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One-Way Slab Minimum Thickness Calculator (ACI 318)

Get the minimum slab thickness ACI 318 allows without an explicit deflection calculation, based on span and support condition.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
19.69 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Minimum slab thickness

0.7 ft

Check your inputs

This is the code's deflection-exempt minimum thickness for normal-weight concrete with Grade 60 rebar — it doesn't replace a strength (moment/shear) design, and lightweight concrete or other rebar grades require an adjustment factor per ACI 318.

Equivalent in inches
8.44 in
Span/depth divisor used
28

For the dimensions entered, expect a minimum slab thickness of 0.21. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • ACI 318 Table 9.3.1.1 minimum thickness for solid one-way slabs not supporting or attached to partitions likely to be damaged by deflection: simply supported = span/20, one end continuous = span/24, both ends continuous = span/28, cantilever = span/10

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Frequently asked questions

Can I use a thinner slab than this minimum?
Only if you perform an explicit deflection calculation showing the actual deflection stays within code limits — this table exists specifically so designers can skip that calculation when they meet these minimums.
Why does a cantilever need a much thicker slab per unit span?
A cantilever has no support at its free end, so it deflects far more than a slab supported (or continuous) at both ends for the same span — hence the much more conservative span/10 ratio versus span/28.
Does this account for the actual load on the slab?
No — this table is a deflection-control shortcut based only on span and support condition, independent of load. The slab's thickness must still separately satisfy strength requirements for its actual applied loads.