Concrete math is unforgiving because the pour cannot pause: order short and you are hand-mixing while the truck waits. These calculators size the pour, the rebar, the footing, and the block wall before the truck is booked — and the bid tools price the job after.
An 80-lb bag yields 0.60 cu ft — 45 bags per cubic yard, roughly $250 of bags at typical prices against $130–160 a yard for ready-mix. Below about half a yard, bags win on short-load fees; past a yard, the truck wins on price and your back. A 10×12 pad at 4 inches is 1.5 yards — truck territory.
For a 4-inch residential slab, #3 or #4 rebar on 18–24 inch centers both ways is the common spec — the rebar calculator turns slab dimensions and spacing into stick counts and weight. Wire mesh is the lighter alternative for patios; driveways and shop floors want the bar.
Because the blocks get counted and the cores don't. An 8-inch block wall swallows roughly a cubic yard of fill per 100 sq ft of wall when cores are poured solid — the block-fill calculator sizes grout or concrete for the cores separately from the block count itself.