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Concrete for a 20x30 Slab
A 20x30 slab (600 sq ft) 4 inches thick needs 7.41 cubic yards (200 cubic feet) of concrete, or 334 bags of 80 lb premix. Order about 10% extra: 8.15 cubic yards or 367 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.
20x30 slab: concrete by thickness
| Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags | Order (+10%) |
|---|---|---|---|---|---|
| 4 inch | 200 | 7.41 | 334 | 445 | 8.15 yd³ · 367 × 80 lb |
| 5 inch | 250 | 9.26 | 417 | 556 | 10.19 yd³ · 459 × 80 lb |
| 6 inch | 300 | 11.11 | 500 | 667 | 12.22 yd³ · 550 × 80 lb |
| 8 inch | 400 | 14.81 | 667 | 889 | 16.3 yd³ · 734 × 80 lb |
Volume = 20 ft × 30 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 20x30 slab?
A 20x30 slab is 600 square feet. At 4 inches thick that is 600 × 4 ÷ 12 = 200 cubic feet, or 7.41 cubic yards (5.66 m³). At 6 inches it is 11.11 cubic yards. Order about 10% more, 8.15 cubic yards at 4 inches, to allow for an uneven base and spills.
How many bags of concrete for a 20x30 slab?
For a 4 inch 20x30 slab you need 334 bags of 80 lb, 445 bags of 60 lb or 667 bags of 40 lb premix, before any extra. With 10% extra that is 367 bags of 80 lb. An 80 lb bag makes about 0.6 cubic feet.
Should I use bags or ready-mix for a 20x30 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, 8.15 yards costs about $1,223–$1,630, before any short-load fee or pump. Check your local supplier's price.
How thick should a 20x30 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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