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, step-by-step 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.
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.
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.
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).
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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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 right-hand fields in the upper portion of the calculator, and then display the steps and formulas used in the area just below this button.