Foundation Concrete

Calculating Concrete for a Strip Footing

A strip footing is a continuous concrete foundation that runs around a building’s entire perimeter, carrying the structure’s weight down into the ground below the frost line. Unlike a flat slab, which fills a whole rectangular area, a footing is a long, thin prism that only follows the building’s outer edge — so its volume comes from a different formula: perimeter × width × depth, rather than length × width × depth over an entire floor.

Enter the foundation’s perimeter directly, or switch to Rectangular Building and enter a simple building’s outer length and width instead — the calculator works out perimeter = 2 × (length + width) automatically. Check Include a Stem Wall if the project also has a vertical foundation wall on top of the footing (common wherever a basement or crawlspace needs the foundation to reach above grade).

The Formula

1. Footing volume: $\vE{\text{Footing Volume}} = \vA{\text{Perimeter}} \times \vB{\text{Footing Width}} \times \vC{\text{Footing Depth}}$
2. Stem wall volume (only when included) — the same shape, running the same perimeter: $\vF{\text{Stem Wall Volume}} = \vA{\text{Perimeter}} \times \vD{\text{Wall Thickness}} \times \vG{\text{Wall Height}}$
3. Total volume: footing volume plus stem wall volume, converted to cubic yards — the unit ready-mix concrete is ordered in.
4. Amount needed: the total, rounded up to the nearest whole cubic yard.
5. Recommended order: the rounded amount plus a waste margin (10% by default, adjustable above) — the same standard rule-of-thumb the Concrete Calculator uses, covering spillage and uneven trenching.
6. Estimated weight and bag counts (40/60/80 lb) follow the same conventions as the Concrete Calculator: about 4,050 lb per cubic yard, and bag yields of 0.30/0.45/0.60 cubic feet respectively.

A footing also needs to reach below the local frost line so seasonal ground freeze-thaw can’t heave it — this calculator flags whether the entered depth clears the commonly-cited 12 in frost-line floor many (not all) climates sit at or above, but always check your own local building code, since the real requirement varies significantly by region.

Worked Example

A 40 ft × 30 ft rectangular building, with a 16 in wide, 8 in deep footing:

1. Perimeter: $2 \times (40 + 30) = \vA{140}$ ft.
2. Footing volume: $\vA{140} \times \left(\vB{16} \div 12\right) \times \left(\vC{8} \div 12\right) \approx \vE{124.4}$ cubic feet, which is about 4.61 cubic yards.
3. Amount needed: rounded up to 5 cubic yards.
4. Recommended order: $5 \times 1.1 = 5.5$ cubic yards.
5. Estimated weight: $4.61 \times 4{,}050 \approx 18{,}700$ lbs.

That’s a typical single-story home’s perimeter, with a footing depth below the commonly-cited 12 in frost-line floor — worth checking against local code before pouring.

Key Factors to Consider

• Soil bearing capacity, not just frost depth, determines a footing’s real required width and depth. A structural engineer sizes a footing based on the local soil’s ability to bear the building’s weight without settling unevenly — poor or expansive soil can require a wider or deeper footing than a frost-line check alone would suggest.
• Interior load-bearing walls sometimes need their own footings, separate from the perimeter footing. This calculator covers the perimeter footing that follows the building’s outer edge — a structure with interior load-bearing walls may need additional footings under those walls too, which aren’t captured by a perimeter-only calculation.
• Footing width is typically wider than the wall it supports, to spread the load. A footing is commonly designed wider than the foundation wall sitting on top of it, distributing the building’s weight over a larger area of soil — check your structural plans for the specific footing width required, rather than assuming it matches the wall thickness.
• This calculator estimates material volume only — reinforcement and drainage are separate, necessary considerations. Rebar reinforcement and a footing drain (to direct groundwater away from the foundation) are both standard parts of real foundation construction that this calculator doesn’t estimate material for.

Common Mistakes

• Confusing this with the Concrete Calculator’s slab mode. A slab fills a flat area; a strip footing only follows the building’s outer edge. Use this calculator for the foundation footing itself, then the Concrete Calculator separately for any slab poured inside it.
• Forgetting the stem wall. A foundation with a basement or crawlspace usually needs a stem wall on top of the footing — leaving it unchecked here would understate the total concrete needed for that kind of project.
• Skipping the rebar and frost-depth questions. This calculator estimates material volume only — reinforcement requirements and the required frost-line depth both vary by local building code and should be confirmed with a structural engineer or building department before pouring.

Useful to Know

• Ordering ready-mix concrete for a foundation pour usually has a minimum-load requirement from the supplier — a small footing’s calculated volume can end up costing the same as a larger order anyway, worth checking before assuming the calculated amount is exactly what arrives.
• A footing poured in cold weather may need extra curing time or an accelerator additive to reach strength before backfilling or framing begins — a scheduling factor separate from the volume this calculator estimates.
• This calculator pairs naturally with the Concrete Calculator (for any flat slab poured inside the foundation) and the Rebar Calculator (for the reinforcement this material-volume estimate doesn’t cover).