Diameter to Radius Calculator
This calculator converts between a circle's diameter and its radius, and works out the circumference and area at the same time, so you get all four key measurements from one figure. Diameter and radius are easy to mix up on a spec sheet, tyre sidewall or product listing, and getting the wrong one can throw out a calculation for pipe fittings, wheel clearances, garden beds, pools or manufacturing drawings. To use it, choose whether you are converting diameter to radius or radius to diameter, enter your measurement, and select the unit - millimetres, centimetres, metres, inches or feet. The calculator instantly returns the radius (centre to edge), the diameter (edge to edge through the centre), the circumference (distance around the circle) and the area (space inside the circle), all shown in the unit you entered. A full breakdown and the exact formulas used are listed below the results so you can see the working. Because the radius is always exactly half the diameter, the conversion carries no rounding error, though circumference and area figures are rounded slightly for display using full-precision pi. If you are working to tight tolerances, such as pipe fittings or structural components, treat the results as a helpful working figure and confirm critical measurements with a physical check or a qualified tradesperson.
1. Enter Your Measurement
2. What You Get
Enter the diameter or radius of any circle and this tool converts it to the other measurement straight away. It also works out the circumference (distance around the circle) and the area (space inside the circle) using the same unit you entered.
This is useful for pipe sizing, wheel and tyre measurements, round garden beds, pizza sizes, or any job where a spec sheet gives you one measurement and you need the other.