How many bags of concrete you really need, and the point where ready-mix gets cheaper

Quikrete's data sheet for its standard Concrete Mix lists yields of about 0.30 cubic feet for a 40-pound bag, 0.45 for 60 pounds and 0.60 for 80 pounds. Divide the pour volume in cubic feet by the yield. The 10 x 10 slab at 4 inches is 33.3 cubic feet, or 55.6 bags of 80-pound mix. Sakrete's estimating guide says to add 10% to every estimate, which brings the slab to 62 bags, or 4,960 pounds of dry mix to lift.

Running short in the middle of a pour is how small jobs go wrong. Quikrete tells users to fill forms to full depth and work continuously from one end, because partial fills can leave cold joints. It also calls for at least 1 cubic foot of mixer capacity for each 80-pound bag in a batch and 6 to 9 pints of water per bag.

Then price it both ways. BuildHowMuch found Home Depot selling Quikrete's 80-pound bag for $6.47 in September, about $291 for the 45 bags in a yard. Angi puts ready-mix at $110 to $150 a yard plus a short-load fee of $40 to $60 a yard for orders below a full truck of about 10 yards. At those rates, the 1.35 yards the slab needs with 10% extra cost roughly $200 to $285 from a truck, against about $400 for 62 bags.

Fee structures vary, so ask the plant how it bills small orders. HomeGuide says some suppliers charge $30 to $60 for each yard below the truck's minimum rather than each yard delivered. For the 1.35-yard slab against a 10-yard minimum, that comes to $260 to $519 in fees alone, more than the bags.