What Is a Monolithic Slab Foundation?
A concrete slab footing calculator helps you estimate the combined concrete needed for a monolithic foundation — where the floor slab and perimeter footings are poured as a single, continuous concrete unit. The slab interior is typically 4 inches thick; the edges are thickened downward to 12–18 inches to create integrated footings that support the wall loads above. “Monolithic” means poured in one pour — no cold joint between slab and footing.
Monolithic slabs are widely used in the South and Southwest where frost lines are shallow (under 12 inches) and soil is stable. They are cost-effective for garages, workshops, and slab-on-grade homes because a single pour eliminates the need for a separate footing excavation and pour. The thickened perimeter footing typically contains two horizontal runs of #4 rebar, while the slab interior uses #3 rebar at 18 inches OC or welded wire mesh. Per IRC R403.1, the minimum footing depth must extend to undisturbed soil below the frost line.
How to Use This Concrete Slab and Footing Calculator
This concrete slab and footing calculator estimates the combined concrete volume for a monolithic slab with thickened edge footings. Enter the slab dimensions (length, width, depth), then enter the perimeter footing dimensions (width, total depth, and how many sides have a footing). The calculator returns the total volume, split into slab and footing portions, plus bag count and cost estimate. Use it for garage slabs, workshop floors, slab-on-grade foundations, and any project where the slab and footings are poured together.
How to Calculate Concrete for a Slab with Footings
A monolithic slab has two concrete volumes to calculate:
Slab Volume = Length × Width × Slab Depth
Footing Volume = Perimeter × Footing Width × (Footing Depth − Slab Depth)
The footing depth is measured from the top of the slab to the bottom of the footing. The slab already occupies the upper portion, so the extra concrete needed for the footing is only the difference in depth.
Step-by-Step Example: 24×24 Garage Slab
- Slab: 24 ft × 24 ft × 4 in (0.333 ft) = 192 ft³ = 7.11 yd³
- Perimeter: (24 + 24 + 24 + 24) = 96 ft (all 4 sides)
- Footing extra depth: 12 in total − 4 in slab = 8 in (0.667 ft)
- Footing width: 12 in (1 ft)
- Footing volume: 96 × 1 × 0.667 = 64 ft³ = 2.37 yd³
- Total: 7.11 + 2.37 = 9.48 yd³ (before waste)
- With 10% waste: 10.43 yd³ — order 10.5 yd³ from the ready-mix plant
When to Use a Monolithic Slab with Footings
A monolithic (single-pour) slab with thickened edges is a cost-effective foundation for certain projects:
- Garages and workshops — the most common application; the thickened edge supports wall framing while the interior slab serves as the floor
- Slab-on-grade homes — in warm climates (shallow frost line), the entire house foundation can be a monolithic slab with thickened perimeter and interior bearing footings
- Sheds and outbuildings — smaller structures often don't need a full frost-depth foundation, making a monolithic slab ideal
- Not recommended for — multi-story buildings, areas with deep frost lines (36+ inches), expansive clay soils, or sloped sites where a stem wall is needed
For projects where the slab and footings are poured separately, use our concrete slab calculator and concrete footing calculator individually.
Tips for Pouring a Monolithic Slab
- Prepare the subgrade first — compact the soil, add 4 inches of compacted gravel, and install a vapor barrier (6-mil poly) before placing any concrete
- Form the thickened edge carefully — use 2×12 lumber (or wider) for forms so the footing depth is captured; the form height equals the total footing depth, not the slab thickness
- Place rebar in both sections — the slab gets welded wire mesh or #3 rebar on centers; the thickened edges get at least two horizontal #4 bars plus any required vertical dowels
- Pour continuously — the entire slab and footings must be poured in one session to create a monolithic bond; do not allow a cold joint between the slab and footing sections
- Order ready-mix for anything over 2 yd³ — a 24×24 garage slab with footings is 10+ cubic yards, well beyond the practical limit for bag concrete
Sources & References
- IRC Section R403: Footings — International Residential Code — International Code Council
- ACI 318-19: Building Code Requirements for Structural Concrete — American Concrete Institute
- ASTM C94/C94M: Standard Specification for Ready-Mixed Concrete — ASTM International
How Much Does a Monolithic Slab with Footings Cost?
A monolithic slab is one of the most cost-effective foundation types because it combines the floor slab and footings into a single pour, saving on labor and forming costs. Ready-mix concrete runs $130–$165 per cubic yard delivered. For a 24×24-foot garage slab with thickened edge footings — roughly 10.5 cubic yards with waste — the concrete alone costs $1,365–$1,730.
Total installed costs, including excavation, gravel base, vapor barrier, rebar, forming, finishing, and concrete, typically range from $6–$12 per square foot. That puts a 576-square-foot (24×24) garage slab at $3,450–$6,900 fully installed. Smaller projects like a 12×16 workshop slab (192 sq ft) run $1,150–$2,300. Key cost variables include local concrete prices, soil conditions (rocky soil adds excavation cost), and whether you need a permit and engineering stamp — budget $200–$500 for permits in most areas. DIY homeowners who handle their own forming and finishing can save 40–50% on labor, but a monolithic pour this size requires a crew of 3–4 people to place and finish the concrete before it sets.