Buildculus
Concrete Footing Calculator
Size a wall or pad footing, dial in the waste factor, and walk away with bag counts and a total pour volume ready to print.
Most pours need 5–10% extra to cover spillage, uneven subgrade, and formwork. Skipping this often means a costly second trip.
Enter dimensions to calculate.
| Footing size (L × W × T) | Volume | 60 lb bags | 80 lb bags |
|---|---|---|---|
| Wall footing, short run: 10 ft × 16″ × 8″ | 9.8 cu ft | 22 | 17 |
| Wall footing: 20 ft × 16″ × 8″ | 19.6 cu ft | 44 | 33 |
| Wall footing, long run: 40 ft × 16″ × 8″ | 39.1 cu ft | 87 | 66 |
| Wide wall footing: 20 ft × 24″ × 12″ | 44.0 cu ft | 98 | 74 |
| Pad footing, 2×2 ft: 2 ft × 24″ × 10″ | 3.7 cu ft | 9 | 7 |
| Pad footing, 3×3 ft: 3 ft × 36″ × 12″ | 9.9 cu ft | 22 | 17 |
Multiply by your footing count for the full job — or use the calculator above for exact numbers.
How to measure
For a wall footing, measure the total run of the trench in feet or meters — along the centerline, adding up every straight segment around the perimeter. Width and thickness come off the plan or the trench itself, in inches or centimeters; measure the trench where it's widest, since over-excavation is concrete you'll pour whether you planned to or not. For pad footings, enter one pad's dimensions and use the count field for how many identical pads you're pouring.
The math
A rectangular footing is a prism, so volume is length × width × thickness × number of footings. Convert every measurement into the same units before multiplying — width and thickness are usually in inches while length is in feet — then convert the result into cubic yards or cubic meters. All the footings are summed into one total before bag counts are rounded up, the same way a single continuous pour would be batched.
Why add a waste percentage?
Trench pours waste more concrete than formed work. Trench bottoms are rarely flat, so the footing runs thicker in the low spots; sidewalls cave and over-excavate past the plan width, and every extra inch of width is concrete the plan never counted; and dry soil absorbs water and paste from the mix. The default 10% covers a clean machine-dug trench in firm soil — bump it to 15% for hand-dug trenches, loose or sandy soil, or a first pour.
Bagged mix vs. ready-mix
Bagged concrete is convenient for small pours under roughly half a cubic yard. Above that, ready-mix delivered by truck is almost always cheaper per cubic yard and avoids cold-joint problems from mixing one bag at a time.
Frequently asked questions
- How is footing concrete calculated?
- A rectangular footing is a simple prism: length × width × thickness, multiplied by the number of identical footings in the job. Convert every measurement into the same units before multiplying — footing width and thickness are usually quoted in inches, so divide each by 12 to get feet — then divide the total cubic feet by the yield printed on the bag's spec sheet (an 80-lb bag of concrete mix yields about 0.6 cubic feet; check the manufacturer's data sheet). Add a waste allowance and round up, since you can't buy a partial bag. The calculator above does all of this for any dimensions, count, and waste factor.
- I'm pouring a pier or post base — should I use this calculator?
- No — use our concrete post-hole calculator instead. Round piers, sonotube pours, and post bases are cylinders, and that calculator handles the π × r² × depth math, post displacement, and per-hole counts directly. This calculator covers rectangular pad and wall footings. If your design combines a rectangular pad with a pier on top, run the pad here and the pier there, and add the bag counts.
- How wide and thick does a footing need to be?
- It depends on the load and the soil's bearing capacity. The International Residential Code sets minimum footing sizes in Section R403 based on the number of stories, construction type, and soil class — and local amendments frequently override the base tables. As a practical matter, light-frame residential wall footings commonly run 16–24 inches wide and 8–12 inches thick, but that's a starting point for conversation with your building department, not a design. Anything carrying real structure should be sized from the adopted code or by an engineer.
- How deep does a footing have to be?
- The bottom of an exterior footing must sit below the local frost line — IRC Section R403.1.4 covers minimum depth — and on undisturbed soil or properly compacted fill, never on topsoil or loose backfill. Frost depth varies enormously by region, from effectively zero along the Gulf Coast to four feet or more in the northern Midwest, and the adopted value is set by your local jurisdiction. Call your building department for the number before you dig; it usually drives the excavation far more than the footing thickness itself.
- Do footings need rebar?
- Usually, yes. Two continuous horizontal bars — typically #4 — near the bottom of a wall footing is the common residential pattern, and pad footings often take a grid in each direction. Requirements come from the adopted code and the soil conditions: IRC Chapter 4 and ACI 332 address residential footing reinforcement, and seismic or expansive-soil areas mandate more. Rebar chairs or dobies keep the steel off the soil so it ends up inside the concrete, not under it.
- How much waste should I add for a footing pour?
- Ten percent is a safe default and is what this calculator uses out of the box. Footings poured into earth trenches lose more concrete than formed work: trench bottoms are rarely flat, sidewalls over-excavate past the plan width, and dry soil wicks water and paste out of the mix. For neatly formed footings you can drop toward 5%; for hand-dug trenches, sandy or crumbly soil, or a first pour, 10–15% is more realistic.
- Bagged mix or ready-mix for footings?
- Footing volume adds up faster than people expect — a continuous wall footing around even a small addition can pass a cubic yard, which is the usual break-even point where ready-mix becomes cheaper than bags. More importantly, a structural wall footing should be poured in one continuous operation; mixing dozens of bags one at a time risks cold joints between batches. Bags make sense for isolated pad footings and short runs, while for a full-perimeter footing you should price a ready-mix delivery first.
- What if my footings are different sizes?
- Run the calculator once per size group — all footings that share the same length, width, and thickness — and add the bag counts together. Because each run rounds up to whole bags separately, the summed total can come out a bag higher than a single combined pour would need; treat that as part of your buffer rather than a number to shave. If the groups will be poured as one ready-mix delivery, add the cubic-yard volumes instead and let the supplier round the load.