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Concrete calculator
Type the size of the slab or the holes: cubic yards and bag counts for every bag size appear at once, with the extra a ready-mix plant would add.
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Concrete to order
1.28 cu yd
1.23 cubic yards in the forms (33.3 cubic feet) plus 4%
| 80 lb bags | 58 |
| 60 lb bags | 78 |
| 50 lb bags | 93 |
| 40 lb bags | 116 |
Bag counts use the manufacturer’s yields (an 80 lb bag makes 0.60 cubic feet). Past about one cubic yard, a ready-mix truck usually beats bags; trucks carry 8 to 12 cubic yards. How this is calculated.
A 10 by 10 foot slab poured 4 inches thick holds 33.3 cubic feet of concrete, or 1.23 cubic yards, and with the 4% margin the National Ready Mixed Concrete Association puts at the low end of its advice you should plan on 1.28 cubic yards: 58 bags of 80 lb, 78 bags of 60 lb or 116 bags of 40 lb. Those counts use the yields QUIKRETE prints for its Concrete Mix No. 1101, 0.60 cubic feet for an 80 lb bag and 0.30 for a 40 lb bag. Push the slab to 10 by 12 feet and the order grows to 70 of the big bags. A full cubic yard takes 45 bags of 80 lb, which is the point where a truck from a ready-mix plant, carrying 8 to 12 cubic yards, starts to make more sense than mixing by hand. Type the shelf price of a bag or your plant's price per yard and the calculator multiplies it for you.
From slab size to bag count
The math has three steps. Multiply length by width in feet, then by the thickness in inches divided by 12: that gives cubic feet. Divide by 27 for cubic yards, the unit every ready-mix plant quotes. For bags, divide the cubic feet by what one bag makes once mixed with water. QUIKRETE publishes those yields on its data sheet: 0.60 cubic feet for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. Each count is rounded up, because a store will not sell you two thirds of a bag.
The table runs the same arithmetic for common pours at 4% over the plan volume. The last line is a single car pad thickened to 6 inches, the kind of slab that carries a vehicle rather than a grill.
| Pour | In the forms | Order (+4%) | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 4 x 4 ft, 4 in | 0.20 cu yd | 0.21 cu yd | 10 | 13 | 19 |
| 10 x 10 ft, 4 in | 1.23 cu yd | 1.28 cu yd | 58 | 78 | 116 |
| 10 x 12 ft, 4 in | 1.48 cu yd | 1.54 cu yd | 70 | 93 | 139 |
| 12 x 12 ft, 4 in | 1.78 cu yd | 1.85 cu yd | 84 | 111 | 167 |
| 16 x 16 ft, 4 in | 3.16 cu yd | 3.29 cu yd | 148 | 198 | 296 |
| 20 x 20 ft, 4 in | 4.94 cu yd | 5.14 cu yd | 232 | 309 | 463 |
| 12 x 24 ft, 6 in | 5.33 cu yd | 5.55 cu yd | 250 | 333 | 500 |
Why the plant tells you to order extra
The NRMCA guide to ordering ready mixed concrete says to order 4% to 10% more than the plan dimensions give, which is why the calculator starts at 4% and lets you raise it. The reason is mostly the ground. A gravel base that dips a little under the screed swallows concrete you never see. The association’s note on yield discrepancies puts a number on it: one eighth of an inch of extra depth on a 4 inch slab is a 3% shortage. On the 10 by 10 example, the exact plan volume is 56 bags of 80 lb; at 4 1/8 inches it becomes 58. Forms that bow outward, spillage from the wheelbarrow and a little left in the mixer drum add to it.
Post holes, piers and deck footings
Switch the calculator to round holes and it uses the volume of a cylinder instead. A 10 inch hole dug 36 inches deep holds 1.64 cubic feet, so one hole takes 3 bags of 80 lb with the 4% margin, and ten holes take 29, not ten times 3, because the rounding happens once on the total. If you set the post in the hole first, the wood or vinyl takes up some of that space and you will have a bag or two left; most builders keep it for the next hole rather than shave the count. Fence posts are priced with their concrete on the fence cost page, and footing sizes for a deck are on the deck cost calculator.
Bags or a ready-mix truck
Bags win for small jobs because nothing has to be scheduled and no truck has to reach the backyard. They lose on labor: 45 bags of 80 lb make one cubic yard, which is 3,600 lb lifted, poured and mixed, and the first batch starts to set while you mix the tenth. A 12 by 24 foot pad at 4 inches already calls for 167 bags (3.70 cubic yards), and the same pad at 6 inches needs 5.55 cubic yards. At that size, call two plants for a price per yard and ask whether delivery and a short-load charge are included; the calculator multiplies whatever figure you type by the ordered volume, so you can compare it with the bag total on the same screen.
What hiring a concrete crew costs
For driveways and walkways, the 2023 American Housing Survey recorded 978 jobs done by professionals, with a median of $4,400 in August 2026 dollars and half of them between $2,180 and $8,650. Owners who did the work themselves reported a median of $1,145. In the 2022 Economic Census, poured concrete contractors spent 35.3% of the value of their work on materials and 29.0% on payroll and benefits. BLS counted 206,170 cement masons and concrete finishers in May 2025, paid $28.87 an hour on average, with the middle half between $22.60 and $32.15. A finisher's quote covers forms, base, reinforcement, finishing and cleanup, so it will always sit well above the material figure this page gives.