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Concrete for a 20x20 Slab
A 20x20 slab (400 sq ft) 4 inches thick needs 4.94 cubic yards (133.33 cubic feet) of concrete, or 223 bags of 80 lb premix. Order about 10% extra: 5.43 cubic yards or 245 bags.
Cost (optional)
Premixed concrete bags
Cost
- Ready-mix
Under about 1 cubic yard, bags are usually easier and cheaper than a ready-mix truck.
Bag yields: 80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30 (as printed on US bags). Bags are rounded up.
Enter the sizes to see how much concrete you need.
20x20 slab: concrete by thickness
| Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags | Order (+10%) |
|---|---|---|---|---|---|
| 4 inch | 133.33 | 4.94 | 223 | 297 | 5.43 yd³ · 245 × 80 lb |
| 5 inch | 166.67 | 6.17 | 278 | 371 | 6.79 yd³ · 306 × 80 lb |
| 6 inch | 200 | 7.41 | 334 | 445 | 8.15 yd³ · 367 × 80 lb |
| 8 inch | 266.67 | 9.88 | 445 | 593 | 10.86 yd³ · 489 × 80 lb |
Volume = 20 ft × 20 ft × thickness in inches ÷ 12. 1 cubic yard = 27 cubic feet. Bags are rounded up.
Concrete for common slab sizes
About the Concrete Calculator
The concrete calculator works out how much concrete a job needs. Pick the shape (a slab or patio, a wall, a footing, a round column or Sonotube, fence post holes or stairs), enter the sizes in feet and inches or meters and centimeters, and it shows the cubic yards or cubic meters to order, with extra for waste, and how many premixed bags of each size that makes. Add your local prices to compare bags with a ready-mix truck.
How to use it
- 1 Choose the shape: slab, wall, footing, column, post holes or stairs.
- 2 Pick feet and inches or meters and centimeters, and enter the sizes.
- 3 Set how many, and the extra for waste (10% is usual).
- 4 Read the cubic yards to order and the number of 40, 60 or 80 lb bags; add prices to see the cost.
Frequently asked questions
How much concrete do I need for a 20x20 slab?
A 20x20 slab is 400 square feet. At 4 inches thick that is 400 × 4 ÷ 12 = 133.33 cubic feet, or 4.94 cubic yards (3.78 m³). At 6 inches it is 7.41 cubic yards. Order about 10% more, 5.43 cubic yards at 4 inches, to allow for an uneven base and spills.
How many bags of concrete for a 20x20 slab?
For a 4 inch 20x20 slab you need 223 bags of 80 lb, 297 bags of 60 lb or 445 bags of 40 lb premix, before any extra. With 10% extra that is 245 bags of 80 lb. An 80 lb bag makes about 0.6 cubic feet.
Should I use bags or ready-mix for a 20x20 slab?
Above about one cubic yard, ready-mix is usually cheaper and much less work than mixing bags, and the slab is poured in one go. At $150–$200 a cubic yard delivered, 5.43 yards costs about $815–$1,086, before any short-load fee or pump. Check your local supplier's price.
How thick should a 20x20 slab be?
4 inches suits patios, walkways and shed floors. Use 5 to 6 inches for a driveway, garage floor or a hot tub base, and follow local building code and your designer for anything that carries heavy loads.
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