Size the pour, count the bags, and see when a ready-mix truck makes more sense.
slab: length × width × thickness ÷ 27 = cubic yards
post hole: (π × radius² × depth) − post volume, per holeConcrete weighs about 150 lb per cubic foot wet, which is 4050 lb per cubic yard. A quick sanity check for a 4 in slab: one cubic yard covers 81 sq ft, so square footage divided by 81 gives you the yards.
The post itself takes up space in the hole. A 4x4 post in a 12 in hole displaces about 14% of the concrete. This calculator subtracts it, so check whether whatever you compare against does the same — an estimate built on the raw hole volume will have you buying bags you cannot fit in the hole.
Bag yields: an 80 lb bag makes 0.60 cu ft, a 60 lb bag 0.45 cu ft, a 40 lb bag 0.30 cu ft — so 45, 60 and 90 bags respectively make a cubic yard. Fifty-pound bags vary by manufacturer, somewhere between 72 and 75 bags per yard, so check your own bag rather than trusting a single figure.
A 10 ft by 10 ft slab at the standard 4 in thickness is 1.23 cubic yards, or about 56 eighty-pound bags. Add the usual 5–10% waste and you are ordering 1.3 cubic yards. That is right at the point where a ready-mix truck starts to make sense.
45 bags at 80 lb, 60 bags at 60 lb, or 90 bags at 40 lb. Fifty-pound bags vary by manufacturer, somewhere between 72 and 75. That is a lot of mixing either way — it is the main practical reason people switch to ready-mix, quite apart from the cost.
Under about 1.5 cubic yards, buy bags — a truck would add a short-load fee that cancels out its price advantage. Over about 2 cubic yards, order ready-mix: bagged concrete costs roughly twice as much per finished yard, and mixing two yards by hand is a full day of work against a pour that takes under an hour. Between those two figures, get a quote both ways.
4 in for cars and SUVs, which covers most homes. Go to 5–6 in if heavy trucks or a large RV will park on it, and thicken the outside edges by an extra inch or two — edges crack first because they have less soil supporting them from the sides.
About a third of the fence's above-ground height, so 24 in for a 6 ft fence, and three times the post width across. In cold regions the hole should also reach below the frost line, or freezing groundwater will lift the post over a few winters.
About 150 lb per cubic foot, or roughly 4,050 lb per cubic yard wet. Cured concrete is a little lighter at 3,400–3,600 lb per yard as the mix water leaves. Worth knowing before you load bags into a car — 45 eighty-pound bags is 3,600 lb.