Circle Diameter, Radius & Circumference Calculator
Know one measurement of a circle? Find the other two and convert all three between inches, feet, yards, millimeters, centimeters, and meters.
Enter One Measurement
A diameter goes straight across the circle through its center. A radius runs from the center to the edge. Circumference is the distance around the edge.
Enter a positive decimal number. Example: 12.5.
Your Results
The outline is the circumference. The dashed line is the diameter; the shorter line is the radius.
Diameter
Radius
Circumference
Enter a measurement and calculate. Results are lengths in the unit you choose, with the original unit shown below each value.
How to Use the Circle Calculator
Choose the measurement you have, enter its number and unit, then choose the unit you want for the results. The calculator works from a diameter, radius, or circumference; you only need one. All three results use the same selected output unit, and the smaller line under each result shows its value in your original unit.
Measure across the widest part through the center for diameter. If you wrap a tape or string around a round object, that is circumference. Keep the tape level and measure the shape you actually need: the outside edge of a planter and the inside opening of a pipe have different diameters.
Diameter, Radius, and Circumference Formulas
Diameter = 2 × radius = circumference ÷ π
Radius = diameter ÷ 2
Circumference = π × diameter = 2 × π × radius
π (pi) is about 3.14159. The calculator uses JavaScript’s full Math.PI value, then rounds only what it displays. Converting units changes the length unit; it does not change the shape.
Example: A 12-Inch Diameter
A 12-inch circle has a 6-inch radius and a circumference of about 37.6991 inches. Choose centimeters as the output unit and you get a 30.48 cm diameter, 15.24 cm radius, and about 95.7557 cm circumference. Those are the sample settings above.
Which Units Can I Use?
Input and output units can each be inches (in), feet (ft), yards (yd), millimeters (mm), centimeters (cm), or meters (m). The conversions use 1 inch = exactly 25.4 mm, 1 foot = 12 inches, 1 yard = 3 feet, and 1 meter = 1,000 mm. The foot and yard here are the standard international units, which are suitable for ordinary DIY measurements.
Put the Diameter to Work
If you measured the distance around a round post hole or footing, convert circumference to diameter here, then use the diameter in the concrete bag calculator with its round or post-hole option. For a circular planting bed, convert your measurement to feet before entering its diameter in the mulch calculator. Circumference can also help estimate an edging length, but allow extra for overlaps, cuts, and the actual path you lay out. Find other tools in the DIY calculators directory.
Measurement Notes and Sources
This assumes a true circle. Irregular beds, out-of-round objects, and measurements taken at different heights may need several measurements or a different method. Display rounding does not make a rough tape measurement precise. The definitions and circle formula follow NIST’s circumference guide; the exact inch relationship follows NIST’s length guide.