Circle Calculator to Calculate
Area, Circumference, Diameter, Radius

[ Skip to Calculator ] Calcy Pointing Down

The Circle Calculator on this page will calculate three unknown measurements from one known measurement.

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.

Common Circle Measurements

Here is brief description of each of the four common circle measurements.

Circle Diagram 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.
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.

Mini Quick Calcs

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.

Help Convert Feet and Inches to Decimal Number
Feet: Inches:-/
Decimal feet: Decimal inches:
Help Circle Area
A =
Help Circumference
C =
Help Diameter
D =
Help Radius
r =
* Area (a) is always in squared units, e.g., area of 6 = 6 square inches, or 6 square cm, etc.
Help Convert Decimal Number Back to Feet and Inches
Decimal: Feet Inches | Max denominator:
feet, -/ inches

With that, let's use the Circle Calculator to find three missing measurements from one known measurement.

Circle Calculator
Calcy pointing down

Instructions: Enter one known circle measurement (area, circumference, diameter, or radius) and then click the "Calculate Circle Measurements" button to get the other three.

Note that when you type a known circle measurement into an input field, all other fields and results will be cleared of their existing results.

Mouse over the blue question marks for a further explanation of each entry field or button. More in-depth explanations can be found in the glossary of terms located beneath the Circle Calculator.

calcy treasure chest of features Save Your Work!

Just one of the Treasure Chest of Bonus Features you'll enjoy when you support the Ad-Free, Member Version of this site (for as little as 58¢ per month). Watch the Video Tour to see what you're missing!

Calculator Not Working? Please try disabling Ad Block for this page, as it may be blocking the code that runs the calculator.

Help Number of decimal places to round result to:
Help Area (A), as in number of square units:
Help Circumference (C):
Help Diameter (D):
Help Radius (r):
Please grade my work and help me improve:
If you liked this page, please share it, or give it a Like, +1, Tweet, or Pin.
calcy image pointing right

Algebra Calculator Image

Circle Calculator Glossary of Terms

Calcy magnifying glass

Check Out My Other Super
Online Math Calculators
To Help You To
Solve and Learn ...
Super Calcy

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.

Math Tutors

[ Return to Top of Calculator ] [ Return to Top of Page ] Calcy pointing up

> > Circle Calculator

+1 Page Site

Protected by Copyscape Online Plagiarism Detection
Calcy with money image
Top Ten Home
Money Calculators

Follow me on any of the social media sites below and be among the first to get a sneak peek at the newest and coolest calculators that are being added or updated each month.

Monthly What's New Email Update!

Who knows if I will show up in your next search. These monthly updates will insure you'll always know what I've been up to and where you can find me!

Terms, Privacy, and Consent * (all)
Online Pocket Calc

Online math problem solver image
Find a Local Math Tutor Today