The body of static equipment under pressure is in many cases a cylindrical shape. More complex equipment such as destilation columns can also have on or more conical part. LPG tanks however have usually a spherical body.
The Shell Thickness calculation page is to calculate the wall thickness of a cylinder, cone and sphere under pressure without holes. The calculation does not take into account the extra stress around holes for nozzles and is therefore a basic strength calculation. Calculation codes are ASME, Dutch Rules and the EN Euronorm.
Below figure gives the an indication of the dimensions used in the calculations. The calculation also require the user to enter a stress value depending the material. The calculation page has a link to a material property page, but the values on the material pages are for reference only and are not to be used in actual calculations.