How Many Bags of Concrete for a 24×40 Slab?
A 24×40 concrete slab poured at 5 inches thick requires ~16.3 cubic yards (or 734 eighty-pound bags / 978 sixty-pound bags) including 10% waste. Because this exceeds 2.5 yards, ordering ready-mix truck delivery (~1.7 loads) is typically 40% cheaper and saves hours of manual labor.
Interactive Workstation & Complete BOM
Reset to Blank Hub →24x40 Slab Material Takeoff
Project Dimensions & Specs
Client-side reactive engine. Adjust parameters to recalculate quantities immediately.
Takeoff Schedule Summary
| Item | Quantity | Unit | Est. Cost |
|---|---|---|---|
| Ready-Mix (4000 PSI) | 16.3 | yd³ | $2,364 |
| ↳ Bag alternative | 734 | 80-lb | ($4,771) |
| #4 Rebar Grid (12" OC) | 115 | 20' sticks | $1,668 |
| Crushed Stone Base (4") | 18.26 | tons | $639 |
| 10-Mil Vapor Barrier | 2 | roll(s) | $270 |
Material Takeoff & Accessory Checklist
Click any unit price to customize for your local supplier costs.
| Item / Specification | Category | Quantity | Unit Price | Total Est. | Supplier / Order |
|---|---|---|---|---|---|
Ready-Mix Concrete (4000 PSI) Delivered via transit mixer truck, includes 10% safety overage | primary | 16.3 cu yd | $2,608 | Local Supplier | |
#4 (1/2") Grade 60 Rebar (20 ft Sticks) Grid reinforcement spaced 12" on-center both ways | accessory | 115 pieces (20') | $1,668 | Home Depot | |
Rebar Tie Wire (Pack of 500 loop ties) 16-gauge bar ties for intersections + manual tie twister | fastener | 3 packs | $60 | Amazon | |
Heavy Duty Rebar Chairs (2" - 3" height) Maintains proper concrete cover above ground | accessory | 120 units | $114 | Local Supplier | |
Crushed Stone Base (#57 or 3/4" Crusher Run) 4" compacted sub-base for drainage and stability | accessory | 18.26 tons | $767 | Local Supplier | |
10-Mil Heavy Duty Polyethylene Vapor Barrier 10' x 100' roll, prevents capillary moisture ingress into slab | accessory | 2 rolls | $270 | Home Depot | |
1/2" x 4" Foam Expansion Joint Strips Perimeter expansion buffer for sidewalks, footings, and walls | accessory | 3 rolls (50') | $66 | Local Supplier |
Calculation Mechanics: Concrete Slab & Sub-Base
Formulas adhere to ASTM standards, ACI 318 guidelines, and standard construction estimating practice.
Step-by-Step Derivation Breakdown:
- 1Convert depth to feet: 5 inches ÷ 12 = 0.417 ft.
- 2Calculate raw slab volume: 40 ft × 24 ft × 0.417 ft = 400.00 cu ft.
- 3Convert to cubic yards: 400.00 ÷ 27 = 14.815 cu yds.
- 4Apply 10% safety allowance for uneven subgrade and form deflection: 16.3 cu yds total.
- 5Calculate 80-lb pre-mixed bags: 16.3 cu yd ÷ 0.0222 cu yd/bag = 734 bags.
- 6Calculate #4 rebar grid @ 12" on-center: 115 pieces (20ft rods) = ~2300 linear feet.
Field Inspection & Trade Best Practices
- For driveways and slabs taking vehicle loads, 4000 PSI concrete at 5" minimum thickness with #4 rebar is recommended over unreinforced 3.5" flatwork.
- Pouring over a compacted 4" crushed gravel sub-base prevents slab settling, frost heave, and cracking.
- Place control joints at intervals no greater than 2.5 to 3 times the slab thickness in feet (e.g., maximum 10-foot spacing for a 4" slab).
Need a Licensed Subcontractor to Build This?
Request competitive estimates from insured, pre-screened Concrete & Masonry trade contractors near your job site. Real quotes based on your exact calculated takeoff volume.
Frequently Asked Questions for 24×40 Slabs
How many 80 lb bags do I need for a 24x40 slab?
You will need approximately 734 bags of 80 lb concrete mix. This accounts for a standard 10% waste and spillage buffer.
How many 60 lb bags for a 24x40 slab?
If using 60 lb bags, you will need approximately 978 bags. 1 cubic yard requires 60 bags of 60 lb mix.
How much rebar is needed for a 24x40 concrete slab?
With a standard #4 (1/2") rebar grid spaced 12 inches on center in both directions, you need approximately 115 sticks of 20-foot rebar and 1025 tie points.
How much gravel base is required?
A recommended 4-inch crushed stone sub-base requires ~18.26 tons (13.04 cubic yards) of #57 gravel or crusher run.