DIY Cost Tools
Circle Calculator
GEOMETRY CALCULATOR
Enter one known circle measurement to calculate its radius, diameter, circumference, and area.
How it works
Every circle measurement can be converted to radius. The calculator then applies the standard diameter, circumference, and area formulas.
Formulas
Diameter d = 2r
Circumference C = 2πr
Area A = πr²
Example
A circle with radius 5 units has diameter 10 units, circumference about 31.4159 units, and area about 78.5398 square units.
Important limits
- Results use JavaScript’s value of π and are rounded only for display.
- Area is expressed in square units; all other measurements are linear units.
- The calculator assumes a perfect circle.
Frequently asked questions
How do I find radius from circumference?
Divide circumference by 2π.
How do I find radius from area?
Divide area by π and take the square root.
Using circle measurements correctly
Start by confirming whether the known measurement is a radius or a diameter. The radius runs from the center to the edge, while the diameter crosses the entire circle and equals twice the radius. Area is reported in square units; circumference, radius, and diameter use linear units.
The calculator uses π, so rounded results will vary slightly depending on the number of decimal places displayed. For material estimates, measure the actual object in more than one direction because real patios, tabletops, and openings may not be perfectly circular. When buying edging, flooring, or fabric, add an appropriate waste allowance and use the supplier’s packaged coverage rather than relying on the geometric result alone.
Review notes
Formulas follow standard Euclidean circle geometry. Last reviewed September 30, 2026.