How to use the minimum slab thickness calculator
Slab thickness has to clear two gates: serviceability (deflection) and flexural strength. ACI 318-19 Table 7.3.1.1 gives minimum thicknesses for nonprestressed one-way slabs that let you skip a deflection calculation: L/20 simply supported, L/24 one end continuous, L/28 both ends continuous, and L/10 for a cantilever. For yield strength other than 60,000 psi the table value is multiplied by (0.4 + fy/100,000).
The second gate is strength. This tool takes the thickness, adds self-weight at 150 pcf normalweight concrete, factors the loads as 1.2D + 1.6L, and applies the ACI approximate moment coefficients (1/8 simple, 1/10 one end continuous, 1/11 both ends continuous, 1/2 cantilever). With an economical reinforcement ratio of 0.0075 and the flexural strength reduction factor φ = 0.90, it back-solves the effective depth and adds 1.25 in for cover plus half a bar diameter. Thicknesses round up to the nearest 0.5 in.
Typical residential and office slabs are governed by the deflection table, while heavily loaded warehouse or parking slabs can be governed by flexure. Prestressed members, two-way slabs, and slabs carrying deflection-sensitive finishes fall outside the table and need a direct deflection check. Treat the output as a preliminary sizing aid and have a licensed structural engineer confirm the final thickness and reinforcement.
Frequently Asked Questions
Only for one-way slabs not supporting or attached to partitions and finishes likely to be damaged by large deflections, per ACI 318 Table 7.3.1.1. Otherwise a direct deflection check including long-term creep and shrinkage is required.
Use the center-to-center distance between supports; for a cantilever, enter the clear projection from the fixed end. Two-way slabs follow ACI 318 Section 8.3.1.2 instead of this table.