Estimate ready-mix or bagged concrete for continuous strip footings — enter the total length of all runs, set the width and depth, and get the order quantity with overage priced in.
Quick answer: Multiply total length × width × depth in feet, divide by 27. A 40-ft run at 16 in wide × 8 in deep: (40 × 1.333 × 0.667) / 27 = 1.32 cubic yards → 60 eighty-pound bags net (about 66 with a 10% waste allowance). Ready-mix delivery usually wins above ~1–2 yards.
Ballpark, not gospel. Trench conditions change the real pour volume, and ready-mix prices vary by market — confirm sizing against your local building code and pricing with your supplier before ordering.
| Project | Width | Depth |
|---|---|---|
| Garden / landscape wall | 16 in | 8 in |
| Detached garage / shop | 18–20 in | 10 in |
| House / basement wall | 20–24 in | 12 in |
Typical ranges, not building code — footing width is usually about twice the wall thickness. Frost depth varies by region and footings must bear below the local frost line, so check your local code before sizing or pouring.
Straight answers, no fluff. Then run your own numbers above.
Multiply the total footing length × width × depth in feet and divide by 27. A 40-ft run at 16 in wide × 8 in deep: (40 × 1.333 × 0.667) / 27 = 1.32 cubic yards, or 60 eighty-pound bags net. Add up every footing run into one total length before you calculate.
Depths of 8 to 12 inches are typical for residential footings, but the governing factor is frost depth: footings must bear below the local frost line, which varies widely by region. Check your local building code before you dig or pour — an undersized or shallow footing is an expensive fix later.
Footing width is typically about twice the wall thickness, with 16 to 24 inches covering most residential walls. Wider footings spread the load over more soil, which matters on soft or clay ground. The table above lists typical sizes by project.
A footing is a continuous strip of concrete under a foundation or bearing wall that spreads the wall's load onto the soil; a slab is a flat concrete surface you walk or drive on. Footings are poured first, then walls are built on them. They are sized differently, so use this calculator for footings and the concrete calculator for slabs.
Below about 1 to 2 cubic yards, mixing bagged concrete on site is usually practical; above that, ready-mix delivery is typically cheaper per yard and goes down faster. Footing trenches are awkward to work in, so ready-mix also wins on labor. The calculator shows both bag counts and ready-mix cost.
Add the lengths of every run together and enter the total — the calculator only needs one length number. Keep width and depth consistent across runs; if a run has different dimensions, calculate it separately and add the yards together.
A 4- to 6-inch layer of compacted gravel under the footing is typical practice: it levels the trench bottom, improves drainage, and gives a firm bearing surface. It does not replace proper depth — the footing still has to sit below the frost line.
Order 5 to 10 percent extra. Trench walls cave, forms flex, and trench bottoms are never perfectly flat, so footings consume more concrete than the math says. The calculator above applies a 10 percent waste allowance by default.
Pouring a slab instead of a footing? Use the concrete calculator — and the concrete-bag calculator prices small bag-only jobs. Sizing the gravel base under the footing? See how much gravel under a concrete slab.