Strip Footing Calculator — ft to cu yd, bags, cost

How to Calculate Concrete for a Strip Footing

A strip footing (also called a continuous or wall footing) is the concrete band poured beneath a load-bearing wall to spread the wall load onto the soil. Volume is the cross-section (width × depth) carried along the total run, so a long, narrow band consumes ready-mix fast — a 40 ft × 16″ × 10″ footing alone is about 1.6 yd³. Order ready-mix when the pour exceeds roughly 1 yd³; below that, bagged concrete is usually cheaper and avoids a short-load fee. The 5–10% waste allowance covers over-excavated trench walls and uneven subgrade — bump it to 15% for hand-dug trenches in soft ground. If you are sizing a foundation from scratch, work out the wall footing here, then add the column footings with the spread footing calculator and any deep supports with the concrete pier calculator.

Formulas

Volume: V = total length × width × depth. For a closed rectangle the total length = 2 × (L + W); L and U layouts sum their straight legs. Volume in yd³ = V(ft³) ÷ 27. An 80-lb bag yields ≈ 0.6 ft³ (0.022 yd³); a 60-lb bag ≈ 0.45 ft³ (0.017 yd³), so a yard of concrete is roughly 45 × 80-lb bags. Concrete weighs ≈ 150 lb/ft³ (4,050 lb/yd³). For an isolated bag-count check use the concrete bag calculator.

Reinforcement from the structural drawings

Strip footings carry longitudinal rebar (#4 or #5) over the full length with lap splices at 40 bar diameters (40d_b) — about 20″ for #4 bar. Standard 20-ft bars set the splice count. Optional #3 stirrups at 12″–24″ on center tie the cage together where the footing also acts as a grade beam. Minimum cover is 3″ for concrete cast against soil from the structural drawings §20.6.1. A reinforced wall footing spanning soft spots is essentially a shallow grade beam; for pile-supported foundations size the cap with the pile cap calculator.

Ordering tips & jobsite notes

In 2025 USA pricing, ready-mix is roughly $160–220/yd³ delivered, residential footing labor runs $8–15 per linear foot, and #4 grade-60 rebar is $0.80–1.40/lf at the yard. The single biggest estimating mistake on a wall footing is forgetting the short-load fee — most plants tack on $80–150 if you order under ~3 yd³, so a 40 ft × 16″ × 10″ footing (≈ 1.6 yd³) is often cheaper in 80-lb bags than from the truck. Don't forget the keyway: a 2×4 chamfer pressed into the wet top gives the stem wall a positive shear key. Per IRC §R403.1, the trench bottom must rest on undisturbed soil below the local frost line, and minimum cover to rebar from concrete cast against soil is 3″ (ACI 318-19 §20.6.1).

FAQ

How wide should a strip footing be? Typically twice the wall width — a 16″ footing under a standard 8″ block wall. The IRC allows 12″ for light one-story residential loads on firm soil.
How deep should a strip footing be? The poured thickness is 8″ minimum (10–12″ standard), but the underside must sit below the local frost line — 24–48″ in cold climates. Footing depth here is the concrete thickness, not the trench depth.
Do I need rebar in a strip footing? Plain footings are permitted for some light residential cases, but two continuous #4 bars are standard practice and required wherever the footing bridges variable bearing or seismic detailing applies.