A 50-foot continuous wall footing at 24 inches wide and 12 inches deep needs 3.70 cubic yards of concrete — 4.07 yards once you add 10% waste, which means a ready-mix order, not a bag project. Multiply 4 yards (rounded to your supplier’s minimum increment) by your local per-yard ready-mix price, add any short-load fee, and you have your material budget.
That’s the answer. Here’s how to get to it for your own dimensions — and how to know when your footing is small enough that bags make sense after all.
How to measure a footing
Three numbers, but measure each one the right way:
Length. For a straight run, measure end to end. For a perimeter footing — a garage, an addition, a full foundation — measure along the centerline of the trench, not the outside face. Measuring the outer perimeter double-counts every corner by the footing’s full width; on a rectangular layout that’s an extra 4 × width of phantom length.
Width. Measure the trench at its widest point. Trenches dug in real soil aren’t uniform, and concrete fills whatever hole exists, not the hole on your drawing.
Depth. Use your plan depth — the designed thickness from the bottom of the trench to the top of the pour. Footing depth is governed by frost depth and soil conditions in your jurisdiction; IRC Section R403 sets the minimums, and your local amendments may go deeper. Confirm with your building department before you dig.
The math
Volume is length × width × depth, with everything in feet first:
- 24 in ÷ 12 = 2 ft wide; 12 in ÷ 12 = 1 ft deep
- 50 × 2 × 1 = 100 cubic feet
- 100 ÷ 27 = 3.70 cubic yards (about 2.83 m³)
Round up once, at the end of the whole job — not once per wall segment. If your footing has four runs, sum the volumes and then round; ceiling each segment separately can quietly add a bag or two you don’t need. The concrete footing calculator handles this rounding for you and does the imperial–metric conversion in both directions. For a job with footings of different sizes, run each size group separately — the calculator’s FAQ covers how the rounding works out.
Why you add waste — and how much
Trenches waste more concrete than forms do. The bottom of a machine-dug trench is never dead flat, buckets over-excavate at corners, and dry soil wicks water and paste out of the mix at the interface. That volume is gone before you ever screed.
Use 10% for a machine-dug trench in stable soil — the default in the footing calculator. Go to 15% for hand-dug trenches or sandy, sloughing soil, where over-excavation and absorption both run higher. On the 50-foot footing, 10% takes you from 100 to 110 cubic feet — 4.07 yards. Order 4 to 4.5 depending on your supplier’s increments; running a half-yard short on a continuous footing means a cold joint in a structural element, which is a far more expensive problem than a little extra concrete.
Why this footing is a truck, not a pallet
Run the same 110 cubic feet through bag math and the answer makes itself. An 80-lb bag of concrete mix yields about 0.60 cubic feet (per Quikrete’s published data sheet), so this pour is 184 eighty-pound bags with waste — 167 net — or 245 sixty-pound bags. That’s over seven tons of material to haul, stage, mix, and place fast enough that a 50-foot monolithic pour doesn’t cold-joint between batches. At 3.7+ yards — roughly half a ready-mix truck — the truck wins on cost, consistency, and your back.
When bags do make sense: deck pads
Scale down and the logic flips. Say you’re pouring eight 2 ft × 2 ft pad footings, 12 inches deep for a deck: 8 × (2 × 2 × 1) = 32 cubic feet net — about 1.19 yards. That’s below most ready-mix minimums or deep into short-load-fee territory. At 5% waste (formed pads, not open trench), you need 56 eighty-pound bags — two pallets, mixable one pad at a time with no cold-joint risk because each pad is its own pour. If your deck design uses round piers or post bases instead of square pads, the post-hole calculator handles the cylinder math — and our 10×36 post-hole walkthrough works the bag counts for the common pier sizes.
Footing first, then everything on top
The footing is the first pour, not the only one. If a slab follows — garage floor over the stem wall, shed slab inside the footprint — size it separately with the concrete slab calculator; footing and slab waste behave differently and shouldn’t share one estimate. Our 10×10 slab walkthrough covers the slab side — base prep, joints, and the bags-vs-truck call at slab scale.
For your own dimensions — any length, wall or pad, imperial or metric — the concrete footing calculator gives you volume, bag counts, and weight in one pass, with waste applied the same way this article does it.