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The slab's clear span length.
For a cantilever, this is the cantilever's projection length.
How the slab is supported at each end.
'Continuous' means the slab continues over the support into an adjacent span, rather than ending there.
Minimum slab thickness
0.696 ft
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
- 8.36 in
- Span/depth divisor used
- 28
They open the calculator with your figures already in it
One-Way Slab Minimum Thickness Calculator (ACI 318): 0.6964 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ACI 318 Table 7.3.1.1 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
Inputs used
- Clear Span
- 19.5 ft
- Support Condition
- Both ends continuous
Intermediate steps
- Equivalent in inches
- 8.36 in
- Span/depth divisor used
- 28
Confidence note: 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.
What this calculation does not cover
- This is a deflection-control ratio, not a design. It says nothing about whether the slab can carry its load: bending and shear capacity, reinforcement area, bar spacing and cover are separate calculations, and on a heavily loaded floor it is strength that ends up setting the depth, not this ratio. Fire-resistance rating and the depth needed to bury conduit or drainage can also demand more than the figure here.
- The ACI table behind this applies only to slabs that do not support, and are not attached to, partitions or other construction likely to be damaged by large deflections. A full-height block wall landing mid-panel, a rigid partition head, bonded stone or glazing on this slab puts you outside the table, and the explicit deflection check it exists to excuse becomes mandatory again.
- The divisors are written for solid, nonprestressed one-way slabs in normal-weight concrete with Grade 60 reinforcement, and nothing is adjusted when you fall outside that. Lightweight concrete and other bar grades carry modification factors in ACI 318 that this calculator does not apply, and post-tensioned slabs, two-way slabs and flat plates, ribbed and waffle floors, and composite metal-deck slabs are not covered by the table at all.
- It sizes one span with one support condition. On a continuous floor the deepest bay sets the whole slab, and this will not find that bay for you — the edge bay, continuous at one end only, is usually the one that governs. The answer is a bare minimum rather than a pour dimension, so round it up to a thickness you would actually build.
- This is ACI 318's number. Eurocode 2, CSA A23.3 and AS 3600 each control slab deflection through their own provisions and will not produce the same depth, so a thickness taken from here is an ACI figure and is worth labelling as one on any drawing that crosses a border.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- ACI 318 Table 7.3.1.1 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
Which documents these citations point at
Standards referenced: ACI 318 (American Concrete Institute, United States).
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