Bishop-Pellini curve is modified for calculating the riser dimensions of steel castings when the shape factor is small. The relation is given by :
    V_r/V_c=5/(L+W)⁄(T)
  where V_r is the volume of riser,
    V_c is the volume of castings,
    L, W, and T are the length width, and thickness of the castings respectively.
  Rewriting this, the following formula is obtained.
    [Written in non-displayable characters.]
  where D is the diameter of the riser,
    H is the height of the riser,
    k is the ratio of H to D.
  By using this formula, authors made a slide rule. It is useful for calculating the diameter and height of riser when the length, width and thickness of castings are given.