Why the bag count is never what the label implies
An 80 lb bag of concrete mix yields about 0.60 cubic feet once mixed. That number is on the bag, but people rarely use it, because thinking in cubic feet is awkward when you measured the slab in feet and the thickness in inches. The conversion that matters is that a slab's volume in cubic feet equals length times width times thickness divided by twelve. A 10 by 12 pad at 4 inches is 10 x 12 x 0.333, which is 40 cubic feet, or 1.48 cubic yards. At 0.60 cubic feet per bag that is 67 bags before any waste allowance. That is roughly a ton and a half of dry mix, which is the point at which most people stop and call a truck.
Ready-mix is quoted in cubic yards and there are 27 cubic feet in a yard. Plants sell in quarter-yard steps and nearly all of them charge a short-load fee under one yard, sometimes a flat charge that exceeds the price of the concrete itself. That fee is the reason this calculator shows both routes side by side rather than picking one for you.
What the waste allowance is actually covering
The default here is 10 percent, which is a middle figure rather than a safe one. What it covers depends on what you are pouring onto.
| Situation | Typical allowance | Where it goes |
|---|---|---|
| Formed pour on gravel, screeded level | 5% | Spillage, what stays in the mixer or chute |
| Slab on prepared subgrade | 10% | Low spots, slight over-excavation, edge slump |
| Footings in trenched dirt | 15% or more | Trench walls sloughing, uneven bottom |
| Post holes, auger-dug | 15-20% | Bell-out at the bottom, collapsed sides |
The asymmetry matters more than the exact percentage. Running out mid-pour leaves a cold joint you cannot undo, and a second truck means a second delivery charge. Ordering a quarter yard more than you need costs a few dollars and a wheelbarrow of cleanup. Round up.
Post holes and tube forms
Set the shape to circle and enter the diameter of the hole or tube in inches. A 12 inch tube form 4 feet deep takes 3.14 cubic feet, which is a little over five 80 lb bags per pier. Multiply by the number of piers with the quantity field rather than working one out and doing the multiplication in your head, because the waste allowance compounds across them and the rounding to whole bags happens only once at the end.
Auger-dug holes in soft ground are rarely the diameter of the auger. If the sides sloughed, measure the actual hole at the top and bottom and use the larger figure, or push the waste allowance to 20 percent.
What this does not tell you
Volume is the easy part. Thickness, reinforcement, base preparation and control joint spacing are what determine whether the slab cracks, and none of them come out of a volume calculation. A driveway slab carrying vehicles is a different specification from a shed floor even at the same square footage. If the pour is structural, or it supports something that would be expensive to have fail, the sizing belongs with someone who can see the site and the soil.
The mix itself also matters and is not modelled here. Bagged mixes vary in yield between manufacturers by a few percent, fast-setting formulations yield differently again, and a plant's mix design will not match a bag. The yields used here are the common nominal figures: 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. Check the bag you are actually buying if the count is tight.
Questions people ask
How many 80 lb bags make a cubic yard?
Forty-five. A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 cubic feet, so 27 divided by 0.60 gives 45 bags. That is 3,600 lb of dry mix, which is why a yard is the practical ceiling for bag work. For 60 lb bags it is 60 bags per yard, for 50 lb bags 72, and for 40 lb bags 90. Those figures are before any waste allowance, so treat them as the floor rather than the order quantity.
Is it cheaper to use bags or order ready-mix?
Bags win below about half a yard and ready-mix wins above about one yard, with the middle depending entirely on your plant's short-load policy. The comparison people get wrong is that they price the concrete and forget everything else. A yard of bag mix is 45 bags to load, haul, lift and open, plus a mixer rental, plus water, plus the hours. A truck arrives mixed and leaves in fifteen minutes. Call the plant and ask two things: the price per yard, and the short-load fee schedule. Some waive it above a certain quantity, some charge per yard under the minimum, and some have a flat delivery charge that changes the answer completely.
What thickness should a slab be?
It depends on the load, and this calculator does not decide it for you. As rough orientation for what people commonly build: a shed floor or garden pad is often 3 to 4 inches, a walkway 4 inches, a driveway or garage floor 4 to 6 inches over a compacted base, and anything carrying heavy vehicles more than that with engineered reinforcement. Thickness alone does not prevent cracking. Base preparation, reinforcement and control joints cut at the right spacing matter as much or more, and a thick slab on a poorly compacted base will still fail. For anything structural, get the specification from someone who can inspect the site.
Does the calculator include rebar or mesh displacement?
No, and it does not need to. Reinforcement displaces a fraction of a percent of the volume, far less than the waste allowance already covers. What does change the volume meaningfully is thickened edges, which are common on slabs that carry a wall or a vehicle. If your pour has a thickened perimeter, calculate the main slab here, then calculate the edge beam separately as a long narrow rectangle and add the two together.