Circle Calculator
Enter any one of the radius, diameter, circumference and area to find the other three.
Circle formulas
- Diameter = radius × 2
- Circumference = diameter × π = radius × 2 × π
- Area = radius × radius × π
To work back from the circumference, radius = circumference ÷ (2 × π). To work back from the area, radius = √(area ÷ π).
Pi (π) is an endless non-repeating number. This calculator uses an approximation of π to 60 decimal places. The 60 digits are the digits of π used in the calculation; they do not mean the result is accurate to 60 digits. Results are shown with 7 significant digits, and the high-precision copy has 15 significant digits (both are far coarser than the error of the approximation of π). Values that involve π (the circumference and the area) are "rounded values" that differ slightly from the true value, and the page labels them "(rounded value)". Converting between the radius and the diameter does not use π, so it is exact.
Examples
- Radius 5 cm: diameter 10 cm, circumference about 31.41593 cm, area about 78.53982 cm²
- Diameter 10 cm: radius 5 cm (the same circle)
- Circumference 100 cm: radius about 15.91549 cm, diameter about 31.83099 cm, area about 795.7747 cm²
- Area 100 cm²: radius about 5.641896 cm, diameter about 11.28379 cm, circumference about 35.44908 cm
Length units and area units
The length (radius, diameter, circumference) is shown in the unit you choose (mm, cm, m, km, inches, feet or yards), and the area in the square of that unit (such as cm²). When you enter an area, enter it in the square of the chosen unit. For example, when the unit is cm, the area is in cm². To convert between area units such as square meters, square feet or acres, use the Area Converter.
Notes
- Enter a number greater than 0, with up to 9 digits before the decimal point and 6 decimal places. Use a period as the decimal separator; commas are accepted only as thousands separators (1,234.5).
- Results are shown with 7 significant digits (very small or large values use scientific notation). You can copy the value as displayed, or with 15 significant digits. The kind of value copied is shown under the buttons.
- The results can differ slightly from a calculation that uses 3.14 for π.
- To convert lengths, use the Length Converter; for volumes, the Volume Converter.
Frequently asked questions
Should I use 3.14 for pi?
This calculator uses π to 60 decimal places, which is more accurate than 3.14. If a school problem says to use 3.14, the answer will be slightly different, so do that calculation separately.
Is finding the radius from the area also accurate?
The square root is approximated to 40 decimal places, which does not affect the displayed 7 digits or the copied 15 digits.
I mix up the radius and the diameter
The radius is the distance from the center to the edge. The diameter is the distance across the circle through the center, which is twice the radius. Choose which one you know with the buttons at the top.