Slabs, footings, piers and stairs. Add every pour on the job, get one order and a materials list you can hand to the supply house.
Every shape resolves to cubic feet, then divides by 27 for cubic yards. Waste is applied once, to the combined total, so adding pours never compounds the allowance.
Length × width × thickness. A 4″ slab is 0.333 ft thick, a 6″ slab is 0.5 ft.
Total run × width × depth. Measure the run along the centerline of the trench and the corners take care of themselves.
π × radius² × height, multiplied by the number of piers. Tube forms are sized by inside diameter, so a 12″ tube is 12″ of concrete.
Each step is treated as a slab sitting on the one below it, so step n carries n × rise of depth. Solid stairs poured on grade, no void underneath.
| Bag | Yield | Bags per yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Yields are for standard sand-mix and high-early bagged concrete. Fast-setting and countertop mixes run different, so check the bag.
Half a yard is the practical ceiling for bags — that is roughly 23 eighty-pound bags, or most of a morning. Past a yard, a short load is cheaper than the labor even after the short-load fee, and you get a consistent mix instead of 45 separate ones.
Suppliers sell in quarter-yard increments and charge a short-load fee under 3 to 4 yards, so it is worth checking whether rounding up to the fee threshold costs less than paying it.
Forty-five. An 80 lb bag yields about 0.60 ft³ and a yard is 27 ft³.
Ten percent covers spillage and normal subgrade variation. Go to fifteen on rough or hand-dug subgrade where the bottom is uneven. Five is enough on formed work over compacted gravel.
Almost always subgrade. An extra half inch of depth across a 20×20 slab is a third of a yard on its own. Screed the base before you order.