Concrete Trench Footing Calculator
Estimate concrete for a trench-poured strip footing dug directly into the ground.
Concrete needed
2.30 yd³
for 40 ft of trench footing
- Trench cross-section
- 216 in²
- Cubic feet
- 62.1 ft³
- Cubic yards
- 2.30 yd³
- Bags equivalent
- 104 bags
Short answer
A 40-foot trench footing, 12 inches wide and 18 inches deep, needs about 2.3 cubic yards of concrete once a 15% waste allowance is added. Trench pours run wider and deeper than they look on paper because dug soil walls are rarely perfectly straight, so a higher waste margin than a formed slab is standard practice.
Typical trench footing sizes by wall type
| Wall type | Typical width | Typical depth |
|---|---|---|
| Shed or fence wall footing | 12 in | 12 in |
| Garage or small building wall | 16 in | 8–12 in |
| House foundation wall (non-frost area) | 16–24 in | 8–12 in |
| Frost-protected footing (cold climates) | 16–24 in | Per local frost depth, often 24–42 in |
How to plan and pour a trench footing
- Check your local building code for the required footing width, depth, and frost depth for your wall type.
- Dig the trench to the required width and depth, keeping the bottom as level and the walls as straight as practical.
- Enter your trench dimensions above, using a higher waste percentage (10–15%+) than you would for a formed slab.
- Place any required rebar along the length of the trench, supported off the bottom with spacers.
- Pour directly into the trench, using the soil itself as the form, and screed the top level.
- Let the footing cure for several days before building on top of it, per your project’s schedule and local code.
Worked example
- Trench size
- 40 ft long × 12 in wide × 18 in deep
- Volume
- (40 × 12) in × 12 in × 18 in = 103,680 in³ = 60.0 ft³, plus 15% waste = 69.0 ft³
- Cubic yards
- 69.0 / 27 = 2.56 yd³
Common mistakes
- Using the same waste percentage as a formed slab, which underestimates material for an irregular, hand-dug trench.
- Digging the trench narrower or shallower than code requires, which can fail inspection or under-support the wall above.
- Not accounting for cave-ins or oversized digging in loose soil, which increases the actual concrete needed beyond the planned dimensions.
- Skipping rebar in a footing that will support a wall, risking cracking under load.
Assumptions
- Standard premix concrete at roughly 150 lb per cubic foot for the bag-equivalent reference figure.
- Bag yields follow manufacturer averages: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³.
- Trench cross-section is treated as a uniform rectangle along its full length.
- Does not include rebar, forms at the ends, or excavation/spoil removal cost.
Frequently asked questions
How wide and deep should a trench footing be?
It depends on the wall and local frost depth — common residential footings run 12 to 24 inches wide and 8 to 12 inches deep in non-frost areas, with much deeper footings required where frost protection is needed. Always check local code.
Why does a trench footing need more waste allowance than a slab?
A formed slab has precise, straight edges, while a hand-dug or machine-dug trench almost never comes out perfectly uniform. Soil walls bulge, cave in slightly, or run a bit oversize, so 10 to 15% or more overage is common.
Does a trench footing need rebar?
Most footings supporting a structural wall include horizontal rebar for tension strength. Check your local code and engineer’s specification for the required size and placement.