A 40 × 30 ft house on a 20 × 10 in continuous footing with an 8 in stem wall 24 in tall needs 140 feet of footing and 14.1 cubic yards of concrete: 194 cubic feet in the footing and 187 in the stem. With 10 % waste that is a 15¾-yard ready-mix order. Two #4 bars in the footing, one at the top of the stem, corner bars and #4 dowels every 48 in come to 37 bars of 20 ft, about 494 lb. The calculator above works this out for any rectangle, a rectangle with an interior bearing wall, or any footing length.
How the calculator works
Choose the layout and enter the building size, or the total footing length for an irregular outline. Enter the footing width and thickness and, if there is one, the stem wall. Then the bars, how you buy the concrete, the wall load and the soil. The calculator gives the concrete, the bars to buy, the forms, and checks the footing against the IRC minimums and the soil.
Footing length
Footings are measured along their centerline: 2 × (40 + 30) = 140 ft for the default house. On the centerline the corners take care of themselves, since what the outside corner gains the inside corner loses. An interior bearing wall across the width adds its length, 30 ft here.
Concrete
The footing is length × width × thickness, the stem length × thickness × height. 140 × (20/12) × (10/12) = 194.4 ft³ for the footing and 140 × (8/12) × 2 = 186.7 ft³ for the stem, 381 ft³ or 14.1 yd³ in all. Add 5 to 10 % for trench walls that are not straight and what stays in the chute.
Reinforcement
Horizontal bars run along each side. A 40 ft side is longer than a 20 ft bar, so each bar along it needs two lap splices, each 40 bar diameters (20 in for #4) by default; a 30 ft side needs one. Corner bars are bent L-bars at each corner and tee with legs one lap long each way, so the steel is continuous around the corner. Dowels tie the stem wall to the footing: each has a 90° hook at the bottom with an extension of 12 bar diameters (ACI 318-19 Table 25.3.1), sits on 3 in of cover and runs up to 2 in below the top of the stem. The bars to buy come from a cutting plan, offcuts included.
What the IRC requires
The prescriptive footing sizes in IRC Tables R403.1(1) to R403.1(3) depend on the number of stories, the wall construction, the snow load and the soil, so the calculator takes the width from your drawings instead of guessing it. It checks the absolute minimums the IRC sets for every footing:
- Size. At least 12 in wide and 6 in thick (R403.1.1).
- Projection. The footing extends at least 2 in beyond the face of the wall and not more than its own thickness (R403.1.1). A 20 in footing under an 8 in wall projects 6 in each side.
- Depth. The bottom of an exterior footing is at least 12 in below undisturbed ground and below the frost line (R403.1.4). Frost depth is set locally; ask your building department.
- Soil. The pressure under the footing stays within the presumptive load-bearing value of the soil: 1,500 psf for clay and silt, 2,000 for sand, 3,000 for gravel (Table R401.4.1).
Pressure on the soil
The concrete alone weighs 150 lb per cubic foot. The default footing and stem weigh 408 lb per foot of length, spread over a 20 in footing: 245 psf. Enter the wall load per foot from the structural drawings, the weight of the walls, floors and roof the footing carries, and the calculator adds it before comparing with the soil.
Common mistakes
- Measuring the outside perimeter. 2 × (L + W) on the outside face overstates concrete by four corner blocks; use the centerline.
- Forgetting the stem. On a crawl space foundation the stem wall can hold as much concrete as the footing.
- Bars without corner bars. Lapping straight bars at a corner is not continuous reinforcement; bent bars are.
- Ignoring frost depth. A footing above the frost line heaves.
For the slab inside the footing, use the Concrete Slab Calculator; for the bars alone, the Rebar Calculator; for a block stem wall, the Concrete Block Calculator.