About

A monolithic foundation (or slab-on-grade) combines the structural footing and the floor slab into a single concrete pour. This method is efficient for stable soils and flat terrain, significantly reducing construction time compared to stem-wall foundations.

The calculation is twofold: first, the volume of the main floor slab, and second, the "thickened edge" or footing trench around the perimeter. Accurately estimating this volume is tricky because the footing often has sloped sides (battered) or irregular trenching. This tool uses a standard rectangular approximation for the footing cross-section to provide a safe, sufficient estimate for ordering concrete.

Formulas

We calculate the slab as a full rectangle, then add the extra volume required for the thickened edge (subtracting the slab thickness already counted).

Slab Volume:
V_s = L × W × T_slab

Footing Length:
P = 2(L + W)

Footing Cross-Section (Added Part):
A_f = W_foot × (D_total − T_slab)

Total Volume:
V_tot = V_s + (P × A_f)

Note: This method slightly overestimates corners (counting them twice in the perimeter), which acts as a built-in waste buffer (~2-3%).

Reference Data

Feature	Monolithic Slab	Stem Wall & Slab	Notes
Pour Type	Single pour	Two or three pours	Mono is faster.
Frost Depth	Shallow (warm climates)	Deep (cold climates)	Mono suitable for frost-protected shallow foundations (FPSF).
Cost	Lower	Higher	Less labor in Mono.
Suitability	Flat ground	Sloped ground	Stem walls handle slopes better.
Rebar	Heavy perimeter reinforcement	Vertical & Horizontal	Mono relies on edge stiffness.
Insulation	Under & Edge	Perimeter	Critical for efficiency.
Typical Edge Depth	12-24 inches	Varies	Depends on load.
Typical Slab Depth	4-6 inches	4-6 inches	Standard floor load.

Frequently Asked Questions

Total depth is measured from the top of the finished slab down to the bottom of the trench. If your slab is 4 inches and your trench digs down another 12 inches from the subgrade, the Total Depth is 16 inches.

Yes, but with precautions. Concrete must be kept above 50°F (10°C) to cure properly. You may need insulated blankets, heated enclosures, or additives (accelerators) in the mix.

A common rule of thumb for estimation is roughly 80kg of steel per cubic meter of concrete for residential foundations, though this varies by engineering requirements (e.g., #4 bars at the perimeter and wire mesh in the field).

Calculate using the average width. If the top of the footing is 18" wide and the bottom is 12" wide, use 15" as the width in this calculator.