How Many Bags of Concrete for a 18×20 Slab?
A 18×20 concrete slab poured at 4 inches thick requires ~4.89 cubic yards (or 221 eighty-pound bags / 294 sixty-pound bags) including 10% waste. Because this exceeds 2.5 yards, ordering ready-mix truck delivery (~0.5 loads) is typically 40% cheaper and saves hours of manual labor.
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Project Dimensions & Specs
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Takeoff Schedule Summary
| Item | Quantity | Unit | Est. Cost |
|---|---|---|---|
| Ready-Mix (4000 PSI) | 4.89 | yd³ | $709 |
| ↳ Bag alternative | 221 | 80-lb | ($1,437) |
| #4 Rebar Grid (12" OC) | 44 | 20' sticks | $638 |
| Crushed Stone Base (4") | 6.85 | tons | $240 |
| 10-Mil Vapor Barrier | 1 | roll(s) | $135 |
Material Takeoff & Accessory Checklist
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| 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 | 4.89 cu yd | $782 | Local Supplier | |
#4 (1/2") Grade 60 Rebar (20 ft Sticks) Grid reinforcement spaced 12" on-center both ways | accessory | 44 pieces (20') | $638 | Home Depot | |
Rebar Tie Wire (Pack of 500 loop ties) 16-gauge bar ties for intersections + manual tie twister | fastener | 1 packs | $20 | Amazon | |
Heavy Duty Rebar Chairs (2" - 3" height) Maintains proper concrete cover above ground | accessory | 45 units | $43 | Local Supplier | |
Crushed Stone Base (#57 or 3/4" Crusher Run) 4" compacted sub-base for drainage and stability | accessory | 6.85 tons | $288 | Local Supplier | |
10-Mil Heavy Duty Polyethylene Vapor Barrier 10' x 100' roll, prevents capillary moisture ingress into slab | accessory | 1 rolls | $135 | Home Depot | |
1/2" x 4" Foam Expansion Joint Strips Perimeter expansion buffer for sidewalks, footings, and walls | accessory | 2 rolls (50') | $44 | 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: 4 inches ÷ 12 = 0.333 ft.
- 2Calculate raw slab volume: 20 ft × 18 ft × 0.333 ft = 120.00 cu ft.
- 3Convert to cubic yards: 120.00 ÷ 27 = 4.444 cu yds.
- 4Apply 10% safety allowance for uneven subgrade and form deflection: 4.89 cu yds total.
- 5Calculate 80-lb pre-mixed bags: 4.89 cu yd ÷ 0.0222 cu yd/bag = 221 bags.
- 6Calculate #4 rebar grid @ 12" on-center: 44 pieces (20ft rods) = ~880 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).
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Frequently Asked Questions for 18×20 Slabs
How many 80 lb bags do I need for a 18x20 slab?
You will need approximately 221 bags of 80 lb concrete mix. This accounts for a standard 10% waste and spillage buffer.
How many 60 lb bags for a 18x20 slab?
If using 60 lb bags, you will need approximately 294 bags. 1 cubic yard requires 60 bags of 60 lb mix.
How much rebar is needed for a 18x20 concrete slab?
With a standard #4 (1/2") rebar grid spaced 12 inches on center in both directions, you need approximately 44 sticks of 20-foot rebar and 399 tie points.
How much gravel base is required?
A recommended 4-inch crushed stone sub-base requires ~6.85 tons (4.89 cubic yards) of #57 gravel or crusher run.