What a bag actually yields
Yield is printed on the bag, not implied by the weight. Construclytic’s planning yields are 0.30 ft³ for 40 lb, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb. Real yield moves with slump and aggregate, so counts always round up.
Post holes and sonotubes are the classic bag job: a few 80 lb sacks per hole, mixed next to the pier. A 10 × 10 patio at 4 in is already ~1.2 yd³ net — 55-plus 80 lb bags before waste.
When the truck is the cheaper crew
Past about 1 yd³, bag mixing is a labour decision. You still have to place, screed and finish inside the same window as a small load. Plants sell a minimum; a leftover quarter-yard on a truck is usually cheaper than a half-day of mixing.
Use bags when you cannot get a chute to the pad, when you are filling discrete holes, or when the spec names a bagged product. Use the slab or footing calculator first so the bag table is waste-in, not a guess.
Do it in order
- Calculate net volume from the real shape, then apply waste.
- Read the bag table next to the ready-mix order. If 80 lb bags exceed roughly 45, you are in truck territory.
- Replace the default yield with the selected product’s labelled yield before you buy.
Worked example — 1.5 yd³, no waste
A 12 ft diameter, 4 in circular pad with 0% waste orders 1.5 yd³. That is 135 of 40 lb or 68 of 80 lb bags. Most crews will call the plant.
Planning boundary
Bag counts are planning yields. They are not a mix design, a PSI guarantee or a labour-hour estimate.
Can I mix bag brands in one pour?
Not if you care about colour or set. Stay on one product and one water window.
Does bag size change the cubic yards?
No. Volume is volume. Bag size only changes how many sacks you lift.
What about volumetric mix trucks?
They meter on site. You still start from a waste-adjusted volume; you do not skip the takeoff.