Nothing stalls a weekend concrete pour like running short on material — or paying for a full yard you never needed. Ready-mix suppliers sell concrete by the cubic yard, bagged concrete is rated by yield per bag, and the math between the two comes down to two simple formulas. This guide walks through both with a worked example you can copy for your own slab, patio, or driveway.
Formula 1: Cubic Yards for a Slab
Concrete volume in cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27. Dividing by 12 converts inches to feet; dividing by 27 converts cubic feet to cubic yards, since one cubic yard holds exactly 27 cubic feet. Always add 10% for waste — uneven subgrade and form spread quietly eat volume. You can skip the arithmetic with a free cubic yard calculator that applies the waste factor automatically.
Worked Example: 12×16 ft Patio at 4 Inches
12 × 16 × 4 ÷ 12 = 64 cubic feet. 64 ÷ 27 = 2.37 cubic yards. Add 10% waste and you need about 2.6 cubic yards — so tell the batch plant 2.6 yards (most suppliers round to the nearest quarter yard). Ordering the exact 2.37 figure is the classic beginner mistake; the truck always comes up a little short against real-world subgrade.
Formula 2: Bags From Cubic Yards
Bag yields are standardized: one 80-lb bag of premix yields about 0.6 cubic feet, which works out to 45 bags per cubic yard. A 60-lb bag yields 0.45 cu ft (60 bags per yard), a 50-lb bag 0.375 cu ft (72 per yard), and a 40-lb bag 0.3 cu ft (90 per yard). Multiply your yardage by the bags-per-yard figure — our 2.6-yard patio needs 2.6 × 45 = 117 eighty-pound bags. See the full bag size and coverage chart for water amounts and pallet counts per bag size.
Ready-Mix Truck or Bags?
The contractor rule of thumb: once a project passes roughly 1.5 to 2 cubic yards (about 70–90 eighty-pound bags), a ready-mix truck becomes cheaper and dramatically less labor than hand-mixing. Below that threshold, bags from the home center win on convenience. One warning: most ready-mix suppliers charge a short-load fee of $120–$250 on orders under 4–5 yards, so factor that into small truck orders.
Estimating the Cost
Budget roughly $125–$150 per cubic yard for the concrete itself in most US markets, plus delivery and a 10% contingency for over-excavation. Our 2.6-yard patio therefore lands around $350–$450 in material before any labor. Run the full estimate — yardage, bag counts, and cost — on the free concrete calculator.
Three Mistakes to Avoid
- Skipping the waste factor and ordering exact theoretical volume.
- Measuring thickness in inches but forgetting to divide by 12.
- Using only 10% waste for hand-dug footings or post holes — irregular digging needs 15%.
Measure twice, pour once — and let the calculator double-check your math before you call the supplier.
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