Concrete calculator
A 10 × 10 ft slab 4 in thick takes 1.23 cubic yards of concrete. With 10% extra that is 1.36 yards, or 62 bags of 80 lb mix. Choose what you are pouring and enter your measurements.
How the calculator works
Concrete is ordered by volume. The calculator turns your measurements into cubic feet, divides by 27 to get cubic yards, and then counts bags from the yield printed on each bag size. You can type lengths in feet, inches, yards, meters or centimeters, and fractions such as 3 1/2 work too.
- Slab, patio or driveway: length × width × thickness.
- Footing: length × width × depth, times the number of footings. For a wall footing, use the total length of the run.
- Round column or hole: π × (diameter ÷ 2)² × height, times the number of columns.
- Steps: each step is a solid block from the ground up, so the first step is one rise tall, the second two rises, and so on. A top landing adds its own depth at full height.
- Wall: length × height × thickness.
The extra allowance covers spillage, forms that bow out, and ground that is lower than you measured. The National Ready Mixed Concrete Association advises ordering at least 4 to 10 percent more than the measured volume, and notes that a slab only 1/8 in thicker than planned on a 4 in pour is already 3 percent short. The calculator adds 10% unless you change it.
Worked example
10 ft × 10 ft × 4 in = 10 × 10 × 0.333 ft = 33.33 cu ft
33.33 cu ft ÷ 27 = 1.23 cu yd
With 10% extra: 36.67 cu ft = 1.36 cu yd
36.67 cu ft ÷ 0.60 cu ft per 80 lb bag = 61.1, so 62 bags
If you order ready-mix, round up to what your supplier sells; the calculator shows the volume rounded up to the next quarter yard.
Cubic yards for common slab sizes
Exact volumes, before any extra. Add your waste allowance on top.
| Slab | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 4 × 4 ft | 0.20 | 0.25 | 0.30 |
| 8 × 8 ft | 0.79 | 0.99 | 1.19 |
| 10 × 10 ft | 1.23 | 1.54 | 1.85 |
| 10 × 12 ft | 1.48 | 1.85 | 2.22 |
| 12 × 12 ft | 1.78 | 2.22 | 2.67 |
| 12 × 16 ft | 2.37 | 2.96 | 3.56 |
| 16 × 16 ft | 3.16 | 3.95 | 4.74 |
| 12 × 24 ft | 3.56 | 4.44 | 5.33 |
| 20 × 20 ft | 4.94 | 6.17 | 7.41 |
| 24 × 24 ft | 7.11 | 8.89 | 10.67 |
Bags in a cubic yard
Bagged concrete mix is sold by weight, but what matters is the volume it makes once mixed. These yields are from the QUIKRETE Concrete Mix data sheet; Sakrete lists the same 0.6 cu ft for its 80 lb bag.
| Bag | Yield | Bags per cu yd | Dry mix to carry |
|---|---|---|---|
| 40 lb | 0.3 cu ft | 90 | 3,600 lb |
| 50 lb | 0.375 cu ft | 72 | 3,600 lb |
| 60 lb | 0.45 cu ft | 60 | 3,600 lb |
| 80 lb | 0.6 cu ft | 45 | 3,600 lb |
| 90 lb | 0.675 cu ft | 40 | 3,600 lb |
A cubic yard from 80 lb bags means lifting 45 bags and 3,600 lb of dry mix. For more than a yard or so, ready-mix delivered by truck is usually the practical choice; ask your local plant about its minimum order and short-load charges before you decide.
How thick should it be?
The International Residential Code asks for concrete floor slabs at least 3½ in thick and spread footings at least 6 in thick, and its footing tables start at 12 in wide. Patios and walks are commonly poured 4 in thick. Driveways and slabs that carry heavier loads are often thicker, and your local building department can tell you what it requires.
Sources
- QUIKRETE Concrete Mix (No. 1101) data sheet, The QUIKRETE Companies, revised October 2022. Bag yields: 40 lb 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60, 90 lb 0.675; about 140 lb per cubic foot.
- Sakrete High-Strength Concrete Mix 80 lb product page, Sakrete (Oldcastle APG), current listing. An 80 lb bag yields 0.6 cu ft, the same as QUIKRETE Concrete Mix.
- CIP 8: Discrepancies in Yield (Concrete in Practice series), National Ready Mixed Concrete Association, 2016 edition. Order at least 4 to 10 percent more than the measured volume; 1/8 in extra on a 4 in slab is a 3 percent shortfall.
- International Residential Code, sections R403.1.1 and R506.1, International Code Council, 2021 edition. Spread footings at least 6 in thick with projections of 2 in or more; Table R403.1 widths start at 12 in; concrete floor slabs at least 3.5 in thick.