Circle value calculator
Pi Calculator
Enter one value—radius, diameter, circumference, or area—and calculate the remaining circle values in the same unit.
Input
Use one value at a time. When you edit one field, the other value fields clear automatically.
Results keep the unit you provide. Read area as the squared version of that unit.
Results
Formula guide and usage notes
How to Use the Pi Calculator Correctly
If you know one circle value—radius, diameter, circumference, or area—you can calculate the remaining values in the same unit. The unit field is a display label, not a unit converter, so treat all entered values as the same length unit.
What you can enter
The calculator supports four starting points. When you type in one field, the other input fields are cleared so you do not accidentally calculate from mixed sources.
The distance from the center of the circle to its edge. It is the core value used by the formulas.
The full width across the circle. Radius is half of diameter.
The total length around the circle. Use it for edges, rims, or curved outlines.
The flat space covered by the circle. Read the result as the square of the selected unit.
How to use it
- Enter one known value: radius, diameter, circumference, or area.
- If needed, type a display unit such as in, ft, cm, or m in the unit field.
- Press Calculate to see the input basis, radius, diameter, circumference, and area.
- Copy a single result or use Copy all to copy the full set of values.
Formulas used
All calculations use π = Math.PI. Displayed results may be rounded to a readable number of digits.
d = 2rr = d ÷ 2C = 2πrA = πr²r = C ÷ 2πr = √(A ÷ π)
Examples
Entering 5 as the radius returns diameter 10 in, circumference 31.415927 in, and area 78.539816 in².
If you only know the diameter, the radius is calculated as 5 in and the remaining values are filled from that basis.
When area is entered, the calculator finds radius with √(A ÷ π), then calculates circumference and diameter.
Changing the unit label changes the text shown with the result. It does not convert inches to centimeters or centimeters to inches.
Important notes
- Enter only positive numbers. Negative numbers, zero, and empty values cannot be used as the calculation basis.
- The unit field is only a label. Mixing units in your input will produce values that are off by the same mismatch.
- For fabrication, construction, or design work, add material thickness, cutting allowance, and measurement tolerance separately.
- On-screen decimals are formatted for readability. If you need more significant digits, copy the result and verify the precision you need.
Frequently Asked Questions
Does the unit field convert values automatically?
No. The unit field is a display label added to the results. Enter radius, diameter, and circumference in the same length unit; area is shown as the square of that unit.
Can I calculate from diameter only?
Yes. When you enter diameter, the calculator derives radius as half of the diameter, then calculates circumference and area from that radius.
Which value of π does the calculator use?
It uses the browser Math.PI value. Results are formatted for readability, so use appropriate precision and tolerance checks for fabrication or design work.
Can it work backward from circumference or area?
Yes. From circumference it uses r = C ÷ 2π, and from area it uses r = √(A ÷ π), then calculates the remaining circle values.