Concrete & Masonry
Concrete Slab Calculator
Measure the inside of your forms, choose the slab thickness, and get the concrete you need in cubic yards and in 60 or 80 lb bags.
Worked example
- Volume = 12 ft x 10 ft x (4 in / 12) = 40 cu ft.
- Cubic yards = 40 / 27 = 1.48 cu yd.
- Add 10% for waste: 44 cu ft = 1.63 cu yd.
- 80 lb bags yield about 0.6 cu ft each: 44 / 0.6 = 73.3, round up to 74.
Common sizes at a glance
| Size | Concrete | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 x 10 ft, 4 in | 1.36 cu yd | 62 | 82 |
| 10 x 20 ft, 4 in | 2.72 cu yd | 123 | 163 |
| 12 x 16 ft, 4 in | 2.61 cu yd | 118 | 157 |
| 20 x 20 ft, 4 in | 5.43 cu yd | 245 | 326 |
| 24 x 24 ft, 5 in | 9.78 cu yd | 440 | 587 |
| 30 x 40 ft, 6 in | 24.44 cu yd | 1,100 | 1,467 |
Measure inside the forms
Take length and width from the inside faces of the form boards, not from the outside of the stakes. On a slab that is not quite square, measure both long sides and use the larger number.
For an L-shaped slab, split it into two rectangles, run each one, and add them together. That is more accurate than measuring the outer box and guessing what to take away.
Thickness changes everything
Thickness has the biggest effect on the total. Going from 4 in to 5 in adds a quarter more concrete, and 6 in adds half again. The common sizes table shows this: a 20 x 20 ft slab at 4 in needs 5.43 cu yd.
The right thickness depends on what the slab carries and on local rules. The local building code and a licensed professional have the final word on thickness, base and reinforcement for a garage or anything structural.
The base is not as flat as it looks
A compacted gravel base that dips half an inch across the slab can add more concrete than you expect. Rake and tamp the base, then check depth with a string pulled tight across the forms at several points.
If many spots read deeper than your planned thickness, raise the waste allowance a few points or fix the base before the truck or the bags arrive.
Mistakes that send you back to the store
- Measuring the planned slab instead of the actual form layout.
- Forgetting thickened edges or a turned-down perimeter, which need extra concrete.
- Setting waste to zero because the math looked exact.
- Mixing up 60 and 80 lb bag counts when buying.
What comes next
Once the volume is set, plan the steel and the cost. The rebar calculator counts bars for a grid over the same slab, and the concrete slab cost calculator turns the yards into a material and labor estimate using your own price.
Questions
How much concrete is a 10 x 10 slab at 4 inches?
About 1.36 cu yd with waste included, which is 62 bags of 80 lb or 82 bags of 60 lb.
Does the result include waste?
Yes. The worked example shows 1.48 cu yd before waste and 1.63 cu yd after adding 10%.
Is it cheaper to mix bags for a big slab?
Compare your own bag price times the bag count with your local ready-mix price per yard. On larger slabs the labor of mixing by hand also adds up fast.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.