rounded to 2 decimal places and π was rounded to 3.14.

Result in other units of area:

A circle of radius = 9 or diameter = 18 or circumference = 56.55 mm has an area of:

2.545 × 10^-10 square kilometers (km²)
0.0002545 square meters (m²)
2.545 square centimeters (cm²)
254.5 square millimeters (mm²)
9.8263 × 10^-11 square miles (mi²)
0.000304379 square yards (yd²)
0.00273942 square feet (ft²)
0.394476 square inches (in²)

Use the this circle area calculator below to find the area of a circle given its diameter, or other parameters. To calculate the area, you just need to enter a positive numeric value in one of the 3 fields of the calculator. You can also see at the bottom of the calculator, the step-by-step solution.

Formula for area of a circle

Here a three ways to find the area of a circle (formulas):

Circle area formula in terms of radius

A = πr²

Circle area formula in terms of diameter

A = π(d2)²

Circle area formula in terms of circumference

A = C²4π

See below some definitions related to the formulas:

Circumference

Circumference is the linear distance around the circle edge.

Radius

The radius of a circle is any of the line segments from its center to its perimeter. The radius is half the diameter or r = d2.

Diameter

The diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. The diameter is twice the radius or d = 2·r.

The Greek letter π

π represents the number Pi which is defined as the ratio of the circumference of a circle to its diameter or π = Cd . For simplicity, you can use Pi = 3.14 or Pi = 3.1415. Pi is an irrational number. The first 100 digits of Pi are: 3.1415926535 8979323846 2643383279 5028841971 6939937510 5820974944 5923078164 0628620899 8628034825 3421170679 ...

Note:

If you input the radius in centimeters, you will get the answer in square centimeters (cm²), if in inches, will get the answer in square inches (in²) and so on ...

Circumference is often misspelled as circunference.

Popular Circle Area Calculations

Calculate area for different circle dimensions:

Related Calculators

Circle Circumference Calculator — perimeter from radius or diameter
Cube Root Calculator — find side length from volume
Cube (n³) Calculator — useful for volume of a cube
Decimal to Percent — ratios and proportions