How Much Concrete for a 30x40 Shop Slab?
Concrete needed
24.44 cubic yards
A 30 by 40 foot shop slab at six inches, with ten percent on top for waste.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Length30 ft
- Width40 ft
- Thickness6 in
- Waste allowance10 %
Concrete needed24.44 cubic yards
| Volume | 600 cu ft |
|---|---|
| Cubic yards before waste | 22.22 |
| Cubic yards with 10% waste | 24.44order to this number |
| Order in quarter yards | 24.5 cu yd |
| 60 lb bags, if bagging it | 1,467 |
| 80 lb bags, if bagging it | 1,100 |
The material list this produces
| Ready mix concrete, cubic yards | 24.5 |
|---|---|
| Rebar or mesh, sq ft of slab | 1,200 |
| Form lumber, linear ft | 140 |
Thickness compared
Thickness is the one number on this page worth arguing about, so here is the same slab at each of the usual four.
| Thickness | Concrete needed |
|---|---|
| 4 in | 16.3 cubic yards |
| 5 in | 20.37 cubic yards |
| 6 in | 24.44 cubic yards |
| 8 in | 32.59 cubic yards |
What else matters here
Twelve hundred square feet at six inches is twenty four and a half yards, which is three truckloads at a typical nine or ten yard load. That is no longer a pour you organize on the morning of, it is a pour you schedule.
With three trucks the whole job turns on spacing. Fifteen to twenty minutes between them keeps a continuous edge without leaving a truck sitting and building up a load charge. Tell the dispatcher the spacing you want rather than the times you want, because they are managing a plant, not your slab.
At this size, rent the power trowel before you need it and put somebody on the slab whose only job is watching the surface. A pour this big changes state across its own area, so one end can be ready to trowel while the other is still bleeding, and chasing that is what the extra person is for.
What this calculator assumes
- One cubic yard is 27 cubic feet. A 4 inch slab is one third of a foot thick, not 4.
- A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.60. Bags are for small pours only; past about one cubic yard, ready mix is cheaper and far less work.
- Ready mix is usually sold in quarter yard steps with a short load fee below one yard.
Questions people ask
How many yards of concrete for a 30x40 slab?
22.22 cubic yards at six inches, 24.44 with a ten percent waste allowance. At four inches it is 16.3 with waste, but a shop floor at four inches will not take forklift or trailer traffic.
How many trucks is a 30x40 slab?
Three at a typical nine yard load, or two and a half if your supplier runs ten yard trucks. Order them spaced fifteen to twenty minutes apart so the edge never sets between loads.
How thick should a pole barn floor be?
Six inches for general shop use with vehicles and equipment. Go to eight, with an engineered rebar layout, if a loaded forklift or a lift will work on it.
The concrete calculator takes any dimensions you like and hands back the same list.