Circle Calculator to Calculate Area, Circumference, Diameter, Radius

Circle Calculator Sign

This 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, the calculator also includes 4 mini calculators you can access by tapping the plus (+) icon on each circle measurement line ...

  • Circle Area Calculator
  • Circumference Calculator
  • Diameter Calculator
  • Radius Calculator

... and 2 conversion calculators for converting between tape measurements (feet/inches) and decimal numbers (one button appears at the top of the calculator, and the other appears at the bottom).

If you are unfamiliar with the common measurements used in plane circle geometry, it may help to visit the Learn tab to learn about them before using the calculator.

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Circle Calculator

Find the three missing circle measurements from one known measurement: area, circumference, diameter, or radius.

Special Instructions

Learn More

Selected Data Record:

A Data Record is a set of calculator entries that are stored in your web browser's Local Storage. If a Data Record is currently selected in the "Data" tab, this line will list the name you gave to that data record. If no data record is selected, or you have no entries stored for this calculator, the line will display "None".

DataData recordData recordSelected data record: None
Convert Ft/In to DecimalConvert Feet/Inches to DecimalConvert Feet/Inches to DecimalConvert Feet/Inches to Decimal

Enter the feet, inches, and fraction of an inch, and then click the Convert to Decimal button. You can then copy the appropriate decimal and paste into the circle calculator.

Feet:
Inches:
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Feet:
Inches:

Convert Feet/Inches to Decimal:

If you need to convert a tape measurement into a decimal number, tap the button in this row to expand the converter.

Dec places:Decimal places:Number of decimal places:Number of decimal places to round result to:

Number of decimal places:

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):

Select the known value and just start typing.

Circle Area Calculator
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A =

Mini Circle Area Calculator:

Select the known circle measurement from the drop-down menu and then enter the selected measurement in decimal format. The circle's area (A) will be calculated as you type. Click the Clear button to clear both fields.

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. Enter only digits 0-9 and a decimal point (if applicable). Non-numeric characters will not compute.

Or, tap the plus (+) to expand the mini calculator if you need to find the area.

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.).

 
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Circumference (C):
Circumference Calculator
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C =

Mini Circumference Calculator:

Select the known circle measurement from the drop-down menu and then enter the selected measurement in decimal format. The circle's circumference (C) will be calculated as you type. Click the Clear button to clear both fields.

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. Enter only digits 0-9 and a decimal point (if applicable). Non-numeric characters will not compute.

Or, tap the plus (+) to expand the mini calculator if you need to find the circumference.

 
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Diameter (D):

Select the known value and just start typing.

Diameter Calculator
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D =

Mini Diameter Calculator:

Select the known circle measurement from the drop-down menu and then enter the selected measurement in decimal format. The circle's diameter (D) will be calculated as you type. Click the Clear button to clear both fields.

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. Enter only digits 0-9 and a decimal point (if applicable). Non-numeric characters will not compute.

Or, tap the plus (+) to expand the mini calculator if you need to find the diameter.

 
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Radius (r):

Select the known value and just start typing.

Radius Calculator
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d =

Mini Radius Calculator:

Select the known circle measurement from the drop-down menu and then enter the selected measurement in decimal format. The circle's radius (r) will be calculated as you type. Click the Clear button to clear both fields.

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. Enter only digits 0-9 and a decimal point (if applicable). Non-numeric characters will not compute.

Or, tap the plus (+) to expand the mini calculator if you need to find the radius.

 
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If you would like to save the current entries to the secure online database, tap or click on the Data tab, select "New Data Record", give the data record a name, then tap or click the Save button. To save changes to previously saved entries, simply tap the Save button. Please select and "Clear" any data records you no longer need.

Convert Decimal to Ft/InConvert Decimal to Feet/InchesConvert Decimal to Feet/InchesConvert Decimal to Feet/Inches

Enter the decimal number, tap Feet or Inches, select the maximum denominator, and then click the Convert to Feet/Inches button. The converter will round the fraction portion to the selected denominator and then reduce it to simplest form.

Decimal:
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Type:
Max Denom:Max Denom:Max Denominator:Maximum Denominator:
Feet:
Inches:
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Convert Feet/Inches to Decimal:

If you need to convert a decimal to a tape measurement, copy a decimal from the above results and tap the button to expand the converter.

Decimal number:

Enter the decimal number you would like to convert into a tape measurement.

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Help and Tools

Learn

Common circle measurements and how they are found.

Circle Diagram Depicting Each Measurement and Formula

Here is a brief, color-coded description of each of the four common circle measurements.

Circle DiagramArea
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.
Circle DiagramCircumference
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.
Circle DiagramDiameter
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).
Circle DiagramRadius
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 calculator results.

Back to CalculatorBack to CalculatorBack to Circle CalculatorBack up to Circle Calculator

Adjust Calculator Width:

Move the slider to left and right to adjust the calculator width. Note that the Help and Tools panel will be hidden when the calculator is too wide to fit both on the screen. Moving the slider to the left will bring the instructions and tools panel back into view.

Also note that some calculators will reformat to accommodate the screen size as you make the calculator wider or narrower. If the calculator is narrow, columns of entry rows will be converted to a vertical entry form, whereas a wider calculator will display columns of entry rows, and the entry fields will be smaller in size ... since they will not need to be "thumb friendly".

Show/Hide Popup Keypads:

Select Show or Hide to show or hide the popup keypad icons located next to numeric entry fields. These are generally only needed for mobile devices that don't have decimal points in their numeric keypads. So if you are on a desktop, you may find the calculator to be more user-friendly and less cluttered without them.

Stick/Unstick Tools:

Select Stick or Unstick to stick or unstick the help and tools panel. Selecting "Stick" will keep the panel in view while scrolling the calculator vertically. If you find that annoying, select "Unstick" to keep the panel in a stationary position.

If the tools panel becomes "Unstuck" on its own, try clicking "Unstick" and then "Stick" to re-stick the panel.