BuildCalc Guides · Concrete
How Many Bags of Concrete for a 20×20 Slab?
A 20 by 20 foot pad is two-car garage territory. At this size, bagged concrete is a thought experiment — the right answer is to order ready-mix and plan the day around the truck arriving.
The math
- Volume in cubic feet = 20 × 20 × (4 ÷ 12) = 133.33 ft³ at 4 inches, or 200 ft³ at 6 inches.
- Cubic yards = ft³ ÷ 27. 133.33 ÷ 27 = 4.94 yd³ net.
- Add waste. 4.94 × 1.05 = 5.19 yd³ to order. At 6 inches you are at 7.78 yd³.
- Bags (theoretical) at 0.6 ft³ per 80 lb: ~234 bags at 4 inch, ~350 bags at 6 inch. Don't do this.
By slab thickness
| Thickness | ft³ | yd³ | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3" | 100.0 | 3.70 | 175 | 234 |
| 4" | 133.3 | 4.94 | 234 | 312 |
| 6" | 200.0 | 7.41 | 350 | 467 |
Includes a 5 percent waste factor. Bag columns are theoretical.
Planning the pour
- Order the right mix. 4,000 PSI for a garage or driveway slab. 3,500 PSI minimum if it is a covered patio.
- Round up to whole yards. 5.19 yd³ usually gets ordered as 5.5 yd³. Suppliers will deliver in 0.25 yd³ increments but you do not want to be short with a wet form ready to pour.
- Time of day. Schedule an early morning pour in summer — concrete sets faster in heat. In freezing weather, schedule mid-morning so the slab gets at least 6 hours above freezing before night.
- Truck access. A standard concrete truck needs about 11 ft of width and 14 ft of vertical clearance. Confirm overhead wires and tree branches before the truck shows up.
Calculate for your own slab
The numbers above are for a 20′ × 20′ × 4″ pad. For any other size, depth or shape — including thickened edges and round columns — BuildCalc's concrete calculator handles fractional input, waste factor and live material cost.
Open the concrete calculator →FAQ
How many cubic yards of concrete is a 20x20 slab?+
A 20 by 20 foot slab at 4 inches thick is about 4.94 cubic yards of net concrete. With a 5 percent waste allowance, order 5.19 cubic yards. At 6 inches thick (standard for a two-car garage) you are at 7.78 cubic yards.
How thick should a 20x20 garage slab be?+
6 inches is the standard for a two-car garage or workshop floor. Step up to 8 inches only for heavy commercial vehicles or stored equipment. 4 inches is for a patio or non-vehicle pad.
How many 80 lb bags would a 20x20 slab take?+
At 4 inches: about 234 bags. At 6 inches: about 350 bags. These are theoretical — almost nobody bags a 20×20. The math is here as a reality check on why ready-mix is the only sane choice at this size.
How much does a 20x20 concrete pad cost?+
Ready-mix runs about $150 to $200 per cubic yard delivered in most US markets, so a 4 inch 20×20 (5.19 yd³) is roughly $780 to $1,040 in material. A 6 inch garage slab (7.78 yd³) is $1,170 to $1,560 in material. Forms, rebar, vapor barrier, gravel base and finishing labor are separate — total installed cost typically runs $4,000 to $7,000.
Do I need a thickened edge on a 20x20 slab?+
Yes — a free-standing 20×20 should have a 12 inch deep by 6 inch wide turn-down edge around the perimeter. That is about 80 linear feet × 0.5 ft² = 1.5 cubic yards extra, on top of the slab. Skip the thickened edge only when the pad butts up against an existing foundation.
How many people do I need to pour a 20x20 slab?+
At least four. One on the chute or wheelbarrow, one screeding, one floating and one cutting in edges. With 5+ yards arriving on a truck you have under 90 minutes to place and screed before the concrete starts setting — being short-handed costs you the pour.
Related guides
- How many bags of concrete for a 16×20 slab? — single-car detached garage footprint.
- How many bags of concrete for a driveway? — typical driveway sizes and yards needed.
- How many bags of concrete for a 12×12 slab? — small patio version of the same math.