How Many Bags of Concrete for a 20×24 Slab?
A 20 × 24 ft slab (480 ft²) at 4″ thick needs 6.52 yd³ of concrete — 5.93 yd³ net plus 10% waste. Order 6.75 yd³ ready-mix — plants ticket in ¼-yd³ and most charge a short-load fee under ~5 yd³.
20×24 at other thicknesses
| Thickness | Volume | Order (yd³) | 80 lb bags |
|---|---|---|---|
| 4″ (this page) | 5.93 yd³ | 6.52 yd³ | 294 |
| 5″ | 7.41 yd³ | 8.15 yd³ | 367 |
| 6″ | 8.89 yd³ | 9.78 yd³ | 440 |
Other slab sizes
- How Many Bags of Concrete for a 10×20 Slab?200 ft²
- How Many Bags of Concrete for a 16×16 Slab?256 ft²
- How Many Bags of Concrete for a 20×20 Slab?400 ft²
- How Many Bags of Concrete for a 24×24 Slab?576 ft²
- How Many Bags of Concrete for a 24×30 Slab?720 ft²
- How Many Bags of Concrete for a 30×30 Slab?900 ft²
Order 6.75 yd³ ready-mix — plants ticket in ¼-yd³ and most charge a short-load fee under ~5 yd³.
- Net volume (160.0 ft³)
- 5.93 yd³
- Waste 10%
- + 0.59 yd³
- Ready-mix ticket
- 6.75 yd³
- Approx. weight
- 24,000 lb
- 80 lb bags
- 294
- 60 lb bags
- 392
- 40 lb bags
- 587
- Materials + delivery$125–$180 / yd³ · Sep 2026
- $944 – $1,315
- Installed (with labor)typ. $3,513
- $2,384 – $5,155
Quantities follow the ticket. Links may earn us a commission at no cost to you; they never change a number.
What will this concrete job cost?
See the installed price range for these quantities and get up to 3 quotes from local contractors. Free, no obligation. Also: Driveway cost.
Different size or depth?
Open the full concrete calculator to change every input, pick a material and see truckloads.
How to calculate concrete by hand
Cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27 · 80 lb bags = ft³ ÷ 0.60
Worked example
- Area: 20 ft × 17 ft = 340 ft².
- Volume: 340 ft² × (4 in ÷ 12) = 113.3 ft³ = 4.20 yd³.
- Waste 10%: 113.3 × 1.10 = 124.7 ft³ = 4.62 yd³ → ready-mix ticket 4.75 yd³.
- Bags instead: 124.7 ÷ 0.60 = 208 × 80 lb bags (or 277 × 60 lb, 416 × 40 lb).
- Above 1 yd³, ready-mix costs less than bags even with a short-load fee — order the truck.
- 1 yd³ of concrete
- 45 × 80 lb · 60 × 60 lb · 90 × 40 lb bags
- 1 yd³ at 4″ thick
- ≈ 81 ft²
- 1 yd³ at 6″ thick
- ≈ 54 ft²
- 80 lb bag yield
- 0.60 ft³ · ≈ 1.8 ft² at 4″
Questions crews ask
How many bags of concrete do I need?
Multiply length × width × thickness (in feet) for cubic feet, add 10% waste, then divide by the bag yield: 0.60 ft³ per 80 lb bag, 0.45 per 60 lb, 0.30 per 40 lb. A 10 × 10 ft slab at 4″ is 33.3 ft³ → 36.7 with waste → 62 × 80 lb bags.
How many 80 lb bags of concrete make a yard?
Exactly 45: a cubic yard is 27 ft³ and an 80 lb bag yields 0.60 ft³. It takes 60 × 60 lb bags or 90 × 40 lb bags. At $6–$7 a bag that is $270–$315 per yard in bags versus $125–$180 for ready-mix, which is why trucks win above a yard.
How much does a yard of concrete cover?
81 ft² at 4 inches thick, 65 ft² at 5″, 54 ft² at 6″. Divide 324 by the thickness in inches to get square feet per cubic yard.