A 10×12 concrete slab 4 inches thick takes 74 bags of 80 lb concrete mix, or 98 bags of 60 lb mix — that is 1.63 cubic yards including 10% extra. Change the size, thickness, or extra in the QtyLab concrete calculator (prefilled for 10×12) and it recounts as you type.
The math for a 10×12 slab, 4 in thick
- Area = 10 ft × 12 ft = 120 sq ft
- Volume = 120 sq ft × 4 in ÷ 12 = 40 cu ft
- Add 10% extra = 40 × 1.10 = 44 cu ft
- Cubic yards = 44 ÷ 27 = 1.63 yd³
- 80 lb bags = 44 ÷ 0.60 cu ft per bag = 73.3 → 74 bags
- 60 lb bags = 44 ÷ 0.45 cu ft per bag = 97.8 → 98 bags
The 10% extra covers a subgrade that is a little low in spots, spillage, and forms that bow out. Without it the slab is exactly 40 cu ft (1.48 yd³): 67 bags of 80 lb or 89 bags of 60 lb — no margin at all.
10×12 slab bags by thickness (10% extra)
| Thickness | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 in | 1.63 | 74 | 98 |
| 5 in | 2.04 | 92 | 123 |
| 6 in | 2.44 | 110 | 147 |
| 4 in, no extra | 1.48 | 67 | 89 |
Other slab sizes (4 in thick, 10% extra)
| Slab | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10×10 | 1.36 | 62 | 82 |
| 10×12 | 1.63 | 74 | 98 |
| 12×12 | 1.96 | 88 | 118 |
| 12×16 | 2.61 | 118 | 157 |
| 10×20 | 2.72 | 123 | 163 |
Bags or ready-mix?
74 bags of 80 lb mix is 5,920 lb of concrete to haul, mix, and place before it starts to set. Above about 1 cubic yard, get a delivered ready-mix quote for the same yardage and compare it to the bag total before you buy. If you do use bags, line up a mixer and helpers so the pour goes in one go.
Gravel base and forms
If your slab plan calls for a gravel base, a 4 in layer under a 10×12 slab is 1.63 yd³ — about 2.28 tons at 1.4 tons per yard with 10% extra (open the gravel calculator prefilled). The slab edge is 44 ft, which is about six 2x4x8 form boards plus stakes.
Mistakes that throw the count off
- Using inches as feet. 4 in is 0.333 ft. Multiplying 120 sq ft × 4 gives 480 “cu ft” — twelve times too much.
- Mixing up bag yields. An 80 lb bag makes about 0.60 cu ft; a 60 lb bag about 0.45 cu ft. Dividing by the wrong one is off by a third.
- No extra. Subgrade is never perfectly flat. Ordering the bare 67 bags often leaves the last corner short.
- Forgetting thickened edges or footings. Anything deeper than the main slab is extra concrete — run it as a separate shape in the calculator.
- Not checking the bag label. Yields vary a little by brand and mix. The label number wins.
Shopping list for a 10×12 slab, 4 in thick
- 74 bags of 80 lb concrete mix (or 98 × 60 lb) — shop 80 lb concrete mix
- 6 2x4x8 form boards + stakes for 44 ft of slab edge — buy locally
- Reinforcement (wire mesh or rebar), if your plan or local code calls for it — buy locally
- 1 mixing tub + mortar hoe (or rent a mixer) — shop mixing tubs
- 1 magnesium float + edger — shop floats and edgers
- 1 screed board or straight 2×4 to strike off the surface — buy locally
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More on bag math: how many bags of concrete do I need (bags per yard + slab chart). Base under the slab: how much gravel do I need.
FAQ
How many 80 lb bags of concrete do I need for a 10×12 slab?
A 10×12 slab 4 inches thick is 40 cubic feet. With 10% extra that is 44 cubic feet, or 74 bags of 80 lb mix at about 0.60 cubic feet per bag. Without the extra it is 67 bags.
How many 60 lb bags of concrete for a 10×12 slab?
At 4 inches thick with 10% extra, a 10×12 slab takes 98 bags of 60 lb mix (about 0.45 cubic feet per bag). At 6 inches thick it takes 147.
How many yards of concrete is a 10×12 slab?
A 10×12 slab 4 inches thick is exactly 1.48 cubic yards, or 1.63 cubic yards with 10% extra. At 5 inches it is 2.04 yards and at 6 inches 2.44 yards, both with 10% extra.
How many bags of concrete for a 10×12 slab 6 inches thick?
A 6 inch 10×12 slab is 60 cubic feet, or 66 with 10% extra. That is 110 bags of 80 lb mix or 147 bags of 60 lb mix (2.44 cubic yards).
Should I use bags or ready-mix for a 10×12 slab?
A 4 inch 10×12 slab is 1.63 cubic yards — 74 bags of 80 lb mix, almost 6,000 lb to mix by hand or with a rented mixer. Above about 1 cubic yard, get a ready-mix quote for the same yardage and compare it with the bag total.
Bag yields (80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft) are typical figures printed on premixed concrete bags — check your label. All counts are from the QtyLab concrete and gravel calculators. Estimates for planning only, not structural advice — check local code for thickness, base, and reinforcement.