A sheet packaging material (M, M') for producing a sealed package of a pourable food product, comprising: - at least one first crease line (65; 66) and at least one second crease line (68; 69); - at least one third crease line (63; 67) which intersects said first crease line (65; 66) in at least one first point (91; 94; 101; 104) and said second crease line (68; 69) in at least one second point (92; 93; 102; 103); - at least one fourth crease line (61; 62) transverse to said first crease line (65; 66) and to said second crease line (68; 69); - at least one first panel bounded, at least in part, by said first crease line (65; 66), said second crease line (68, 69) and said third crease line (63; 67); said first panel being adapted to define a lateral wall of said finished package once said sheet packaging material (M, M') has been folded; - at least one fifth crease line (70, 73; 74, 77), which extends between said first point (91, 94; 101, 104) and said fourth crease line (61; 62); - at least one sixth crease line (71, 72; 75, 76), which extends between said second point (92, 93; 102, 103) and said fourth crease line (61; 62); and - at least one second panel (160, 161; 170, 171) bounded by said fifth crease line (70, 73; 74, 77), said sixth crease line (71, 72; 75, 76) and a portion of said third crease line (63; 67) interposed between said first point (91; 94; 101; 104) and said second point (92, 93; 102, 103); said second panel (160, 161; 170, 171) being adapted to define at least part of a first folded flap (26; 32) of said finished package once said sheet packaging material (M, M') has been folded; said first point (91; 94; 101; 104) and said fourth crease line (61; 62) being spaced by a first distance; said second point (92, 93; 102, 103) and said fourth crease line (61; 62) being spaced by a second distance; said first crease line (65; 66) and said second crease line (68; 69) being spaced by a third distance; characterized in that H 1 + H 2 < B where: H1 is said first distance, H2 is said second distance and B is said third distance, wherein H 1 = H 2 = H where: H is the distance between said third crease line (63; 67) and said fourth crease line (61; 62), and wherein H ≥ B / 2,30. - at least one first crease line (65; 66) and at least one second crease line (68; 69); - at least one third crease line (63; 67) which intersects said first crease line (65; 66) in at least one first point (91; 94; 101; 104) and said second crease line (68; 69) in at least one second point (92; 93; 102; 103); - at least one fourth crease line (61; 62) transverse to said first crease line (65; 66) and to said second crease line (68; 69); - at least one first panel bounded, at least in part, by said first crease line (65; 66), said second crease line (68, 69) and said third crease line (63; 67); said first panel being adapted to define a lateral wall of said finished package once said sheet packaging material (M, M') has been folded; - at least one fifth crease line (70, 73; 74, 77), which extends between said first point (91, 94; 101, 104) and said fourth crease line (61; 62); - at least one sixth crease line (71, 72; 75, 76), which extends between said second point (92, 93; 102, 103) and said fourth crease line (61; 62); and - at least one second panel (160, 161; 170, 171) bounded by said fifth crease line (70, 73; 74, 77), said sixth crease line (71, 72; 75, 76) and a portion of said third crease line (63; 67) interposed between said first point (91; 94; 101; 104) and said second point (92, 93; 102, 103); said second panel (160, 161; 170, 171) being adapted to define at least part of a first folded flap (26; 32) of said finished package once said sheet packaging material (M, M') has been folded;