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.
For your convenience, this page also includes 4 mini circle measurement conversion calculators and 2 tape measurement conversion calculators (feet, inches to decimal number, and decimal number to feet, inches).
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 (like perimeter), 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.
If you just need to quickly calculate the answer to a specific problem, use the appropriate circle solver below to find the answer.
Or, if you need to convert a tape measurement into to a decimal number, or visa versa, use the appropriate conversion calculator below.
Otherwise, if you would like to find all 3 missing dimensions at once, control the rounding, and see the formulas and the work, scroll down and use the full Circle Calculator.
With that, let's use the Circle Calculator to find three missing measurements from one known measurement.

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