Every bag of pre-mixed concrete looks similar on the pallet, but the three common sizes yield very different volumes and cost different amounts per cubic foot. Choosing well is the difference between a sensible materials bill and paying double for the same slab — not counting the lifting.
Yields, memorise these three
An 80 lb bag yields about 0.60 cubic feet of concrete. A 60 lb bag yields about 0.45. A 40 lb bag yields about 0.30. That means a cubic yard (27 cu ft) is roughly 45, 60 or 90 bags respectively — the numbers that should drive every bag-versus-delivery decision.
Price per cubic foot, not per bag
The 80 lb bag is almost always the cheapest per cubic foot, and the 40 lb bag the most expensive — often close to double. Smaller bags make sense for small, staged jobs (setting posts one weekend at a time) or where lifting 80 lb repeatedly is unrealistic. For anything bigger, price the 80 lb option first.
The ready-mix crossover
Count the bags for your pour. Somewhere around 80 to 100 bags — roughly two cubic yards — a ready-mix truck usually becomes cheaper before you even value your own labour, and it delivers one consistent mix that can be placed in an hour instead of a day of mixing. Between 40 and 80 bags is the judgement zone: access, help available and how fast you can work decide it.
Mixing and curing basics that affect quantity
Mix to the water quantity printed on the bag — extra water makes placing easier and the finished concrete weaker, permanently. Have everything ready before the first mix: an out-of-level base found mid-pour is fixed with your concrete, at bag prices. Keep the finished slab damp for several days and it will cure markedly stronger.
Frequently asked questions
Can I mix different bag brands in one slab?
It is workable but not ideal — mixes vary slightly in colour and strength. For a visible slab, one brand and one batch throughout looks better.
How many bags do I need for a fence post?
Typically one to two 50 lb bags of fast-setting post mix per post, depending on hole size. A hole three times the post width and about a third of the post length deep is the standard target.