How many bags of concrete do I need for a slab?
By ConcreteLab · Published June 10, 2026 · Updated June 10, 2026
Divide the slab's waste-adjusted volume in cubic feet by the bag yield — about 0.60 cubic feet per 80-lb bag — and round up to whole bags: a 10 × 10 ft slab poured 4 inches thick works out to 62 80-lb bags, 82 60-lb bags, or 123 40-lb bags including a 10% waste factor.
The three-step method behind every bag count
Every bag estimate is the same three steps. First, compute the slab's volume in cubic feet: length × width × thickness, with the thickness converted from inches to feet. A 10 × 10 ft slab poured 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.33 cubic feet. Second, add a waste factor — this site's calculator defaults to 10%, which takes the example to 36.66 cubic feet. Third, divide by the yield of your bag size and round up to whole bags: at roughly 0.60 cubic feet per 80-lb bag, the slab lands at 62 bags. The same waste-adjusted volume takes 82 60-lb bags or 123 40-lb bags, because smaller bags hold proportionally less mix.
The yields are manufacturer numbers, not guesses. QUIKRETE's product data for ordinary pre-mixed concrete (Concrete Mix No. 1101, checked 2026-06-10) puts an 80-lb bag at about 0.60 cubic feet, a 60-lb bag at about 0.45, and a 40-lb bag at about 0.30 — a consistent 0.0075 cubic feet per pound. Those are the exact constants this site's calculator uses, so every count in this guide is the calculator's own output, not a rounded rule of thumb.
Bag counts for common slab sizes
All of the following include the default 10% waste factor and were computed with this site's engine at a 4-inch pour depth. A 4 × 4 ft stoop pad (5.33 cubic feet formed) needs 10 80-lb bags, 14 60-lb bags, or 20 40-lb bags. A 5 × 10 ft walkway section (16.67 cubic feet) needs 31, 41, or 62. An 8 × 10 ft shed base (26.67 cubic feet) needs 49, 66, or 98. The 10 × 10 ft patio (33.33 cubic feet) needs 62, 82, or 123. And a 12 × 14 ft patio (56 cubic feet) needs 103, 137, or 206.
Two reference points put those numbers in context. A full cubic yard — 27 cubic feet, the unit ready-mix suppliers quote — takes 45 80-lb bags, 60 60-lb bags, or 90 40-lb bags. And the 8 × 10 ft shed base already sits at 0.99 cubic yards formed, which is exactly where bagged concrete stops being the obvious choice and a ready-mix delivery starts to compete.
Why the waste factor and the round-up are not optional
The formed volume is a lower bound on what the pour will consume. Subgrade is never perfectly flat, so a "4-inch" slab averages thicker than 4 inches wherever the gravel dips; mix is lost to the wheelbarrow, the mixing drum, and spillage at the forms. Running short mid-pour is the expensive failure — a cold joint where fresh concrete meets half-set concrete — which is why adding 5–10% is standard practice across the trade and why this calculator applies the waste factor before it counts bags.
The round-up matters for the same practical reason: you cannot pour 61.1 bags. The calculator always rounds up to the next whole bag, and it rounds once, on the final waste-adjusted volume — not row by row — so the overage stays as small as the bag size allows. On a stoop pad the round-up costs at most one extra bag; unused, unopened bags can generally go back to the store.
The weight check: when bags stop making sense
Bag counts hide a logistics problem, so convert to pounds before committing. The 10 × 10 ft patio's 62 80-lb bags are 4,960 pounds of material to load, haul, and lift; the 12 × 14 ft patio's 103 bags are 8,240 pounds. Every bag has to be mixed with water — by hand or in a small drum mixer — and placed before the earlier batches start to set, because a slab needs to be one continuous pour.
The practical crossover is about one cubic yard. Below it, bags win: no delivery minimum, no short-load fee, no schedule window. Above it, a ready-mix truck delivers one consistent, continuously placed mix and usually costs less per yard than the equivalent 45-plus bags. The bag counts in this guide are still worth computing past that point — they are the clearest way to see why a 2.28-cubic-yard pour like the 12 × 14 ft patio belongs on a truck.
Choosing between 80, 60, and 40 lb bags
For the same slab the total weight of mix is nearly identical whichever size you pick — the choice trades lifting comfort against trip count and mixing batches. The 4 × 4 ft stoop pad illustrates it cleanly: 10 bags at 80 lb, 14 at 60 lb, or 20 at 40 lb for the same 5.86 waste-adjusted cubic feet. Larger bags usually cost less per cubic foot of yield and halve the number of mixing cycles; smaller bags are easier on the back and mix completely in a wheelbarrow.
One caution: these counts apply to ordinary pre-mixed concrete only. Fast-setting post mix, mortar, sand mix, and high-early-strength blends publish their own yields per bag, so running this slab math with a different product's bag count will be wrong. Check the yield on the data sheet for the exact product you are buying, and keep the mix wetness reasonable — yields assume the water ratio printed on the bag.
Questions
- How many 80-lb bags do I need for a 10 × 10 slab?
- 62 bags at a 4-inch thickness, including the calculator's default 10% waste factor — the slab forms 33.33 cubic feet, 36.66 with waste, divided by the 0.60 cubic-foot yield of an 80-lb bag and rounded up.
- How many bags of concrete make a cubic yard?
- 45 80-lb bags, 60 60-lb bags, or 90 40-lb bags. A cubic yard is 27 cubic feet, divided by each bag size's published yield.
- Do these bag counts work for fast-setting concrete?
- No. The counts here use the published yields for ordinary pre-mixed concrete. Fast-setting post mix, mortar, and sand mix have different yields per bag, so check the data sheet for the specific product.
- What slab thickness should I plug in?
- 4 inches is the common convention for patios, walkways, and shed bases; 5 to 6 inches for driveways and vehicle loads. Those are conventions, not a design — structural slabs are governed by local code.