Footings and holes are boxes and cylinders, not slabs, and their volumes are small enough that a small measuring error becomes a large ordering error. Shape by shape, with the post subtracted and the bell counted.

Not slabs: why holes and footings need their own math

Slab arithmetic is area times thickness, and it is forgiving: a small error in thickness changes the volume by the same small proportion, and the waste allowance absorbs the rest. Footings, sonotubes, and post holes are not slabs. They are boxes and cylinders, often several of them, often below ground where they cannot be seen, and their volumes are small — which makes them unforgiving in the opposite direction. A sonotube that is 2 inches deeper than planned takes proportionally more concrete, a footing that is 2 inches wider takes proportionally more, and a dozen post holes each a little oversized add up to a whole extra yard. The waste allowance that saves a slab order does not save a footing order unless the shapes were measured right in the first place, because the allowance is a percentage of a small number, and the errors are absolute.

The second difference is what the concrete is doing. A slab spreads a load over a wide area. A footing spreads a concentrated load — a column, a post, a wall — over enough ground that the soil can carry it without settling. A sonotube is a round footing formed by a cardboard tube, used for deck posts and columns, and it carries the load down to undisturbed soil or below the frost line. A post hole holds a post in the ground, and the concrete around it resists sideways movement and uplift as much as it carries downward load. The volumes follow the shapes, and the shapes follow the loads, which is why this guide works shape by shape, with the soil and frost questions stated alongside the arithmetic rather than after it.

Strip footings: length, width, depth, and the trench reality

A strip footing is a continuous trench of concrete under a wall, and its volume is the simplest in this guide: length times width times depth, all in the same unit, divided by 27 for cubic yards. A footing 30 feet long, 18 inches wide, and 8 inches deep is 30 times 1.5 times two-thirds of a foot, which is 30 cubic feet, or 1.11 cubic yards. Add the 10 percent waste allowance the Concrete Calculator guide uses, and the order is 1.22 cubic yards. The arithmetic is trivial; the measurement is where strip footings go wrong, because a trench is never the neat rectangle the drawing shows. The sides cave, the bottom is uneven, the width wanders, and the depth varies with the ground. Measure the width and depth at several points along the trench and use the largest typical figures, not the smallest or the average of the best ones — concrete fills the trench as dug, not as drawn, and the trench as dug is always a little larger than the drawing.

Depth deserves its own paragraph, because a footing’s depth is decided by two different requirements that are often confused. The structural depth is the thickness of the concrete itself, set by the load and the design. The founding depth is how far below ground the footing sits, set by frost and by soil: the footing must bear on undisturbed soil, below the frost line where the local code sets one, so that freeze-thaw cycles cannot heave it. A footing that is thick enough but founded too shallow will move, however well the concrete was mixed. The local code and the soil report, where one exists, set the founding depth; this guide sets only the volume once the depth is known. Where the ground is fill, organic soil, or otherwise unsuitable, the footing must go deeper or wider, and that is an engineering decision, not an estimating one.

Pad footings: count them, then multiply

A pad footing is an isolated block of concrete under a single column or post, usually square or rectangular, and its volume is length times width times depth for one pad, multiplied by the number of pads. Four pads each 2 feet square and 12 inches deep are 4 times 2 times 2 times 1, which is 16 cubic feet, or 0.59 cubic yards, before waste. The counting is the whole discipline: pads are easy to miscount when a deck or a pergola has a mix of pad sizes, or when corner pads differ from intermediate ones, and a miscounted pad is a whole block of concrete missing from the order. List every pad, with its dimensions, before calculating anything, and calculate each size separately. A pad footing that must resist uplift — a deck post in wind, a fence post in a gate — may need to be deeper or wider than a pad carrying only downward load, and the reinforcement, where specified, sits in the pad before the pour, not after it.

Pads and strips combine more often than guides admit. A wall on a strip footing with columns on pads, a deck with a strip under a stair and pads under the posts, a retaining wall with a strip and periodic pads — each combination is calculated as its parts and added, with one waste allowance on the total, not one per part. The habit that keeps combined orders honest is the same as the habit for pads alone: write the list first, calculate second, and check the list against the drawing third. Concrete ordered from a list is checked; concrete ordered from memory is guessed.

Sonotubes: cylinders, and the diameter trap

A sonotube is a round cardboard form, and the concrete in it is a cylinder: volume equals pi times the radius squared times the height. The trap is the diameter. Tubes are sold by diameter, the formula needs the radius, and halving the diameter is the step most often skipped or doubled. A 12-inch tube has a radius of 6 inches, or half a foot. At 36 inches deep, the volume is pi times 0.25 times 3, which is 2.36 cubic feet, or 0.087 cubic yards. Add 10 percent waste and the order is 0.096 cubic yards — about 4 bags of 80-lb concrete, since one cubic yard takes 45 of them, and 2.36 cubic feet divided by the 0.6 cubic feet each 80-lb bag yields is just under 4, rounded up. A single sonotube is a small order, and small orders are where bags beat trucks: the delivery and short-load considerations in the hub guide decide the crossover, and for one or two tubes the answer is almost always bags.

Two more sonotube realities belong in the estimate. The first is the bell. Many footings are belled at the bottom — flared wider than the tube — to spread the load, and a bell is extra concrete that the cylinder formula does not count. If the footing is belled, measure the bell diameter and depth and calculate the flare as a second, wider cylinder, or add a generous allowance for it, and say which was done. The second is the ground. A tube set in a dug hole wider than the tube leaves a gap that concrete will fill if it is poured carelessly, and backfill that is not compacted will settle and leave the tube proud. Set the tube, brace it plumb, backfill and compact around it before pouring, and pour to the marked height, not to the top of the tube by eye. The concrete in a sonotube is only as straight as the tube that formed it.

The table is computed at the cylinder formula with the 10 percent waste allowance, and bag counts use 0.6 cubic feet per 80-lb bag, the yield the hub guide’s 45-bags-per-yard figure implies. It is a planning table, not a specification: the depth is set by frost, soil, and design, the diameter by the load, and the bell, where one exists, is extra. Recompute from the measured diameter and depth for the job in front of you, and the table is a check, not a substitute.

Post holes: the post takes space too

A post hole is a cylinder with a post in it, and the concrete volume is the cylinder minus the post. The gross cylinder for a 12-inch hole 30 inches deep is pi times 0.25 times 2.5, which is 1.96 cubic feet. A nominal 4-by-4 post actually measures 3.5 by 3.5 inches, and the buried 30 inches of it occupy 0.21 cubic feet, so the net concrete is 1.75 cubic feet, or 0.065 cubic yards — 3 bags of 80-lb concrete, rounded up, before waste, and 3 bags with the waste allowance absorbed by the rounding. Forgetting the post over-orders by about a tenth on a single hole, and by a whole bag across a fence line of a dozen posts, which is money and material carried home for nothing.

Post holes have their own depth and shape questions. Depth is set by frost and by the height of the fence or structure: a common rule of thumb is one-third of the post buried, with frost depth as the floor, and the local code as the authority where the two disagree. Shape matters more than depth for holding power: a hole that is wider at the bottom than the top — belled, or dug with a clamshell that flares — resists uplift, while a hole wider at the top than the bottom is a cone that frost can lift. Dig the hole straight or belled, never tapered the wrong way, and set the post plumb and braced before pouring, because a post concreted out of plumb stays out of plumb. Backfill the top few inches with soil rather than concrete where the design allows, so water sheds away from the post instead of pooling against it at the concrete line.

Ordering: bags, trucks, waste, and the cold joint

The bag-or-truck decision for footings and holes is decided by total volume and by access, and the hub guide’s bag math governs it: one cubic yard is 45 bags of 80-lb concrete, so a footing order under about a yard is usually bag work, and an order over a few yards is usually truck work, with the short-load fee deciding the exact crossover. What changes the decision for holes and footings is that the work is often spread out — a dozen holes across a yard, a footing trench around a building — and hand-mixing a dozen small batches is slow enough that the pours set at different rates. A footing poured in two sessions has a cold joint where the sessions meet, and a cold joint is a weak plane unless it is planned, keyed, and bonded. Order enough concrete, and enough hands, to pour each footing or each run of holes in one continuous session, and treat the waste allowance as part of that planning, not as an afterthought: the 10 percent covers form flex, uneven ground, and spills, and on holes and trenches it is the minimum, not the target. Measure twice, list every shape, add the allowance, round up to whole bags or to the truck’s minimum, and pour once.

Frequently asked questions

How much concrete do I need for a sonotube?

Use the cylinder formula: pi times radius squared times height. A 12-inch tube 36 inches deep holds 2.36 cubic feet, or 0.087 cubic yards, which is about 4 bags of 80-lb concrete with the 10 percent waste allowance. Halve the diameter to get the radius before calculating, and add extra for a bell at the bottom, which the cylinder formula does not count.

How do I calculate concrete for a footing?

For a strip footing, multiply length times width times depth, all in the same unit, and divide by 27 for cubic yards. A footing 30 feet long, 18 inches wide, and 8 inches deep is 30 cubic feet, or 1.11 cubic yards, and 1.22 cubic yards with the 10 percent waste allowance. Measure width and depth at several points and use the largest typical figures, because a trench is always a little larger than the drawing.

How many bags of concrete are in a cubic yard?

45 bags of 80-lb concrete, at the yield the hub guide works with: each 80-lb bag yields 0.6 cubic feet, and 27 divided by 0.6 is 45. For other bag sizes, divide the pour’s cubic feet by the yield printed on the bag, add the waste allowance, and round up to whole bags. A small footing order is usually bag work; a large one is usually truck work.

How deep should a footing or post hole be?

Deep enough to bear on undisturbed soil and below the local frost line, where one applies, and deep enough for the load and the height of the structure. A common rule of thumb buries one-third of a post, with frost depth as the floor. The local code and the soil conditions set the founding depth; this guide sets only the volume once the depth is known.

Do I subtract the post when calculating concrete for a post hole?

Yes. The concrete volume is the cylinder minus the buried post. A 12-inch hole 30 inches deep is 1.96 cubic feet gross, and a nominal 4-by-4 post, which actually measures 3.5 by 3.5 inches, occupies 0.21 cubic feet of it, leaving 1.75 cubic feet of concrete. Forgetting the post over-orders by about a tenth per hole, which is a whole bag across a fence line.

What is a bell on a footing, and does it change the volume?

A bell is a flare at the bottom of a footing or hole, wider than the shaft above it, that spreads the load over more soil and resists uplift. It adds concrete that the cylinder or box formula does not count, so measure the bell diameter and depth and calculate the flare as a second, wider cylinder, or add a generous allowance for it. A belled footing without the bell in the estimate is a short order.