Add each run of trench, then set the width and depth once — they apply to every run. Measure along the centreline, not the outside edge, or the corners get counted twice.
A strip footing is a long box. Add up the trench length, multiply by the width and depth of the cross-section, and you have the volume. The only real trap is how you measure the length.
Take a 30 × 20 ft building with a 16 in wide footing. Using outside dimensions gives a perimeter of 100 ft, but each corner then gets counted twice — once going along and once going across. The overlap is one footing width squared at each of the four corners.
Measuring along the centreline avoids it automatically. For that building the centreline perimeter is about 94.7 ft, which is roughly 5% less concrete. On a bigger footing the error grows.
A slab sits on a surface you can screed flat. A trench has two sides that slump inward and a bottom that follows whatever the excavator left. Every irregularity fills with concrete, and none of it can be scraped back. Crews who under-order footings end up with a cold joint in a structural element, which is far worse than one in a patio.
| Assumption | Value used |
|---|---|
| Yield of an 80 lb bag of concrete mix | 0.60 ft³ |
| Yield of a 60 lb bag | 0.45 ft³ |
| Yield of a 25 kg bag | 0.0118 m³ |
| Full ready-mix truck | 10 yd³ (7.6 m³) |
| Supplier order increment | 0.25 yd³ |
| Cross-section applied to every run | yes |
Footing dimensions are set by your local building code and the bearing capacity of the soil, not by this calculator. Confirm width and depth with your building department or engineer before you dig.
A common starting rule is twice the width of the wall it carries, with thickness at least equal to the wall width. Soil conditions and local code decide the real number — check both.
No, as long as every length is measured along the centreline. The centreline path already runs continuously through each corner exactly once.
Yes. 24' 6 1/2" works for run lengths, and widths like 1' 4" read the same as 16.