About

A monolithic slab is one of the strongest and most cost-effective foundation types, but material estimation errors can be expensive. Ordering too little concrete results in "cold joints" (structural weaknesses), while ordering too much wastes budget. This tool calculates not just the concrete volume, but the entire ecosystem of materials required: the steel reinforcement (rebar) mesh, the waterproofing membrane area, and the plywood sheets needed for the perimeter formwork.

It applies a standard engineering safety factor of 1.05 (5%) to the concrete volume to account for ground unevenness and pump waste. Rebar calculations assume a dual-layer grid (top and bottom reinforcement) common in load-bearing residential slabs. The results are instantly converted between Metric (m³) and Imperial (yd³) systems to support international blueprints.

Formulas

The calculator uses geometric volume and grid logic for estimation.

V_conc = (L × W × H) × 1.05

Rebar length is calculated for a grid pattern (x and y directions) usually in 2 layers:

L_rebar = 2 × [ (LStep + 1) × W + (WStep + 1) × L ]

Reference Data

Material	Property	Standard Value	Notes
Concrete	Density	2400 kg/m³	~150 lb/ft³
Rebar (10mm / #3)	Linear Weight	0.617 kg/m	Light residential
Rebar (12mm / #4)	Linear Weight	0.888 kg/m	Heavy residential
Plywood Sheet	Standard Size	2.44 × 1.22 m	4x8 ft (Imperial)
Safety Margin	Volume Factor	5%	Added to total

Frequently Asked Questions

Excavations are rarely perfectly flat. A 5% buffer accounts for uneven subgrade, spillage during pouring, and concrete residue left in the pump truck. It prevents the disaster of running out of concrete mid-pour.

No. The calculation provides the pure geometric length required to cover the grid. In practice, rebar comes in fixed lengths (e.g., 6m or 20ft), and you need to overlap ends by 40x diameter. We recommend adding an extra 10-15% to the rebar weight for overlaps and cut-offs.

It calculates the perimeter area of the slab edge (Perimeter × Height) and divides it by the area of a standard plywood sheet strip. It assumes you will cut sheets to fit the height of the slab.