Concrete Footing Calculator: Volume, Bags & Yardage

How the volume is worked out

A footing is a box, so the volume is length times width times depth — with the inches turned into feet first. The default footing is 40 ft long, 18 in wide, and 12 in deep: 18 inches is 1.5 ft and 12 inches is 1 ft, so 40 × 1.5 × 1 is 60.00 ft³. There are 27 ft³ in a cubic yard, so 60.00 divided by 27 is 2.22 yd³ — and ready-mix is sold by the yard, so that is the number you order. The waste allowance is 10% on top: 60.00 times 1.10 is 66.00 ft³, or 2.44 yd³. The bags are the volume divided by the bag yield: a 60-lb bag yields 0.45 ft³, so 60.00 divided by 0.45 is 133.33, rounded up to 134 bags; an 80-lb bag yields 0.60 ft³, so 60.00 divided by 0.60 is 100.00, rounded up to 100 bags. The cost is the bags times the bag price — 100 times $4.50 is $450.00 — against ready-mix at $150 a yard times the unrounded 2.22 yd³, which is $333.33.

The worked example, line by line

Setup: continuous footing, 40 ft long, 18 in wide, 12 in deep, waste 10%, 80-lb bags at $4.50, ready-mix at $150/yd³.

Line 1 — inches to feet. 18 in = 1.5 ft; 12 in = 1 ft.

Line 2 — volume. 40 × 1.5 × 1 = 60.00 ft³.

Line 3 — yards. 60.00 / 27 = 2.22 yd³.

Line 4 — waste. 60.00 × 1.10 = 66.00 ft³ (2.44 yd³).

Line 5 — 60-lb bags. 60.00 / 0.45 = 133.33 → 134 bags; with waste 146.67 → 147 bags.

Line 6 — 80-lb bags. 60.00 / 0.60 = 100.00 → 100 bags; with waste 110.00 → 110 bags.

Line 7 — bagged cost. 100 × $4.50 = $450.00.

Line 8 — ready-mix cost. 2.22 yd³ (unrounded) × $150 = $333.33.

Line 9 — the verdict. $450.00 − $333.33 = $116.67 — ready-mix is cheaper.

Every branch, checked

Small footing, 20 ft × 12 in × 8 in: 12 inches is 1 ft and 8 inches is 0.67 ft, so 20 × 1 × 0.67 is 13.33 ft³, or 0.49 yd³. The bags are 13.33 divided by 0.45, which is 29.63, rounded up to 30 sixty-pound bags; or 13.33 divided by 0.60, which is 22.22, rounded up to 23 eighty-pound bags. A job this small is a bagged job: a ready-mix truck has a delivery fee and often a short-load charge below one yard.

Piers, 4 × 24" × 24" × 12": each pier is 2 ft × 2 ft × 1 ft, which is 4 ft³, and four of them are 16.00 ft³ — 0.59 yd³. The bags are 16.00 divided by 0.45, which is 35.56, rounded up to 36 sixty-pound bags; or 16.00 divided by 0.60, which is 26.67, rounded up to 27 eighty-pound bags.

Wide footing, 40 ft × 24 in × 12 in: 24 inches is 2 ft, so 40 × 2 × 1 is 80.00 ft³, or 2.96 yd³. The bags are 80.00 divided by 0.45, which is 177.78, rounded up to 178 sixty-pound bags; or 80.00 divided by 0.60, which is 133.33, rounded up to 134 eighty-pound bags. Six more inches of width adds a third to the concrete: 60.00 ft³ becomes 80.00 ft³.

Metric, 10 m × 0.5 m × 0.3 m: 10 × 0.5 × 0.3 is 1.50 m³, and 1 m³ is 35.3147 ft³, so 1.50 times 35.3147 is 52.97 ft³ — 1.96 yd³. The bags are 52.97 divided by 0.60, which is 88.29, rounded up to 89 eighty-pound bags.

Reverse — 20 eighty-pound bags on hand: 20 times 0.60 is 12.00 ft³, and an 18-inch by 12-inch footing has a cross-section of 1.5 ft × 1 ft, which is 1.5 ft², so 12.00 divided by 1.5 is 8.00 ft of footing. Allow 10% waste and it is about 7.27 ft.

The honest flip: bagged concrete stops making sense past a yard

Bagged concrete is cheap per bag and expensive per yard: 100 eighty-pound bags at $4.50 are $450.00 for 2.22 yd³, while ready-mix is $333.33 for the same concrete — ready-mix wins by $116.67. The flip is the labour: 100 bags is 8,000 lb of concrete to mix by hand, bag by bag, and a footing pour has to stay wet enough to bond — mixing for hours while the first bags set is how a footing gets a cold joint. The honest rule is that bagged concrete is for jobs under about a yard — the small 0.49 yd³ footing, a few piers — and ready-mix is for everything bigger. The other flip is the waste allowance: 10% sounds generous until the trench is dug a little wide, and a trench dug 2 inches wide over 40 ft is an extra 6.67 ft³ — 11.12% of the whole pour. Measure the trench after it is dug, not before. See the concrete slab calculator for flat pours, and the concrete mix calculator for the cement, sand, and gravel behind a bag.

What the defaults assume

The defaults are a continuous footing 40 ft long, 18 in wide, and 12 in deep, with a 10% waste allowance, 80-lb bags at $4.50 each, and ready-mix at $150 a yard. The calculator assumes a rectangular cross-section dug to the sizes you enter — a trench dug wider or deeper needs more concrete, and the waste allowance is there for exactly that. It assumes the bag yields are 0.45 ft³ for a 60-lb bag and 0.60 ft³ for an 80-lb bag, which are the standard yields for ordinary bagged concrete mix. It assumes piers are square: the width you enter is both sides of the pier. It does not include rebar, formwork, gravel, or labour, and it does not include the ready-mix delivery fee or short-load charge — ask the plant, because both change the bagged-versus-ready-mix verdict on small jobs. The ready-mix cost is worked out from the unrounded yardage — 2.22 yd³ times $150 is $333.33 — because that is what the plant charges for, while the displayed 2.22 yd³ is the rounded figure.

Rounding convention, stated plainly: volumes are rounded to two decimals — 60.00 ft³, 2.22 yd³ — and bag counts are always rounded UP to whole bags, because there are no partial bags: 133.33 becomes 134 and 100.00 becomes 100. The ready-mix cost uses the unrounded yardage — 2.22 yd³ × $150 = $333.33 — so it can differ by a dollar from the rounded yardage times the price. Money is rounded to cents.

Output · formula · meaning

Who uses this

• The homeowner pouring a wall footing: has a 40 ft trench, 18 in wide and 12 in deep, and learns it needs 60.00 ft³ — 100 eighty-pound bags or 2.22 yd³ of ready-mix for $333.33, which is $116.67 cheaper than bagged.
• The deck builder: needs four 24-inch piers and learns they take 16.00 ft³ — 27 eighty-pound bags — so the whole pier job is a bagged job, not a truck.
• The small-job do-it-yourselfer: has a 20 ft footing and learns it is 13.33 ft³ — 23 eighty-pound bags — and that a ready-mix truck would cost more in delivery than the concrete is worth.
• The metric builder: measures in metres and learns a 10 m × 0.5 m × 0.3 m footing is 1.50 m³, which is 52.97 ft³ and 1.96 yd³ — so the bag count and the ready-mix order agree in either unit.

Accuracy & limitations

The math is exact for the numbers you enter: the volume is length times width times depth, with inches turned into feet; the yards are the feet divided by 27; the waste volume is the volume times one plus the waste percent; the bags are the volume divided by the bag yield, rounded up; the costs are the bags times the bag price and the unrounded yards times the ready-mix price. But the inputs carry the assumptions. The trench is never exactly the size you planned: a trench dug 2 inches wide over 40 ft adds 6.67 ft³, which is why the 10% waste allowance exists — measure after digging, not before. The bag yields of 0.45 ft³ and 0.60 ft³ are the standard yields for ordinary bagged mix; a high-strength or fast-setting mix can yield a little less. Piers are assumed square: a round pier, like a sonotube, needs pi times the radius squared times the depth instead — see the concrete tube calculator. The calculator does not include rebar, formwork, gravel, or labour, and it does not include the ready-mix delivery fee or short-load charge, which change the bagged-versus-ready-mix verdict on jobs under a yard. It does not check the footing size against the building code: the width and depth a footing needs depend on the soil and the load, and a footing that is too small can settle. No studies or statistics are cited on this page, because none are needed: every figure is arithmetic on the numbers you enter.