Quick answer: a 10 × 10 ft slab that is 4 inches thick needs 62 bags of 80 lb concrete mix if you add 10% for waste, or 56 bags if you pour it with no allowance at all. In 60 lb bags that is 82 bags, and in 40 lb bags it is 123. Bag counts only work if your slab is exactly the thickness you think it is, so read the section on measuring before you buy.
You can skip the math and use the Concrete Calculator or the Concrete Bag Calculator. The rest of this guide explains where the numbers come from, so you can check them for any slab size.
The formula
Everything starts with volume in cubic feet. Thickness is the part people get wrong, because it is measured in inches and the other two sides are in feet.
Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
Volume with extra = volume × 1.10
Bags = volume with extra ÷ yield per bag, rounded up
A bag’s yield is how much wet concrete it makes. QUIKRETE’s data sheet for its Concrete Mix lists these yields: a 40 lb bag makes about 0.30 ft³, a 50 lb bag about 0.375 ft³, a 60 lb bag about 0.45 ft³, an 80 lb bag about 0.60 ft³, and a 90 lb bag about 0.675 ft³. Other brands of standard concrete mix list similar numbers, but yields differ a little, so read the bag you are actually buying.
Worked example: a 10 × 10 ft patio slab, 4 in thick
- Volume: 10 × 10 × 4 ÷ 12 = 33.33 ft³.
- Add 10% extra: 33.33 × 1.10 = 36.67 ft³.
- 80 lb bags: 36.67 ÷ 0.60 = 61.1, so buy 62 bags.
- 60 lb bags: 36.67 ÷ 0.45 = 81.5, so buy 82 bags.
- 40 lb bags: 36.67 ÷ 0.30 = 122.2, so buy 123 bags.
Without any extra, the exact need is 33.33 ÷ 0.60 = 55.6 bags of 80 lb, which rounds up to 56. A cubic yard is 27 ft³, so this slab is 1.23 cubic yards before extra and 1.36 with it.
Bags needed for common slab sizes
All figures below are for a 4 in slab with 10% extra, rounded up. The last column is a 6 in version of the same footprint with 80 lb bags.
| Slab size | Cubic yards (with 10%) | 80 lb bags | 60 lb bags | 40 lb bags | 80 lb bags at 6 in |
|---|---|---|---|---|---|
| 8 × 8 ft | 0.87 | 40 | 53 | 79 | 59 |
| 10 × 10 ft | 1.36 | 62 | 82 | 123 | 92 |
| 12 × 12 ft | 1.96 | 88 | 118 | 176 | 132 |
| 12 × 16 ft | 2.61 | 118 | 157 | 235 | 176 |
| 20 × 20 ft | 5.43 | 245 | 326 | 489 | 367 |
How heavy is that many bags?
62 bags of 80 lb mix weigh 4,960 lb, about 2.5 tons, and every pound has to be lifted, carried and mixed. QUIKRETE says to allow at least 1 ft³ of mixer capacity for each 80 lb bag, and to start with about 6 pints of water per 80 lb bag, up to about 9 pints if the mix is stiff. For 62 bags that works out to roughly 46 to 70 gallons of water.
That is why bags make sense for small pours and gets hard fast beyond a few cubic yards. A rough decision rule: if the job is more than about 1 cubic yard (45 bags of 80 lb), compare the price of bags against a ready-mix delivery, including any short-load fee your supplier charges. Our guide on how to order ready-mix concrete covers that comparison.
Measure the thickness before you buy
Bag counts depend on thickness more than anything else, and thickness is rarely what you assumed. A few habits that prevent running short:
- Measure the depth in several spots. A slab that averages 4½ inches instead of 4 needs 12.5% more concrete.
- Use the inside dimensions of your forms. Lumber is measured on the inside faces, where the concrete goes.
- Allow for uneven ground. Soft spots and low areas swallow concrete. Extra is cheaper than a second trip.
- Split odd shapes. For an L-shaped patio, calculate two rectangles and add them. The Square Footage Calculator helps with the layout.
Pouring tips that come from the data sheet
QUIKRETE’s data sheet for bagged concrete mix gives a few practical points worth knowing before you mix 62 bags:
- The mix is designed for concrete 2 inches thick or more.
- Dampen the sub-grade first, but don’t leave standing puddles.
- Fill the form to its full depth and work continuously from one end to the other. Partial-depth fills can create cold joints.
- Strike off with a straight board, then float the surface. Cut 1 inch deep control joints every 6 to 8 feet with a grooving tool.
- Curing matters. Keep the surface moist for about 5 days in warm weather (70 °F or higher) or 7 days at 50 to 70 °F, and protect it from freezing for the first 48 hours.
Mixing 62 bags at once is a real job. If you are working by hand, plan helpers and a rhythm: concrete that sits half-poured while you mix the next batch is how cold joints happen.
Common mistakes
- Forgetting the unit mismatch. 4 inches is 0.333 feet, not 4 feet.
- Buying exactly the calculated number. Round up, and add an allowance.
- Mixing bag sizes by weight instead of yield. An 80 lb bag is not twice a 40 lb bag in every brand. Use the yield printed on the bag.
- Skipping the base. The slab thickness here does not include the compacted gravel base underneath. Calculate that separately with the Gravel Calculator.
Frequently asked questions
How many 80 lb bags of concrete for a 10×10 slab?
62 bags at 4 inches thick with 10% extra. Without the extra it is 56 bags. At 6 inches thick it is 92 bags with 10% extra.
How many 60 lb bags of concrete for a 10×10 slab?
82 bags at 4 inches thick with 10% extra (about 74.1 bags, so 75 rounded up, without extra).
How many bags of concrete are in a yard?
45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix, using the QUIKRETE yields above (27 ÷ 0.60 = 45).
Do I need rebar or wire mesh in a 4 inch slab?
That depends on the use, the soil and your local code, so ask your building department or a contractor. This guide only covers how much concrete you need, not how to reinforce it.
Should I add extra concrete?
Yes. Ground is rarely perfectly flat, and forms can bow. A 10% allowance is a common starting point. Running short mid-pour is more expensive than a few extra bags.
Sources
- QUIKRETE, >Concrete Mix Product No. 1101 data sheet: bag yields, mixing water, placement, control joints and curing.
- NIST, >Guide to the SI, Appendix B.8: inch, foot and cubic yard conversions.
Results are planning estimates. Check quantities with your supplier and structural or code requirements with your local building department.