Geometry
Diameter and circumference
The two measurements a tape gives directly. Convert a measured diameter to a radius before using either volume or area.
The Formula
d = 2r, C = 2 x pi x r
The two measurements a tape gives directly. Convert a measured diameter to a radius before using either volume or area.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| d | Diameter | linear units | Twice the radius, the full width through the centre. |
| C | Circumference | linear units | Distance around the widest circle. |
| r | Radius | linear units | Half the diameter. |
What Each Symbol Means
dlinear units
Diameter
Twice the radius, the full width through the centre.
Clinear units
Circumference
Distance around the widest circle.
rlinear units
Radius
Half the diameter.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate the volume of a sphere page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate The Volume Of A Sphere, filed under Math. Every number the guide produces comes from substituting into this one line.