How to Use This Tool
Pick the shape, enter the dimensions, and set your local prices. The volume is exact arithmetic; the interesting part is what it costs in each of the two ways you can buy it.
Bag yield is not on the front of the bag
Bags are sold by dry weight, but what you need is wet volume, and the two are related by a yield figure printed in small type on the back. The three common sizes work out at the same 0.0075 cu ft per pound, so the choice between them is about handling rather than value.
volume 10 x 10 x (4/12) = 33.33 cu ft = 1.235 cu yd +10% = 36.67 cu ft = 1.358 cu yd 80 lb bags 36.67 / 0.60 = 62 bags, 4,960 lb 60 lb bags 36.67 / 0.45 = 82 bags, 4,920 lb 50 lb bags 36.67 / 0.375 = 98 bags, 4,900 lb
The break-even with ready-mix
Ready-mix is priced per cubic yard plus a short-load fee for anything under the truck's minimum, often four or five yards. Bags have no fee but a much higher price per unit volume. That gives a crossover, and it is usually somewhere between one and two cubic yards.
Below the crossover, bags win on money. Above it, the truck wins on money and on everything else. What the arithmetic cannot show is that 60 bags is a full day of mixing for two people, and a slab poured in sections over a day has cold joints where the sections meet.
Post holes deduct the post
A 10 inch hole 36 inches deep is 1.64 cu ft, but if a 4x4 post is sitting in it the concrete only fills the space around the post. On a single hole that is a rounding error; on a 30-post fence it is several bags, so the post size is an input.
Why the waste allowance is not optional
Subgrade is never as flat as the drawing. Forms bow slightly, the gravel base has hollows, and the last barrow always seems to disappear. Five per cent is realistic on a formed and screeded slab, ten per cent on typical ground, and more on a slope. Running out with the pour half finished is the one outcome worth paying to avoid, because a cold joint is permanent.
Strength and curing are separate questions
This tool sizes volume, not mix design. Bagged concrete is usually around 4,000 psi at 28 days, which is fine for slabs and footings. Anything structural, or any slab carrying a vehicle, should be checked against local code rather than a calculator.
