Foundation Calculator
Calculate concrete volume, rebar, vapor barrier sheeting, and ready-mix truckloads for monolithic slabs, thickened-edge foundations, continuous strip footings, and pier columns.
Calculation Results
Total cubic yards of ready-mix concrete to order.
Standard 10-cubic-yard ready-mix transit mixer trucks.
Under-slab poly vapor barrier (includes 15% overlap and tape seams).
Linear feet of #4 rebar running continuously around the thickened edge/strip (2 parallel bar rows).
How This Calculator Works
Follow these structural steps to calculate foundation requirements:
- Select Foundation Style: Monolithic slab with thickened edge is standard for residential garages; uniform slab for sheds; strip footing for stem-wall houses; piers for post-and-beam cottages.
- Enter Dimensions: Input building footprint and trench cross-section.
- Review Complete Takeoff: The tool calculates concrete cubic yards, ready-mix transit trucks, under-slab vapor barrier poly, and continuous footing rebar.
Formula & Calculations
Foundation concrete volume calculations conform to International Residential Code (IRC Chapter 4) and ACI 318 standards:
1. Monolithic Slab with Thickened Edge
Commonly called an "Alaskan Slab" or floating foundation, the structure consists of two geometric zones poured together:
- Center Slab:
Length × Width × Slab Thickness - Perimeter Thickened Edge Beam:
2 × (Length + Width) × Beam Width × Beam Depth - Combined Volume:
Total Cu Ft = Center Slab + Perimeter Trench
2. Continuous Strip / Trench Footing
Volume = 2 × (Length + Width) × Trench Width × Trench Depth
3. Pier Footings (Sonotubes)
Volume = Pier Count × [π × (Diameter / 2)² × Depth]
Step-by-Step Calculation Examples
Two-Car Garage Monolithic Foundation (30ft x 24ft)
4-inch center slab with a 12in wide x 12in deep perimeter thickened edge beam and 10% pour waste.
- foundation_type: monolithic
- length: 30
- width: 24
- slab_thickness: 4
- beam_width: 12
- beam_depth: 12
- waste: 10
- volume_cuyd: 14.18 cu yd
- truckloads: 2
- vapor_barrier: 828 sq ft
- rebar_perimeter: 249 ft
Frequently Asked Questions
What is a monolithic slab with thickened edge?
A monolithic slab (also called a turned-down slab or slab-on-grade) is a foundation where the flat interior floor slab and the deeper exterior perimeter load-bearing footing beams are poured at the exact same time as a single seamless piece of concrete.
How thick should a garage foundation slab be?
The interior floor slab should be 4 inches thick for standard passenger cars, or 5 to 6 inches thick for heavy trucks and workshop machinery. The perimeter edge footing should be at least 12 inches wide and extend down below the local frost depth or undisturbed native soil.
Why is a vapor barrier required under foundation slabs?
Building codes (IRC R506.2.3) require a minimum 6-mil or 10-mil polyethylene vapor retarder beneath residential interior concrete slabs. It blocks subterranean water vapor and radon gas from migrating upward through microscopic concrete pores into finished flooring.
What strength concrete (PSI) is needed for residential foundations?
Per building code, foundation footings and exterior slabs subjected to freeze-thaw cycles require concrete with a minimum compressive strength of 3,000 to 3,500 PSI at 28 days.
How much rebar should be in a footing trench?
Standard residential codes require at least two continuous rows of #4 (1/2-inch) reinforcing rebar running horizontally around the perimeter trench, supported on rebar chairs at least 3 inches above the bottom of the trench.
How many cubic yards of concrete are in a standard ready-mix truck?
A standard transit mixer truck carries 9 to 10 cubic yards of concrete. When planning deliveries, order in full truckload increments to minimize short-load delivery surcharges.