Geometry
Area from the diameter
Equivalent to halving first. Use whichever matches the measurement you actually have.
The Formula
A = pi d^2 / 4
Equivalent to halving first. Use whichever matches the measurement you actually have.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| d | Diameter | length | Twice the radius. This is the quantity usually printed on packaging. |
What Each Symbol Means
dlength
Diameter
Twice the radius. This is the quantity usually printed on packaging.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate the area of a circle 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 Area Of A Circle, filed under Math. Every number the guide produces comes from substituting into this one line.