Formwork for a hanging beam protruding below a floor slab, which comprises a plurality of lateral panels separated from one another by lower panels, which may be affixed to the same in a height-adjustable manner. Each lower panel is composed of two semi-panels, the first semi-panel being longer than the second semi-panel, wherein each semi-panel comprises a connecting transverse edge connected to the transverse edge of the other semi-panel, wherein the two semi-panels may be removably coupled to one another to constitute the lower panel through removable attaching means, configured for their connecting transverse edges to be attached by flat contact, wherein the connecting transverse edge of the first semi-panel forms an obtuse angle with its inner face, and the connecting transverse edge of the second semi-panel forms an acute angle that is supplementary to the aforementioned obtuse angle.