Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 12 ft; Width 20 ft; Thickness 4 in = 0.3333 ft
- Volume = 12 × 20 × 0.3333 × 1 = 80 ft³
- Net volume = 80 ft³ ÷ 27 = 2.963 yd³ (× 0.0283168 = 2.2653 m³)
- Order quantity = 2.963 yd³ × (1 + 10%) = 3.2593 yd³ = 88 ft³
- 80 lb bags = 88 ft³ ÷ 0.6 ft³ per bag = 146.67 → 147 bags (always round up)
- Weight = 88 ft³ × 150 lb/ft³ = 13,200 lb
What this slab is used for
A 12×20 ft slab (240 ft²) is the size of a narrow single-car garage floor, a one-car parking pad, a trailer or boat pad, or a long, narrow workshop. Its 20 ft length fits a full-size pickup, which makes it the first size on this list where vehicles are the main load.
Because cars and light trucks will be on it, 5 in is the usual choice, with 6 in for heavier vehicles or a trailer parked in one spot. A garage floor is often 4 to 6 in with a thickened edge. Driveways and garage floors also benefit from rebar on chairs, and a base of compacted crushed stone.
Concrete for a 12×20 ft slab at each thickness
At 5 in the slab needs 4.07 yd³ with 10% waste, compared with 3.26 yd³ at 4 in. That is a full-size truck order, so bags are not realistic. A garage floor should also slope toward the door, usually about 1/8 to 1/4 in per foot, and sit on a vapor barrier if it is enclosed.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 2.22 | 2.44 | 1.87 | 110 | 147 | 9,900 lb |
| 4 in | 2.96 | 3.26 | 2.49 | 147 | 196 | 13,200 lb |
| 5 in | 3.70 | 4.07 | 3.11 | 184 | 245 | 16,500 lb |
| 6 in | 4.44 | 4.89 | 3.74 | 220 | 294 | 19,800 lb |
The formula is 12 × 20 × thickness ÷ 12 ÷ 27 for the net volume. The order adds 10% for waste and rounds the bag counts up. Use the calculator above for another thickness or waste allowance.
Bags or ready-mix for 12×20 ft?
Bags: 147 bags of 80 lb mix at 4 in, a total of 11,760 lb of dry product. Ready-mix: 3.26 yd³, material about $457–$1,036 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $809–$1,176. Because the order is above two cubic yards, a truck is usually the practical choice. See bagged vs ready-mix.
What to watch for
- A 4 in slab under a vehicle with no reinforcement and a soft base.
- No slope to the door, so water stays in the garage.
- No isolation joint where the slab meets the foundation wall or the apron.
- Forgetting the footing. A garage slab usually has a thickened edge or a stem wall, which is a separate volume to calculate.
Layout and joints
Cut a lengthwise joint down the 12 ft width or at the middle, and cross joints every 10 to 12 ft, which makes panels of about 6×10 ft.
For a 4 in slab, joint spacing of 8 to 12 ft is the usual range, with a 15 ft maximum for any thickness. Read the control joints guide for depth and timing.
Before you order
- Measure the forms, not the excavation, and check the thickness in several places.
- Add the volume of any thickened edge, apron or step separately.
- Plan the base: use the gravel base calculator to estimate stone for 240 ft².
- If the design calls for steel, estimate it with the rebar calculator.
- Put the quantity on the Pour List to combine it with footings and steps.
Frequently asked questions
How many yards of concrete for a 12×20 slab?
2.96 yd³ net at 4 in, 3.26 yd³ with 10% waste.
How many bags of concrete for a 12×20 slab?
147 bags of 80 lb mix or 196 bags of 60 lb mix at 4 in with 10% waste.
How much does a 12×20 slab weigh?
About 13,200 lb (6.60 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 12×20 slab at 6 in?
4.89 yd³ with 10% waste.
How much does a 12×20 concrete slab cost in materials?
About $457–$1,036 for ready-mix concrete at 4 in, before delivery fees, based on the ranges on the cost page (verified 2026-10-04).
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.