How Much Concrete for a 30x40 Slab?
A 30 ft by 40 ft slab poured 4 inches thick needs about 16.3 cubic yards of concrete before waste, and you would order 16.5 yd³ of ready-mix with a 10 percent allowance. That is 1,200 square feet of slab weighing roughly 66,000 lb once cured.
This is an estimate, not professional advice. Check the inputs against your own plans and local building code before you build or order. See terms and disclaimer.
Every number for this 30x40 slab
| Concrete needed | 16.3 yd³ |
| Order ready-mix (rounded to 0.25) | 16.5 yd³ |
| 80 lb bags | 734 |
| 60 lb bags | 978 |
| 40 lb bags | 1,467 |
| Estimated weight | 66,000 lb |
| Volume with waste | 440 ft³ |
| Metric | 12.46 m³ |
How the number is worked out
Volume (ft³) = Length(ft) × Width(ft) × (Thickness(in) ÷ 12) × SlabsWith waste = Volume × (1 + Waste% ÷ 100)Cubic yards = Volume(ft³) ÷ 27Bags = Volume(ft³) ÷ yield (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45, 40 lb ≈ 0.30)Weight (lb) = Volume(ft³) × 150 (normal-weight concrete)- Area = 40 ft × 30 ft = 1,200 ft²
- Volume = 1,200 ft² × (4 in ÷ 12) = 400 ft³
- Cubic yards = 400 ft³ ÷ 27 = 16.3 yd³
- With 10% waste, rounded to the quarter yard the trucks sell in = 16.5 yd³
- Weight = 400 ft³ × 150 lb/ft³ = 66,000 lb
What this number assumes
A 30x40 pour is a shop, garage or barn floor, and at this size the answer is ready-mix and nothing else. The equivalent in bagged mix is about 734 bags of 80 lb concrete, which is several pallets and a full day of mixing before a single square foot is finished. A slab this big has to go down in one continuous pour to avoid a cold joint, and hand-mixing cannot keep up with that.
The 10 percent waste allowance is not padding. It covers subgrade that is not perfectly level, forms that bow slightly under the weight of wet concrete, spillage at the chute and the concrete left in the truck. Running short in the middle of a 1,200 square foot pour means a cold joint and a second short-load fee, which costs far more than the extra quarter yard.
Thickness is the input that moves the number most, and it is set by what drives on the slab, not by preference. Four inches is the residential floor standard; five to six inches is what a slab carrying vehicles, a hoist or heavy equipment needs. The table below shows what each choice costs you in cubic yards.
A 30x40 slab by thickness
Same 1,200 ft² footprint, waste held at 10 percent. Going from 4 to 6 inches adds half again as much concrete, which is the single biggest line on the quote.
| Thickness | Concrete needed (yd³) | Order ready-mix (rounded to 0.25) (yd³) | 80 lb bags | |
|---|---|---|---|---|
| 4 in (this page) | 16.3 | 16.5 | 734 | Use → |
| 5 in | 20.37 | 20.5 | 917 | Use → |
| 6 in | 24.44 | 24.5 | 1,100 | Use → |
| 8 in | 32.59 | 32.75 | 1,467 | Use → |
Different numbers?
Open the Concrete Slab Calculator with this 30x40 slab already loaded and change anything you like. It runs the same formula shown above.
Method & assumptions
Concrete is sold by the cubic yard (ready-mix) or by the bag (pre-mixed). We compute the slab volume in cubic feet, add your waste allowance, then convert: cubic yards = cubic feet ÷ 27.
Bag counts use the yield printed on U.S. pre-mixed concrete: an 80 lb bag yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³ (QUIKRETE and Sakrete spec sheets). We round bags up, because you cannot buy a partial bag, and round ready-mix up to the nearest quarter yard, which is how it is ordered.
At one cubic yard or more, the result flags a ready-mix comparison because the equivalent bag count is large. It does not prove which option is cheaper or feasible. Confirm thickness from the project requirements, then check product yield, plant minimums, delivery, access, placement capacity and order increments.
Sources and references
These references and structured source records support the method and material data used on this page.
- QUIKRETE Concrete Mix yield (spec sheet)
- ACI 332: Residential concrete (slab thickness)
- Concrete slab (Wikipedia)