How Much Concrete Do I Need? Slabs, Footings, and Columns

The first real number on most of my jobs is a concrete number. Footings, a shop slab, a row of sonotube piers — before any of it gets poured, somebody has to decide how much to order. Order short and you get a cold joint or a second delivery charge. Order long and you are paying to truck away mud. After enough pours you learn to get this within a fraction of a yard, and the math behind it is genuinely simple once you see it laid out.

The one formula that covers almost everything

Concrete is sold by volume, so every estimate comes down to volume = length × width × thickness. The only trick is keeping your units honest. Slabs and footings are specified in a mix of units on purpose: length and width in feet, thickness in inches. That is how the trade talks, and it is also where most bad estimates come from, because people multiply feet by inches and get a number that is twelve times too big or too small.

Convert the thickness to feet first. Four inches is 4 ÷ 12 = 0.333 feet. A 12 ft × 12 ft slab at 4 inches is 12 × 12 × 0.333 = about 48 cubic feet. Divide cubic feet by 27 to get cubic yards, because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. So that slab is roughly 1.78 cubic yards. If you would rather not juggle the conversions by hand, the

concrete volume calculator

keeps length and width in feet, thickness in inches, and spits out cubic yards, cubic feet, and cubic metres at once.

Footings: small dimensions, big mistakes

Footings catch people because the numbers feel small. A footing might be 16 inches wide, 8 inches deep, and run 40 feet. Convert: 16 inches is 1.33 feet, 8 inches is 0.667 feet. Volume = 40 × 1.33 × 0.667 = about 35 cubic feet, or 1.3 cubic yards. That is a full short-load minimum at most suppliers, so even a modest footing run is worth ordering by truck rather than mixing by hand.

The bigger footing trap is the subgrade. You almost never dig a footing to exactly its nominal dimensions. The trench is a little wider here, a little deeper there, and all of that extra space fills with concrete. I measure the form, not the drawing, and then I still add a waste percentage on top.

Columns and piers: round volume

Round work — sonotubes, bell piers, deck footings — uses a different shape but the same idea. Volume = π × radius² × height. A 12 inch sonotube is 6 inches of radius, which is 0.5 feet. At 4 feet tall: 3.14159 × 0.5² × 4 = about 3.14 cubic feet per tube, or 0.116 cubic yards. Pouring eight of them? That is roughly 0.93 cubic yards, right at the point where ready-mix makes sense. The calculator has a round-column mode so you enter diameter and height and skip the radius step entirely.

Bags versus ready-mix

Bagged concrete is fine for a few post holes or a small pad. An 80 lb bag yields about 0.60 cubic feet, and a 60 lb bag about 0.45 cubic feet. To fill a single cubic yard with 80 lb bags you need 27 ÷ 0.60 = 45 bags. That is 3,600 pounds of material to haul, open, and mix one at a time. Past about one cubic yard, ready-mix delivery is almost always cheaper and far faster — and it gives you a continuous pour instead of dozens of mixed batches that can set at different rates.

The waste factor that earns its keep

This is the part people skip and regret. I add 5% on clean, formed work over a flat, compacted base, and 10% on anything poured on rough ground or into an over-dug trench. The reason is simple: concrete does not stop flowing where your tape measure says it should. Spillage, a sloppy chute, a subgrade that swallows half a wheelbarrow — it all adds up, and the cost of running out is not just the second delivery fee. A pour that stops and restarts an hour later forms a cold joint, a weak seam where the two batches never bonded. The waste allowance exists to keep the truck on site until the form is full.

A quick worked example

Say I am pouring a 20 ft × 24 ft shop floor at 5 inches thick. Thickness is 5 ÷ 12 = 0.417 feet. Volume = 20 × 24 × 0.417 = 200 cubic feet, divided by 27 is 7.4 cubic yards. Add 7% for waste and I order 7.9, which rounds up to 8 cubic yards at the supplier. That single number — eight yards — is the whole point of the exercise, and it is exactly what the

concrete volume calculator

hands you once you enter the dimensions and a waste percentage.

Order in the supplier's increments

One last practical wrinkle: ready-mix is not sold in arbitrary amounts. Most suppliers batch and bill in quarter-yard or half-yard increments, set a minimum load — often one cubic yard or so — and add a short-load fee below that minimum. So after you calculate your figure and add waste, round up to the next increment your supplier actually pours. On my eight-yard shop floor I am not going to quibble over a tenth of a yard; I order eight and know the form will be full. On a tiny job that calculates to 0.7 yards, I am paying the one-yard minimum either way, which is another reason small pours often tip toward bags. Knowing your supplier's minimum and increment before you call turns a clean volume number into a clean order — and a price you can predict instead of one you find out at delivery.

What the calculator does not do

It gives you gross volume of the form. It does not pick your mix strength — 25 or 32 MPa, 3,000 or 4,000 psi — which changes price but not volume. It does not subtract for large voids or block-outs; calculate those separately. And it cannot see your subgrade. Use the result as your order figure, lean toward the high side of the waste range when the ground is rough, and keep the truck there until the last corner is full.