About

Building a concrete foundation requires more than just mixing cement; it requires a rigid mold, known as formwork, to hold the wet concrete in place until it cures. Formwork is a significant cost component in construction, often requiring careful estimation of plywood sheets, linear feet of lumber for studs and walers, and bracing. Underestimating leads to project delays, while overestimating wastes expensive timber.

This calculator helps contractors and owner-builders determine the precise material list for common foundation types: Slab-on-Grade and Strip Footings. It calculates the contact surface area (CSA) to determine plywood needs and estimates the framing lumber required to support the hydrostatic pressure of wet concrete. It includes a waste factor toggle to account for cutting scraps and site inefficiencies.

Formulas

The calculation begins with the Contact Surface Area (CSA). For a simple slab, this is the perimeter multiplied by height.

CSA=(2L+2W)×Depth

For a strip footing (wall), both sides of the wall require formwork.

CSA_wall=2×Perimeter×Height

The number of plywood sheets is derived from the area of a standard sheet.

Sheets=CSA×(1+Waste)Area_sheet

Stud count assumes a standard spacing (e.g., 16 inches on center).

Studs=PerimeterSpacing

Reference Data

Material	Typical Dimensions (Imp)	Typical Dimensions (Met)	Usage
Plywood / OSB	4' x 8' x 3/4"	1220 x 2440 x 18mm	Sheathing (Face)
Studs	2" x 4"	50 x 100 mm	Vertical Support
Walers	2" x 4" (Double)	50 x 100 mm	Horizontal Reinforcement
Stakes / Kicker	2" x 4" or 1" x 4"	25 x 100 mm	Ground Bracing
Tie Wire	-	-	Holding forms together
Concrete Density	150 lb/ft³	2400 kg/m³	Load Calculation
Standard Spacing	16" OC	400 mm OC	Stud separation
Strip Footing	Varies	Varies	Supports walls

Frequently Asked Questions

For foundation walls and heavy slabs, 3/4-inch (18mm) Plyform or high-density overlay (HDO) plywood is standard to resist bowing under the pressure of wet concrete. Thinner 1/2-inch sheets may be used for very short curbs but require tighter stud spacing.

Yes. While the primary focus is the wooden formwork, the tool calculates the theoretical volume of concrete required to fill the mold. Always add a margin (usually 10%) when ordering concrete to account for ground unevenness and spillage.

Construction materials are rarely used with 100% efficiency. Cuts, mistakes, and odd geometries create scrap. A 5-10% waste factor is standard for simple rectangles, while complex shapes may require 15% or more.

Walers are horizontal beams (usually doubled 2x4s) placed outside the vertical studs. They align the formwork and prevent it from bulging out under the massive hydrostatic pressure of the wet concrete.

Yes, if treated with a form release agent (oil) before pouring. However, the lumber will often be stained and may have nail holes. It is typically reused for structural framing where aesthetics don't matter, or for future formwork.