What is the minimum concrete slab thickness?

• A. 75 mm
• B. 50 mm
• C. 100 mm
• D. 80 mm

Answer: (A)

The minimum thickness of a concrete slab is set by the need to carry expected loads without cracking or excessive deflection, while still being practical to form and finish. For a typical residential slab on grade, 75 mm (about 3 inches) provides enough depth for the concrete to resist bending under normal use and to accommodate finishing practices. A thinner slab, like 50 mm, would be more prone to cracking and failure under everyday loads. A thicker option, such as 100 mm, is usually reserved for exterior slabs or heavier load conditions and isn’t necessary for a standard residential interior slab. An 80 mm slab is thicker than the minimum but not the smallest acceptable thickness, so the minimum practical choice remains 75 mm.