Circle Measurements Calculator
This calculator works out every key measurement of a circle, radius, diameter, circumference and area, from just one value you already know, which is useful whenever you are working from a plan, a pipe spec, a garden design or a maths problem that only gives you a single figure to start with. You choose which measurement you are starting from, enter its value in whatever unit you are using, cm, m, mm, inches or feet, and pick a unit label purely for display. The calculator instantly returns all four measurements together: radius, diameter, circumference and area, each shown in your chosen unit, with area given in square units. It uses the exact value of pi (3.14159265358979) rather than a rounded approximation, so results stay accurate to four decimal places. A worked example below the results spells out the actual arithmetic behind your answer, so you can see how each figure was derived rather than just trusting the output. This makes it handy for checking maths homework, sizing a circular garden bed, pool or tabletop, working out how much edging, fencing or trim you need around a round feature, or converting between measurements on a building plan. Because all four measurements are linked through pi, you only ever need to know one of them; the calculator handles the rest of the geometry for you instantly as you type.
1. Enter a Known Value
2. Circle Formulas Used
d = 2r
C = 2πr
A = πr²
r = C / (2π)
r = d / 2
r = √(A / π)