This free online geometry calculator will take one known circle measurement (area, circumference, diameter, or radius) and calculate the other three.
Plus, unlike other online circle calculators, this calculator will show its work and give a detailed, stepbystep explanation of the formulas and sequence used to arrive at each result.
If you are unfamiliar with the common measurements used in plane circle geometry, please read the following brief explanation before using the calculator.
Here is brief description of each of the four common circle measurements.
Area All of the area within the boundary of the circle, expressed as a square of the unit of measurement (square mm, square feet, etc.). The area can be found by multiplying PI (3.14159) times the radius times the radius again. 

Circumference
The distance around the outward boundary of a circle, expressed as a linear unit of measurement (millimeters, inches, etc.). The circumference can be found by multiplying PI (3.14159) times the diameter. 

Diameter The length of a straight line that intersects the center point of the circle and reaches to each side of the boundary. Diameter can be found by multiplying the radius by two, or by dividing the circumference by PI (3.14159). 

Radius
The length of a straight line drawn from the circle boundary to the center point of the circle. The radius can be found by dividing the diameter by two, or by finding the square root of the quotient resulting from dividing the area by PI (3.14159). 
Note that the formulas used for finding each measurement will be listed in the calculated results.
With that, let's use the Circle Calculator to find three missing measurements from one known measurement.
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Number of decimal places to round result to: Select how many decimal places you would like the result rounded to. Note that you can change the number of places before or after calculating the circle measurement. Also note that the circle calculator will always display the raw results (out to 14 places) in the steps that will appear beneath the calculator.
Area (A): If you know the area of the circle and would like to calculate the circumference, diameter, and radius, enter the area on this line. Note that the area of a circle is expressed as the square of the particular unit of measurement being used (square mm, square inches, etc.).
Circumference (C): If you know the circumference of the circle and would like to calculate the area, diameter, and radius, enter the circumference on this line.
Diameter (D): If you know the diameter of the circle and would like to calculate the area, circumference, and radius, enter the diameter in the field on this line.
Radius (r): If you know the radius of the circle and would like to calculate the area, circumference, and diameter, enter the radius in the field on this line.
Calculate Circle Measurements button: Clicking this button will calculate the missing circle measurements and post the results to righthand fields in the upper portion of the calculator, and then display the steps and formulas used in the area just below this button.
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