Bag counts look like a lookup table and are actually one short calculation: work out the volume, divide by what one bag yields, round up. Everything else is unit conversion.

This guide walks that calculation end to end, explains where the published bag yields come from, and covers the two things that catch people out - the rounding, and the point at which bags stop being the sensible way to buy concrete. The worked figures use the same constants the site's calculators use, so the numbers here and the numbers on a calculator page agree by construction.

Step one: get the volume in cubic feet

Every shape ends in the same place - a volume - and the only trick is getting the units consistent first. Thickness is almost always quoted in inches while length and width are in feet, so the thickness has to be converted before anything is multiplied. Four inches is a third of a foot; six inches is a half.

For a rectangular slab or pad, volume is length × width × thickness, all in feet. A footing is the same prism with different words. A round pier is the area of the circle times the height. A post hole is the hole volume minus the volume the post occupies, which is why a post-hole answer is always slightly smaller than the hole itself suggests.

Cubic feet is the useful working unit because bag yields are published in cubic feet. Cubic yards matter only when you switch to ready-mix, and the conversion is exact: 27 cubic feet to the cubic yard. Cubic metres come from the exact foot-to-metre conversion cubed.

  • Slab or pad: length (ft) × width (ft) × thickness (ft).
  • Round pier or sonotube: π × radius² × height, all in feet.
  • Post hole: hole volume minus the buried volume of the post.
  • Convert inches to feet first - dividing by 12 at the end of a multiplication chain is where errors hide.

Step two: divide by the bag yield

A bag of dry pre-mix yields substantially less placed concrete than its weight suggests, because you are buying dry material that will be compacted with water into a denser solid. The published yields for the common North American sizes are consistent and easy to remember: a 40 lb bag makes about 0.30 ft³, a 60 lb bag about 0.45 ft³, and an 80 lb bag about 0.60 ft³.

Those three numbers are not independent. They are the same density expressed three times - 0.0075 cubic feet of placed concrete per pound of dry mix - which is the constant this site's calculators use. It reproduces all three published figures exactly and extends sensibly to metric bag weights, landing at or just below the manufacturers' published metric yields. Erring slightly low is the safe direction: it orders slightly more material rather than slightly less.

So the bag count is volume ÷ yield, rounded up to a whole bag. A 10 ft × 10 ft slab at 4 in is 33.33 ft³; divided by 0.60 that is 55.6, which rounds to 56 eighty-pound bags. Divided by 0.45 it is 74.1, which rounds to 75 sixty-pound bags. Those are exactly the figures the site's worked example for that slab publishes.

Yields do vary a little between products. The number printed on the bag in front of you beats any published constant, and it is worth checking on a large order where a five per cent difference is several bags.

Rounding, waste, and why the count is never 55.6

You cannot buy 0.6 of a bag, so every bag count rounds up. That single rounding step already builds in a small surplus on most jobs - on average half a bag - which is worth knowing before you add a waste allowance on top of it.

A waste allowance is a separate, deliberate decision. Spillage, over-excavation, an uneven sub-grade and a form that is a little out of true all consume material that never appears in the geometry. On a small pour a modest percentage is prudent; on a large pour the same percentage is a lot of bags, and it is worth being specific about which of those risks actually applies.

There is one failure mode worth naming because it is invisible: floating-point rounding in the calculation itself. Sixty pounds times 0.0075 evaluates to 0.44999999999999996 rather than 0.45, and a naive round-up turns an exact 40-bag job into 41. This site's test suite recomputes the bag count for its slab guides from unrounded dimensions and demands the exact figure, precisely because a phantom bag is the kind of error nobody notices.

When to stop buying bags

Bags are convenient, need no scheduling and let you mix in stages. They are also the most expensive way to buy concrete per unit volume, and mixing them is real physical work: a site mixer of the usual size takes about two 80 lb bags per batch, so a hundred-bag job is roughly fifty batches.

Ready-mix flips every one of those trade-offs. It arrives proportioned and mixed, it is cheaper per cubic yard, and it turns a day of mixing into an hour of placing - but it is sold by the cubic yard with a minimum load and a short-load surcharge below roughly three to four cubic yards, and it arrives on the truck's schedule rather than yours.

The crossover is usually somewhere around one to two cubic yards, and it is as much about labour and continuity as about price. A slab that has to be poured in one continuous placement is an argument for ready-mix regardless of size, because a cold joint through the middle of a slab is a defect you cannot mix your way out of.

Every figure here is a planning estimate. Actual quantities vary with over-excavation, sub-grade loss, spillage, waste and the exact yield printed on your bag.

A worked example, start to finish

Take a 12 ft × 12 ft slab at 4 in. Convert the thickness first: 4 in is 0.3333 ft. Volume is 12 × 12 × 0.3333 = 48 ft³.

In cubic yards that is 48 ÷ 27 = 1.78 yd³ - already past the point where a ready-mix quote is worth getting, though comfortably inside short-load territory.

In bags: 48 ÷ 0.60 = 80 exactly for 80 lb bags, and 48 ÷ 0.45 = 106.7, so 107 sixty-pound bags. Note how much better the 80 lb figure looks purely because the arithmetic lands on a whole number - that is a coincidence of this particular slab, not a property of the bag size.

At roughly two 80 lb bags per mixer batch, 80 bags is around 40 batches. That number, rather than the price, is usually what decides the question.

Frequently asked questions

How much concrete does an 80 lb bag make?

About 0.60 cubic feet of placed concrete. A 60 lb bag makes about 0.45 ft³ and a 40 lb bag about 0.30 ft³. All three follow the same density - roughly 0.0075 cubic feet per pound of dry mix - so they scale consistently. Check the yield printed on your bag, since it varies a little by product.

How many bags of concrete for a 10x10 slab?

A 10 ft × 10 ft slab at 4 in thick is 33.33 cubic feet, which is 56 eighty-pound bags or 75 sixty-pound bags after rounding up. At 6 in thick the same slab is half as much again. The worked example page for that slab shows the full breakdown in every bag size.

Should I add extra for waste?

Rounding up to whole bags already leaves a small surplus - about half a bag on average. Beyond that, a waste allowance covers spillage, over-excavation and an uneven sub-grade, so it depends on the job rather than on a fixed percentage. On a large pour, a blanket allowance becomes a lot of bags.

When is ready-mix cheaper than bags?

Usually somewhere above one to two cubic yards, but the decision is as much about labour as price: bags are cheaper to start and much more work to mix. Ready-mix is sold by the cubic yard with a minimum load and a short-load surcharge below roughly three to four cubic yards. A pour that must be continuous argues for ready-mix at any size.

How do I convert cubic feet to cubic yards?

Divide by 27 - there are exactly 27 cubic feet in a cubic yard. For cubic metres, multiply cubic feet by 0.0283. Bag yields are published in cubic feet, so cubic feet is the natural working unit until you switch to ready-mix, which is quoted by the yard.