Concrete Foundation Calculator
A 30×20 ft slab foundation at 8 inches thick takes 14.81 cubic yards of concrete — roughly two ready-mix truck loads. Enter your foundation footprint and thickness below for volume, waste-adjusted order quantity, and reinforcement guidance.
Pro Tips
- →Foundation thickness varies by soil and load requirements
- →Always check local building codes for specifications
- →Include rebar reinforcement as required by code
- →Consider frost depth requirements in your region
- →Hire a structural engineer for design verification
Includes 10% waste factor
That's typically a professional pour. See costs ↓
4 short emails from Dave: what a fair quote should land at for your slab, the scope changes that swing it ±$500, and whether DIY is actually cheaper at your volume. Reply anytime — he'll review your real quote.
Cost Estimate
Estimated material costs for your project
Recommendation: Ready-Mix Concrete
For projects over 1 cubic yard, ready-mix is typically more economical and easier to work with.
734 bags × 80lb
16.30 cubic yards + delivery
600 sq ft × $3.00–$8.00/sq ft
Prices vary by location and time. Contact local suppliers for accurate quotes.
For general step-by-step instructions, read our complete When To Use Rebar and Permit Requirements.
What This Calculator Covers
Volume estimates for slab-on-grade and monolithic-slab foundations — the two most common residential foundation types. Enter the footprint and thickness; the tool returns cubic yards plus a 10% waste buffer. For stem-wall and crawlspace foundations, run the wall portion separately and add it to the slab volume.
If you only need the footing portion (the thickened concrete strip below a wall), use the footing calculator instead — it handles strip and pad footings with the right depth defaults.
Foundation Volume by House Size
The table below assumes a flat 8-inch slab — typical for residential monolithic-slab foundations. Adjust upward for thickened edges or stem walls.
| House Footprint | Square Feet | Volume (cu yd, 8 in) | Ready-Mix Truck Loads (10 yd) |
|---|---|---|---|
| 24 × 40 ft | 960 | 23.7 | 2.5 |
| 30 × 40 ft (1,200 sq ft) | 1,200 | 29.6 | 3 |
| 30 × 50 ft (1,500 sq ft) | 1,500 | 37.0 | 4 |
| 40 × 50 ft (2,000 sq ft) | 2,000 | 49.4 | 5 |
| 40 × 60 ft (2,400 sq ft) | 2,400 | 59.3 | 6 |
Add 10% to the cubic-yard column when ordering. A standard mixer truck holds about 10 cubic yards; expect a short-load fee on the final partial load.
Thickness, PSI, and Rebar Standards
- Slab thickness — 4 inches is the residential floor minimum, but foundations almost always run thicker. Monolithic slabs use a 4-inch field with a 12-inch thickened edge under bearing walls. Conventional slab-on-grade over a stem-wall footing typically uses a uniform 4–6 inch slab.
- Concrete strength — 3,000 PSI is the residential code floor (IRC). 4,000 PSI is the practical standard for foundations and worth the small upcharge for freeze-thaw climates.
- Rebar — #4 (1/2-inch) bars in a 16-inch grid is typical for the slab field, with two #4s continuous along the perimeter thickened edge. Place bars at mid-slab depth on rebar chairs. Local codes and engineered drawings override these defaults.
- Vapor barrier — 10-mil polyethylene under the entire slab, lapped 12 inches and taped at seams. Skipping this in a heated structure causes flooring failures within a few years.
Reference ACI 332 for residential concrete construction and the local building department's adopted IRC chapter for the prescriptive minimums in your jurisdiction.
Frost Depth and Footing Tie-In
Foundations in cold climates must bear below the frost line — the depth to which winter freezing penetrates the soil. Frost depth ranges from 0 inches in the deep South to 60+ inches in northern Minnesota and Maine.
Monolithic slabs handle this with insulated frost-protected shallow footings; stem-wall systems extend the wall down to bearing depth on a strip footing. The footing portion of either system is sized separately — see the footing calculator for strip-footing volumes by length and width.
Permits, Inspections, and DIY Limits
Residential foundations require a building permit in essentially every U.S. jurisdiction. The permit process includes plan review, soil bearing verification, and inspections at: footing trench, pre-pour rebar, and final.
Forty-plus cubic yards of concrete arriving in 2–6 truck loads is not a DIY pour. The set window between truck arrival and finishing closes fast — a typical 30-yard foundation is 6–8 hours of finishing labor for a 4-person crew. Owner-builder permits exist in most states, but mistakes in foundation work compromise everything above and are extremely expensive to remediate. Hire licensed concrete contractors for any foundation pour over about 5 cubic yards.