How Much Concrete for a 20x20 Slab? Yards by Depth
How much concrete for a 20x20 slab comes to about 4.94 cubic yards at 4 inches thick and about 7.41 cubic yards at 6 inches. A 20 by 20 foot pad is a common size for a two-car parking pad, a detached garage floor, or a large patio, and at this scale it is almost always a ready-mix job. Here is how to figure the order, how much cushion to add, and what goes underneath.
Concrete calculator →Working out the volume
The slab covers 400 square feet. Multiply that by the thickness in feet, then divide by 27 to get cubic yards.
| Thickness | Cubic feet | Cubic yards | Typical order |
|---|---|---|---|
| 4 inches | 133 | 4.94 | 5.25 to 5.5 yards |
| 5 inches | 167 | 6.17 | 6.5 to 6.75 yards |
| 6 inches | 200 | 7.41 | 7.75 to 8 yards |
Ready-mix plants usually sell in quarter-yard or half-yard steps, so the typical order column rounds up to what you would actually ask for. If you are pouring a thickened edge around the perimeter, add that separately (see below).
How much extra to order
On a 400-square-foot slab, a quarter-inch of extra depth across the whole pad adds about 0.3 cubic yards. That is why small grading errors matter more on big slabs. A cushion of 5 to 8 percent is common when the base has been graded to a string line and compacted. If the base is rough or you are not sure, 10 percent is safer. Running short means a second truck, a cold joint, and usually another delivery fee, so most contractors would rather pay for a little leftover.
Thickened edges and footings
Many garage slabs are poured with a thickened edge, sometimes called a turned-down edge or monolithic footing, where the outer foot or so of the slab goes deeper. A 12-inch-wide edge that drops an extra 8 inches around an 80-foot perimeter adds about 80 × 1 × 0.667 = 53 cubic feet, or roughly 2 cubic yards. Local code often decides whether a detached garage needs this or a separate frost footing, so check with the building department before you price the job.
Thickness by use
- Patio or play area: 4 inches on a compacted base is common.
- Parking pad or detached garage with passenger cars: 4 to 5 inches, with many builders choosing 5 for margin.
- Pickups, trailers, or a shop with heavy equipment: 6 inches, and often reinforcement as well.
Our concrete driveway thickness guide covers the reasoning behind these ranges.
Joints on a 20x20 slab
A common rule of thumb puts control joints at no more than two to three times the slab thickness in feet. For a 4-inch slab, that means about 8 to 12 feet apart. A 20x20 slab is usually split into four 10x10 panels with one joint each way. Saw or tool the joints to about a quarter of the slab depth so cracks form there instead of wandering across the surface.
Gravel base
A 4-inch compacted stone base under 400 square feet is about 133 cubic feet, or 4.9 cubic yards. At roughly 1.4 to 1.5 tons per compacted cubic yard, that is about 7 to 7.5 tons. Six inches of base doubles as a buffer on soft or clay soil and takes about 10.5 to 11 tons. The base calculator gives tons for other depths.
Bags are not realistic here
At about 0.6 cubic feet per 80-pound bag, a 4-inch 20x20 slab would need roughly 222 bags, close to 9 tons of dry mix. Even with a mixer and a large crew, keeping that much concrete placed and finished before earlier batches set is very hard. Order a truck, and book a pump or extra wheelbarrows if the chute cannot reach every corner.
Questions people ask
How many yards of concrete for a 20x20 slab 4 inches thick?
About 4.94 cubic yards. Most people order 5.25 to 5.5 yards to cover waste and minor low spots in the base.
How many yards of concrete for a 20x20 slab 6 inches thick?
About 7.41 cubic yards, so an order of roughly 7.75 to 8 yards is typical.
How thick should a 20x20 garage slab be?
Four inches is a common minimum for cars, and many builders use 5 inches. Use 6 inches for heavy trucks or equipment, and follow local code for edges and footings.
How much gravel do I need under a 20x20 slab?
About 4.9 cubic yards for a 4-inch compacted base, or roughly 7 to 7.5 tons of crushed stone.
How many bags of concrete would a 20x20 slab take?
Around 222 bags of 80-pound mix at 4 inches thick. At that size, a ready-mix truck is the practical choice.
Related guides
- Concrete Driveway Thickness: 4, 5 or 6 Inches?
- How Many Bags of Concrete in a Yard? 40, 60, 80 lb
- Gravel Calculator for Driveway & Asphalt Base: How Much Stone
- How Much Does a Ton of Gravel Cover? Depth Chart
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