A self standing bag having two side wall sheets and a bottom sheet. To flatten the bag, the bottom sheet is sandwiched between the two side wall sheets and symmetrically folded along a line parallel to its bottom edge, separating the bag into a four-layer part and a two-layer part. In the two-layer part, the bag includes first sloped sealing lines extending between a first two points on the top edges of the side wall sheets and spaced from the vertical side wall edges by a first distance, to a second two points spaced from the vertical edges of the side wall sheets by a second distance less than the first distance. The four-layer part includes second sloped sealing lines extending between the second two points and a third two points on the bottom edges of the side wall sheets.