Get the ready-mix order in cubic yards and the bag counts if you’re mixing it yourself — with waste already added so you don’t come up a wheelbarrow short at the worst possible moment.
Slab dimensions
ft
ft
in
Concrete needed
—
Cubic feet—
80 lb bags—
60 lb bags—
40 lb bags—
Bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45, 40 lb ≈ 0.30. Ready-mix trucks typically have a 1-yard minimum and charge short-load fees under 4–5 yards — get the yardage right before you call.
The math
Concrete is bought by volume. Multiply the slab's footprint by its thickness in feet, then convert to the units the plant sells in:
cubic feet = L × W × (thickness ÷ 12) cubic yards = cubic feet ÷ 27
Worked example
A 12 × 10 ft shed slab at 4 in thick is 40 cu ft — 1.48 yards, call it 1.63 with 10% waste. As bags, that's a brutal 74 × 80 lb bags (nearly 3 tons to mix by hand). This is the calculation that tells you when to stop being a hero and order the truck: most people's crossover is somewhere around 1 to 1.5 yards.
Why the waste allowance isn't optional
Subgrades are never perfectly flat, forms bow, spillage happens, and a slab that averages 4.25 in instead of 4 quietly eats 6% more mud. Running out mid-pour means a cold joint — a visible, weaker seam — so the 10% cushion is the cheapest insurance on the job.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45 bags. One 80 lb bag yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet. That’s around 3,600 lbs of material — why bagged concrete only makes sense for small pours.
How thick should a concrete slab be?
4 inches is standard for patios, sidewalks and shed slabs; 5–6 inches where vehicles will park or drive. Thickness is the biggest driver of volume, so a 6-inch slab needs 50% more concrete than a 4-inch one.
Should I order ready-mix or mix bags?
Under roughly 1 cubic yard, bags are practical. Above that, ready-mix wins on cost per yard, consistency, and your back — just plan the pour, because the truck’s clock starts when it arrives.