Concrete Calculator
Work out how much concrete you need for a slab, in cubic yards, cubic feet and bags, with its weight and a ready-mix price. Buying a little extra lowers the risk of running out.
Your result
Enter your numbers and press the button. Your result appears here.
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Cost, rebar, blocks and more
How it works
Cubic feet = length × width × thickness (in) ÷ 12. Cubic yards = cubic feet ÷ 27. Bags = cubic feet ÷ bag yield.
Example: a 10×10 ft slab 4 in thick is 33.3 ft³; with 10% extra that is 1.36 yd³, or 62 bags of 80 lb concrete.
Common slab sizes at 4 in thick
| Slab | Concrete | 80 lb bags | 60 lb bags | Ready-mix |
|---|---|---|---|---|
| 10×10 ft | 1.36 yd³ | 62 | 82 | $163 – $231 |
| 12×12 ft | 1.96 yd³ | 88 | 118 | $235 – $332 |
| 12×20 ft | 3.26 yd³ | 147 | 196 | $391 – $554 |
| 20×20 ft | 5.43 yd³ | 245 | 326 | $652 – $923 |
| 20×30 ft | 8.15 yd³ | 367 | 489 | $978 – $1,385 |
| 30×40 ft | 16.30 yd³ | 734 | 978 | $1,956 – $2,770 |
With 10% extra. Ready-mix at $120 to $170 per cubic yard (HomeAdvisor).
Concrete mix ratios
| Grade | Cement : sand : aggregate |
|---|---|
| M5 | 1 : 5 : 10 |
| M7.5 | 1 : 4 : 8 |
| M10 | 1 : 3 : 6 |
| M15 | 1 : 2 : 4 |
| M20 | 1 : 1.5 : 3 |
| M25 | 1 : 1 : 2 |
Nominal mixes by volume, as used for the M grades.
Common questions
What is concrete?
Concrete is a composite of coarse aggregate (sand, gravel or crushed stone) bound together by cement, usually Portland cement, which hardens over time. Cement is also used in mortar and plaster.
How do I get concrete?
Buy it in 40, 60 or 80 lb bags for small jobs, or have ready-mix delivered by truck for larger ones. It must be placed before it starts to harden, and some mixes are made to set quickly.
How long does concrete take to harden?
Concrete gains strength by curing. It typically reaches over 90% of its final strength within four weeks and keeps strengthening for much longer. Keeping it damp in the early days, with a curing compound, ponding or plastic sheeting, helps it reach full strength.
What is a typical concrete mix?
Mixes vary with the job, but a common guideline is 1 part cement, 2 parts sand and 4 parts aggregate, or 1 : 1.5 : 3 for a stronger mix.
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb or 90 bags of 40 lb, based on Quikrete yields of 0.60, 0.45 and 0.30 ft³ per bag.
Sources
- Quikrete, Concrete Mix data sheet (bag yields): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf
- HomeAdvisor, Concrete cost per yard: https://www.homeadvisor.com/cost/outdoor-living/deliver-concrete/