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.
Take the tables off your phone
The Field Reference Pack$15
NEC ampacity & voltage-drop tables, stair and rafter math, concrete/rebar takeoff, drywall, BTU, board feet, excavation — plus estimate sheets. 11-page PDF for the truck.
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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.
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