Reusable modular formwork for making columns, comprising: at least two modular elements, wherein each modular element has linear development and it is made from plastic material, shaped as part of the perimeter of a column, said modular element with linear development comprising at least one wall with a smooth internal side as bottom of the formwork and with a not flat outer side, said not flat outer side further comprising edge ribs (Mo) and intermediate ribs (Mn), wherein the edge ribs (Mo) of the at least one wall present coupling surfaces (Me) for corresponding modular elements with linear development, so as to make up a section or closed portion of said reusable modular formwork and wherein said modular element with linear development has holes (Mf) on the edge ribs (Mo), preferably circular in shape with two or more diametrically opposed grooves, and wherein said holes (Mf) are located along the edge ribs (Mo) so that, when various modular elements with linear development are coupled or aligned, said holes (Mf) in the various modular elements with linear development are aligned along said coupling surfaces (Me); at least two closing keys inserted through said holes (Mf), for fastening said various modular elements with linear development; each of said modular elements being composed of a semicircular plastic panel comprising on the convex not flat outer side said intermediate ribs (Mn) and said edge ribs (Mo), wherein the edge ribs (Mo) are placed along the perimeter of the panel of the modular element and each of said edge ribs (Mo) having greater thickness (S1, S2) and height (H1, H2) than the intermediate ribs (Mn), and wherein vertical edge ribs (MoV) are disposed along the vertical edge of the panel and horizontal edge ribs (MoH) are disposed along the horizontal edge of the panel; characterized in that the height of the vertical edge ribs (MoV) is greater than the height of the horizontal edge ribs (MoH). at least two modular elements, wherein each modular element has linear development and it is made from plastic material, shaped as part of the perimeter of a column, said modular element with linear development comprising at least one wall with a smooth internal side as bottom of the formwork and with a not flat outer side, said not flat outer side further comprising edge ribs (Mo) and intermediate ribs (Mn), wherein the edge ribs (Mo) of the at least one wall present coupling surfaces (Me) for corresponding modular elements with linear development, so as to make up a section or closed portion of said reusable modular formwork and wherein said modular element with linear development has holes (Mf) on the edge ribs (Mo), preferably circular in shape with two or more diametrically opposed grooves, and wherein said holes (Mf) are located along the edge ribs (Mo) so that, when various modular elements with linear development are coupled or aligned, said holes (Mf) in the various modular elements with linear development are aligned along said coupling surfaces (Me); at least two closing keys inserted through said holes (Mf), for fastening said various modular elements with linear development; each of said modular elements being composed of a semicircular plastic panel comprising on the convex not flat outer side said intermediate ribs (Mn) and said edge ribs (Mo), wherein the edge ribs (Mo) are placed along the perimeter of the panel of the modular element and each of said edge ribs (Mo) having greater thickness (S1, S2) and height (H1, H2) than the intermediate ribs (Mn), and wherein vertical edge ribs (MoV) are disposed along the vertical edge of the panel and horizontal edge ribs (MoH) are disposed along the horizontal edge of the panel;