How much concrete do I need for a 20×20 slab?
A 20 × 20 ft slab poured 4 inches thick needs 5.43 cubic yards of concrete, which is 245 bags of 80 lb mix or 326 bags of 60 lb. That includes 10 percent extra for an uneven base; the exact volume is 4.94 cubic yards (133.3 cubic feet). At 6 inches, for a driveway or garage, it is 8.15 cubic yards or 367 bags of 80 lb.
Concrete for a 20×20 slab by thickness
| Thickness | Exact volume | Order (10% extra) | 80 lb bags | 60 lb bags | 50 lb bags | 40 lb bags |
|---|---|---|---|---|---|---|
| 4 in | 4.94 cu yd (133.3 cu ft) | 5.43 cu yd | 245 | 326 | 392 | 489 |
| 5 in | 6.17 cu yd (166.7 cu ft) | 6.79 cu yd | 306 | 408 | 489 | 612 |
| 6 in | 7.41 cu yd (200.0 cu ft) | 8.15 cu yd | 367 | 489 | 587 | 734 |
Bag counts are rounded up and include the same 10 percent allowance. One 80 lb bag yields 0.60 cubic feet; 60 lb yields 0.45; 50 lb yields 0.375; 40 lb yields 0.30.
Change the numbers
The calculator below is preset for this size. Adjust anything and the results update instantly.
Bags or a ready-mix truck for a 20×20 slab?
Order ready-mix. 245 bags of 80 lb would weigh 9.8 tons and take 12 hours to mix, which is not practical and would leave cold joints. A truck delivers 5.43 yards for about $679 to $951, and at this volume you will usually clear the short-load minimum. Book a pump if the truck cannot get within 20 feet of the forms.
Everything else a 20×20 slab needs
- Gravel base: 4 in of compacted crushed stone over 400 sq ft is 5.43 cubic yards, about 8.1 tons. See the gravel calculator.
- Reinforcement: #4 rebar on a 16 in grid, about 585 linear feet (30 stock bars of 20 ft), or 9 sheets of 5 × 10 ft wire mesh. The rebar calculator prices it out.
- Forms: 80 linear feet of 2 × 4 (11 eight-footers) with a stake every 2 ft, about 40 stakes.
- Vapor barrier: 460 sq ft of 6 mil plastic if the slab is for a shed, garage, or anything enclosed.
- Control joints: cut or tool 2 joints so no panel is longer than 10 ft. Cut within 12 hours of finishing.
- Expansion joint: 1/2 in fiber strip wherever the slab meets a house, driveway, or other concrete.
How the numbers are figured
20 × 20 ft is 400 square feet. Multiply by the thickness in feet (4 in is 0.333 ft) for cubic feet, divide by 27 for cubic yards, and add 10 percent because a gravel base is never perfectly flat. Bag counts divide the cubic feet by each bag's yield. The concrete calculator above does the same math for any size, shape, or waste factor.
Common questions
How many 80 lb bags of concrete for a 20x20 slab?
245 bags at 4 inches thick, 306 bags at 5 inches, or 367 bags at 6 inches, each including 10 percent extra. In 60 lb bags the 4 inch slab takes 326.
How much does concrete for a 20x20 slab cost?
About $1,592 in 80 lb bags at $6.50 each, or $679 to $951 for 5.43 yards of ready-mix before any short-load fee. Having a contractor form, pour, and finish it typically runs $2,400 to $4,000, or $6 to $10 per square foot.
How thick should a 20x20 concrete slab be?
4 inches for a patio, walkway, shed floor, or hot tub pad on well-compacted gravel. Go to 5 or 6 inches if cars will drive on it, and 6 inches with rebar for a garage, workshop with heavy tools, or RV pad. For a 20×20 slab the jump from 4 to 6 inches adds 2.72 cubic yards.