About

The structural integrity of a monolithic slab foundation depends on its ability to distribute the building's load over the soil without excessive settling or cracking. A common error in residential construction is under-engineering the slab thickness for the specific building weight and soil type. While a 100 mm slab might suffice for a shed, a two-story brick house on clay soil requires significantly more mass and stiffness.

This tool estimates the required slab thickness by analyzing the approximate linear load of the walls and the distributed load of the floors, matched against the soil's bearing capacity. It provides a structural cross-section preview and calculates the total concrete volume required for ordering.

Formulas

The simplified heuristic for estimating slab adequacy involves calculating the total building weight ($W_{total}$) and ensuring the pressure exerted does not exceed the soil's safe bearing capacity ($q_a$).

1. Estimated Load ($P$):

{

Wood Frame ≈ 150 kg/m²Aerated Concrete ≈ 600 kg/m²Solid Brick ≈ 1800 kg/m²

2. Pressure Check:

W_totalArea_slab ≤ q_soil

Reference Data

Soil Type	Bearing Capacity (kg/cm²)	Typical Use Case
Solid Bedrock	10.0 - 50.0	High-rise, Heavy Industrial
Gravel / Dense Sand	4.0 - 6.0	Standard Residential (Ideal)
Medium Sand / Dry Clay	2.0 - 3.0	Standard Residential (Common)
Wet Clay / Loam	1.0 - 1.5	Requires Wide Footings / Thick Slab
Silt / Soft Clay	0.5 - 1.0	Risk of Settlement (Requires Engineering)

Frequently Asked Questions

For standard residential structures (garages, small houses), the absolute minimum is usually 100mm (4 inches). However, 150mm (6 inches) is the recommended standard for main dwellings to accommodate reinforcement and prevent cracking under load.

Weaker soils (soft clay, silt) have lower bearing capacities. On these soils, the foundation must be stiffer to bridge soft spots and distribute weight more evenly, often requiring a thicker slab (250mm+) or a ribbed slab design, compared to dense gravel which supports thinner slabs.

No. This tool calculates the concrete geometry (thickness and volume). Reinforcement (rebar) calculations require specific structural analysis based on span and bending moments. See our Slab Reinforcement Calculator for mesh details.

Adding thickness increases stiffness and mass, which is generally safer but more expensive. However, increasing depth beyond 300mm often yields diminishing returns for residential loads. It is often more effective to improve the sub-base (compacted sand/gravel) than to just pour more concrete.