Formula Torus Surface area Radius
$$A ~=~ 4\pi^2 \, r \, R$$ $$A ~=~ 4\pi^2 \, r \, R$$ $$r ~=~ \frac{A}{4\pi^2 \, R}$$ $$R ~=~ \frac{A}{4\pi^2 \, r}$$
Surface area
$$ A $$ Unit $$ \mathrm{m}^2 $$ Surface area of a torus ("donut").
Radius of the cross section
$$ r $$ Unit $$ \mathrm{m} $$ Radius of the cross-sectional area. That is, the radius of the area when the torus is cut into two "half-tubes".
Radius
$$ R $$ Unit $$ \mathrm{m} $$ Radius from the center of the hole of the torus to half of the tube.
Pi
$$ \pi $$ Unit $$ - $$ Pi is a mathematical constant and has the value: \( \pi ~=~ 3.1415926... \)