Boneyard Tools

Concrete slabs: bags, ready-mix and how much to order

From slab dimensions to cubic yards and bag counts, why bag numbers climb fast, and when a ready-mix truck wins on cost and time.

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From dimensions to volume

Concrete is ordered by volume, so every estimate starts with length times width times thickness. The catch is units: length and width are in feet but thickness is usually in inches, so the thickness has to be divided by 12 to convert it to feet before multiplying. A 10 by 10 foot slab at 4 inches works out to 10 times 10 times one third of a foot, or about 33.33 cubic feet. Dividing by 27 turns that into cubic yards, the unit a ready-mix plant uses.

Why bag counts climb so fast

Bagged mix is sold in small yields, so the count grows quickly with volume. A 60 lb bag makes only about 0.45 cubic feet and an 80 lb bag about 0.60 cubic feet, which means a single cubic yard needs roughly 60 or 45 bags. Our tool rounds the count up to whole bags because you cannot pour part of one. A modest 10 by 10 foot patio already lands near 75 sixty-pound bags, which is a lot of mixing, so the numbers make the case for ready-mix on their own.

Bags versus a ready-mix truck

For footings, post holes, small pads and repairs, bags win because you mix only what you need with no minimum order. Once a job passes about one cubic yard, a ready-mix truck usually costs less per yard and pours continuously, which gives a stronger slab with no cold joints between batches. Trucks do carry a delivery fee and often a short-load charge under a full load, so weigh the bag count, your time and the mixing effort against the truck price for jobs in the one to two yard range.

Waste, thickness and a safe order

Real ground is never perfectly flat, so some concrete is always lost to an over-dug edge, a low spot or spillage off the barrow. A waste allowance of 5 to 10 percent covers that, and the calculator defaults to 5 percent. Thickness drives cost as directly as area: going from a 4 inch patio to a 6 inch driveway raises the volume by half for the same footprint. Order slightly long rather than short, because running out mid-pour means a visible seam and a weaker slab.

Frequently asked questions

How long do I have to work with concrete before it sets?

Standard mixes start to stiffen within about 30 to 60 minutes and are hard to work after that, faster in hot weather. Have your forms, tools and helpers ready before you add water or the truck arrives.

Does a thicker slab always need rebar?

Not always, but reinforcement helps control cracking and is common in driveways and structural slabs. Wire mesh or rebar is cheap insurance; check your local code for footings and load-bearing work.

How much water do bagged mixes need?

Follow the bag label, since too much water weakens the cured concrete. As a guide, a 60 lb bag needs roughly 2.5 to 3 quarts of water, but mix to a stiff, workable consistency rather than a soupy one.