How much does a yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches, 54 at 6 inches, and 108 at 3 inches. Here is the full table and a two-way converter.
Coverage converter
square feet covered
Coverage by thickness
| Thickness | Square feet per yard | 80 lb bags per 100 sq ft | Typical use |
|---|---|---|---|
| 2 in | 162 | 28 | Stepping stones, thin pads |
| 3 in | 108 | 42 | Footpaths, shed floors |
| 4 in | 81 | 56 | Patios, sidewalks, standard slabs |
| 5 in | 64.8 | 70 | Light driveways |
| 6 in | 54 | 84 | Driveways, garage slabs |
| 8 in | 40.5 | 111 | Heavy equipment pads |
| 12 in | 27 | 167 | Footings, walls |
Bag counts assume 0.60 cubic feet per 80 lb bag and no waste. Add 10% for spillage and uneven subgrade.
The two conversions you need
Square feet to cubic yards: area × (thickness ÷ 12) ÷ 27. A 200 sq ft slab at 4 inches is 200 × 0.3333 ÷ 27 = 2.47 yd³.
Cubic yards to square feet: yards × 27 ÷ (thickness ÷ 12). One yard at 4 inches covers 81 sq ft.
Bag equivalents
| Bag | Yield | Coverage at 4 in | Coverage at 3 in |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 0.9 sq ft | 1.2 sq ft |
| 60 lb | 0.45 cu ft | 1.35 sq ft | 1.8 sq ft |
| 80 lb | 0.60 cu ft | 1.8 sq ft | 2.4 sq ft |
Coverage is a hyperbola, and that is the trap
Divide 27 cubic feet by the thickness in feet and you get the area. Halve the thickness and you double the coverage. The relationship is exact, which makes it feel trustworthy — and it says nothing at all about whether the slab will hold.
A 10 × 10 patio at 4 inches takes 1.23 cubic yards. The same yardage at 3 inches covers 162 square feet, a 12 × 13 patio. That is the whole reason thickness decisions get made badly: the saving is large and immediate, and the failure arrives two winters later as a crack you cannot explain.
Why four inches became the working default
It is not a code number. It is a practice number that satisfies several constraints at once.
- Flexural capacity. A 4-inch slab on a compacted base carries residential loads — furniture, foot traffic, a riding mower — without the steel doing structural work.
- Cover. Reinforcement needs 1½ to 2 inches of concrete around it. On a 3-inch slab there is no room to hold a mat at mid-depth and still have cover on both faces.
- Joint depth. Control joints are cut to a quarter of the slab depth. At 4 inches that is a 1-inch saw cut, which is easy to hit consistently. At 3 inches it is ¾ inch, and the joint becomes unreliable.
Below 4 inches, slabs belong to footpaths, stepping stones, and shed floors where a crack is cosmetic. Above 6 inches you are usually solving a load problem, and the answer is reinforcement plus thickness rather than thickness alone.
Where the thickness assumption fails
- Thickness is measured from the top of the compacted base, not from the bottom of the excavation.
- The forms set thickness at the edges only. In the middle, the slab is as thick as the base is low. A subgrade an inch low in the centre gives a 5-inch slab there and a short pour everywhere.
- Thickened edges are not in the flat calculation. Slabs that carry a structure or vehicle loads turn the edge down to 8–12 inches, and that concrete is real.
- Joint spacing ties to thickness. Control joints go at two to three times the thickness in feet, so 8 to 12 feet on a 4-inch slab. Space them wrong and the slab chooses its own crack lines.
What a quarter inch is worth
At 400 square feet, pouring 4¼ inches instead of 4 is 8.3 cubic feet — 0.31 cubic yards, or 14 extra 80 lb bags. Thickness errors are the most expensive kind because they stay invisible right up to the moment the concrete is ordered.
Check the base in at least five places with a string line and a tape, and set the forms to the lowest reading rather than the average.
Frequently asked questions
How many square feet does a yard cover?
81 sq ft at 4 inches, 54 at 6 inches, 108 at 3 inches — the number falls as the slab thickens.
How much does an 80 lb bag cover?
About 1.8 sq ft at 4 inches, 2.4 sq ft at 3 inches.
How do I convert square feet to yards?
Area × thickness in feet ÷ 27. A 10 × 10 slab at 4 inches is 1.23 cubic yards.
How many square feet does a yard cover at 2 inches?
162 square feet. At 3 inches it is 108, at 4 inches 81, and at 6 inches 54.
How thick should a concrete patio be?
4 inches on a compacted base for foot traffic and furniture. Go to 5 or 6 inches if vehicles will park on it.
How far apart should control joints be?
Two to three times the slab thickness in feet — 8 to 12 feet for a 4-inch slab. Cut to a quarter of the depth.
Does a thicker slab need less rebar?
No. Thickness and reinforcement solve different problems: thickness carries load in compression and bending, steel controls cracking and provides tensile capacity. Adding thickness without steel still cracks.
Note: Coverage assumes a level subgrade and the stated thickness. Actual yield varies with bag product and placement losses.
Related: Concrete calculator · Bags per yard · All material calculators