Concrete & masonry calculators
Footing Concrete Calculator
Footings are the part of the project nobody sees and everybody depends on. Whether it is a continuous foundation footing, a row of deck post footings, or a dozen fence posts, the question is the same: how much concrete goes in the hole? Under-order and you are mixing emergency bags while the first batch starts setting.
This calculator treats a footing as what it is — a rectangular prism — and gives you cubic yards (rounded up to the nearest quarter-yard, with 5% waste built in) plus the bag count for 80, 60, or 50-lb bags. Bag yields come from the Quikrete spec sheet. One important note: this tool sizes concrete volume only. Footing dimensions, depth, and rebar are code questions — check your local building department before you dig.
- Volume
- 32.0 cu ft
- As ready-mix
- 1.25 cubic yardswith 5% waste, rounded up to ¼ yard
- Or in 80-lb bags
- 54 bagseach yields ~0.6 cu ft (Quikrete #1101)
How we calculate this (with your numbers)
Volume = 24 ft × (16 ÷ 12) ft × (12 ÷ 12) ft = 32.00 cu ft
Cubic yards = 32.00 ÷ 27 = 1.19 yd³ → with 5% waste and rounding up: 1.25 yd³
Bags = 32.00 ÷ 0.6 = 53.3 → rounded up: 54 bags
How the math works
Worked example
A footing 24 ft long, 16 in wide, 12 in deep, using 80-lb bags:
- Volume = 24 × (16 ÷ 12) × (12 ÷ 12) = 24 × 1.333 × 1 = 32 cu ft
- Cubic yards = 32 ÷ 27 = 1.19 yd³ → with 5% waste = 1.24 → order 1.25 cubic yards
- Bags = 32 ÷ 0.60 = 53.3 → 54 eighty-pound bags
Enter these exact numbers into the calculator above and you will get the same result — that is the point. No black box.
Limitations
Frequently asked questions
It depends on your frost line, soil, and what the footing carries — there is no universal number, and local building code governs. As very rough context, residential deck footings are commonly 12 inches wide and dug below the local frost line (often 24–48 inches deep in cold climates). Always check your local code before digging: an undersized footing is a structural problem, not a math problem.
Structural footings (foundations, retaining walls, anything carrying a building) almost always require rebar per code, and usually an inspection before the pour. Simple post footings for a fence or mailbox often do not. When in doubt, ask your local building department — this calculator sizes the concrete, not the steel.
Concrete reaches roughly 70% of its design strength at 7 days (per ACI 308R strength-gain curves) and full design strength at about 28 days. Light framing can typically proceed after a few days; heavy loads should wait. Keep the concrete moist for the first week — curing matters more than most DIYers expect.
Usually one to two 80-lb bags. A typical hand-dug hole (about 10 inches across, 24 inches deep) needs roughly 0.9 cubic feet of concrete after the post itself displaces its share — and an 80-lb bag yields about 0.60 cubic feet (Quikrete #1101 spec sheet). Wider augered holes need more. Buy one extra bag per few posts; dry bags keep for the next project.
Yes — this calculator adds 5% automatically. Footing trenches are never perfectly rectangular: the excavator over-digs, the soil crumbles, and bell-shaped hole bottoms eat extra concrete. For hand-dug post holes in rocky soil, consider 10%.
Related calculators & guides
- Concrete calculator — cubic yards for slabs and flatwork
- Concrete bag calculator — bag counts by volume or dimensions
- Concrete curing timeline — when you can build on it, drive on it, load it
- How thick should a concrete slab be? — 4 vs 5 vs 6 inches by use case