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 Stem Wall Foundation Guide and When To Use Rebar.
Foundation Types Compared
The three residential foundation systems differ in how they spread building loads to the soil — and that determines how much concrete you order. Slab-on-grade pours the floor and the bearing edge in one or two operations; a T-footing with stem wall puts a wide strip footing below grade and a vertical wall on top; pier or post foundations carry the load on isolated columns rather than a continuous edge.
| Foundation Type | When To Use | Concrete Footprint | Relative Volume |
|---|---|---|---|
| Slab-on-grade (monolithic) | Warm/no-frost climates, single story, level lots | Full floor + thickened edge | Highest |
| T-footing + stem wall | Frost climates, crawlspace, sloped lots | Strip footing + wall only | Medium |
| Pier / post foundation | Decks, cabins, additions, poor soil | Isolated columns only | Lowest |
Slab-on-grade uses the most concrete because you pour the entire floor. A T-footing system pours only the perimeter footing and stem wall, then frames a floor above the resulting crawlspace. Pier foundations use the least — sometimes a tenth of a slab's volume — because the concrete is concentrated in discrete footings. For deck and post work, the pier footing calculator and deck footing calculator handle column volumes directly.
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.
Footing Sizing and Frost Depth
Every foundation transfers load through a footing, and the footing must reach soil that will not move. The International Residential Code is explicit here. IRC R403.1.4 requires footings to bear on undisturbed natural soil at least 12 inches below the undisturbed ground surface. IRC R403.1.4.1 then requires that, except for frost-protected shallow foundations, footings in areas subject to freezing extend below the local frost line.
| Frost Zone | Frost Depth | Min. Footing Depth | Typical Stem-Wall Height |
|---|---|---|---|
| Mild South (zone 1) | 0 in | 12 in (per R403.1.4) | 8-16 in |
| Mid-Atlantic / Midwest | 24-36 in | 36 in | 24-36 in |
| Upper Midwest | 42-48 in | 48 in | 36-48 in |
| Far North (MN, ME) | 60+ in | 60+ in | 48-60 in |
Footing width follows the load. A standard one- to two-story house on average soil uses a 16- to 20-inch-wide strip footing, 8 inches thick. Run those numbers through the footing calculator, which handles strip and pad footings with the right depth defaults.
Stem-Wall Volume
In a T-footing system you order concrete in two parts: the strip footing and the stem wall that rises from it to the framing line. Calculate the wall as length × height × thickness, then convert to cubic yards (length-ft × height-ft × thickness-ft ÷ 27).
| Wall Thickness | Wall Height | Volume per 100 lin ft |
|---|---|---|
| 8 in | 24 in | 4.94 cu yd |
| 8 in | 36 in | 7.41 cu yd |
| 8 in | 48 in | 9.88 cu yd |
| 10 in | 36 in | 9.26 cu yd |
For a 30×50 ft house, the stem-wall perimeter is 160 linear feet, so an 8-inch wall 36 inches tall adds about 11.9 cubic yards on top of the strip footing. Add the footing and the wall together to size the full order. The stem-wall foundation guide walks through forming and pour sequence, and tall walls may justify the concrete wall calculator for poured-wall basements.
Rebar and Reinforcement
Foundations are reinforced to control cracking and tie the assembly together. ACI 332-20 (Residential Code Requirements for Structural Concrete) and the adopted IRC chapter set the prescriptive minimums; engineered drawings override them.
| Element | Typical Reinforcement | Placement |
|---|---|---|
| Slab field | #4 bar (1/2 in) at 16 in o.c. each way, or 6×6 W2.9 mesh | Mid-slab on chairs |
| Thickened edge | Two #4 bars continuous | 3 in clear from soil |
| Strip footing | Two #4 bars continuous | 3 in up from bottom |
| Stem wall | #4 verticals at 48 in o.c. + two horizontal #4 | Doweled into footing |
Keep a minimum 3 inches of concrete cover where rebar faces soil. Whether your slab needs bar, mesh, or fiber depends on load and soil — see when to use rebar for the decision.
Slab-on-Grade Thickness and Vapor Control
IRC R506.2.3 sets the minimum thickness of a residential concrete floor slab on grade at 3.5 inches. Most builders pour a clean 4 inches as the working standard and thicken to 5-6 inches under garages, bearing walls, and point loads. The slab sits on a 4-inch compacted gravel base over a 6-mil (or thicker) polyethylene vapor retarder, lapped 6 inches and sealed at seams.
- Base — 4 inches of compacted, washed gravel for capillary break and drainage; see gravel under concrete.
- Vapor retarder — required under heated slabs to stop ground moisture; details in the vapor barrier guide.
- Strength — 3,000 PSI is the IRC residential floor; 4,000 PSI is worth the small upcharge in freeze-thaw country.
Drainage and Footing Drains
Water is what destroys foundations, so drainage is part of the system, not an add-on. Per IRC R405, foundations that enclose habitable or usable space below grade need a footing (perimeter) drain. Lay a 4-inch perforated pipe at or below the bottom of the footing, surround it with washed gravel, wrap it in filter fabric, and slope it to daylight or a sump. Backfill with free-draining material, grade the soil to fall 6 inches over the first 10 feet away from the wall, and waterproof the exterior of any stem wall that retains soil.
Ready to size and price the pour? Run your footprint through the calculator above, size the perimeter with the footing calculator, then estimate installed cost with the concrete cost calculator. For permits and inspections, start with the permit requirements guide, and use how to hire a contractor before any foundation pour over about 5 cubic yards.