How to calculate concrete for a slab
Concrete is ordered by volume, so every slab comes down to length × width × thickness. Measure in feet, convert the thickness from inches to feet (divide by 12), and multiply. Divide the cubic feet by 27 to get cubic yards — the unit ready-mix plants sell in. For bags, divide the cubic feet by what one bag yields.
Worked example — a 10 × 10 ft patio, 4 in thick: 10 × 10 = 100 sq ft. 100 × (4 ÷ 12) = 33.3 cu ft = 1.23 yd³. With 10% extra that is 36.7 cu ft; an 80-lb bag makes 0.60 cu ft, so 36.7 ÷ 0.60 = 61.1 → 62 bags.
How many bags make a cubic yard?
Quikrete and Sakrete publish the same yield for each bag weight in their technical data sheets. A cubic yard is 27 cu ft:
| Bag size | Yield per bag | Bags per cubic yard | Bags per 100 sq ft at 4 in |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | 56 |
| 60 lb | 0.45 cu ft | 60 | 75 |
| 50 lb | 0.375 cu ft | 72 | 89 |
| 40 lb | 0.30 cu ft | 90 | 112 |
Two numbers that circulate online are wrong: a bag does not make half a cubic foot of anything but a 50-lb bag (0.375), and a pallet of forty-two 80-lb bags is 25.2 cu ft — 0.93 yd³, not a full yard. Buy 45 bags per yard plus your overage.
Common slab sizes
| Slab | Cubic yards | 80-lb bags (+10%) | 60-lb bags (+10%) |
|---|---|---|---|
| 10×10 ft, 4 in | 1.23 | 62 | 82 |
| 12×12 ft, 4 in | 1.78 | 88 | 118 |
| 10×20 ft, 4 in | 2.47 | 123 | 163 |
| 20×20 ft, 4 in | 4.94 | 245 | 326 |
| 24×24 ft, 4 in | 7.11 | 352 | 470 |
Bags or ready-mix?
Bags cost more per yard (about $260–$290 a yard in 80-lb bags at October 2026 prices) but have no fees. Ready-mix runs roughly $146–$285 per yard on 2026 plant price sheets — then come the extras: a short-load fee ($50–$200) when you order less than the plant's minimum (often 4–6 yd³), a delivery or zone fee, and a fuel surcharge. That is why small slabs are usually cheaper in bags, and anything over about 2 yd³ is usually cheaper — and far less work — from a truck. The calculator above runs the comparison with prices you can change.
Labor matters too: one 80-lb bag takes 3–4 minutes in a mixer, so 60 bags is a long, hard day for two people, and you must keep placing concrete against wet concrete to avoid cold joints.
How thick should the slab be?
- Walkways, patios, shed bases: 4 in. The IRC minimum for slabs on ground is 3.5 in — the height of a 2×4 form.
- Hot tub pads: 4–6 in; check your spa manufacturer's spec. A filled tub with people can exceed 5,000 lb.
- Driveways and garage floors: 5–6 in, with #4 rebar for heavy vehicles. The IRC calls for 3,500 psi concrete in garage floors where weathering is severe.
Don't forget base, forms and reinforcement
Most slabs sit on 4 in of compacted gravel that extends about 6 in beyond the edges (Quikrete's patio method: place about 5 in, tamp down to 4). Forms are usually 2×4s (3.5 in) or 2×6s (5.5 in) staked every 3 ft. Reinforcement is optional for patios but a grid of #3 bar at 18 in, or #4 at 12 in for driveways, sitting on chairs in the middle-to-upper third of the slab, controls cracking. Cut control joints about every 6–8 ft (Quikrete) or groove panels every 3–4 ft on walks (Sakrete).
Pouring the floor of a heated room? The model IRC (R506.2.3 in 2021, R506.3.3 in 2024) puts a 6-mil polyethylene vapor retarder, lapped 6 in, under heated slabs. Patios, walks, driveways, garages and carports are exempt. A few states amend this — New Jersey and Virginia call for a 10-mil ASTM E1745 Class A retarder — so check your local edition.
Mixing and curing
- Use the least water that makes a workable mix: Quikrete lists 0.75 gal to start and 1.125 gal maximum per 80-lb bag. Extra water weakens the slab and causes dusting.
- Keep the slab moist for 5 days above 70°F or 7 days between 50 and 70°F.
- Protect fresh concrete from freezing for at least 48 hours.
- Standard mix takes foot traffic after about 24 hours; wait several days before driving on a slab.
Pouring something else? Use the footing calculator, post hole calculator, Sonotube calculator or concrete steps calculator. Need the gravel separately? Use the gravel calculator.