How Much Concrete for a 16x20 Slab?
Concrete needed
4.35 cubic yards
A 16 by 20 foot slab at four 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.
- Length16 ft
- Width20 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed4.35 cubic yards
| Volume | 106.7 cu ft |
|---|---|
| Cubic yards before waste | 3.95 |
| Cubic yards with 10% waste | 4.35order to this number |
| Order in quarter yards | 4.5 cu yd |
| 60 lb bags, if bagging it | 261 |
| 80 lb bags, if bagging it | 196 |
The material list this produces
| Ready mix concrete, cubic yards | 4.5 |
|---|---|
| Rebar or mesh, sq ft of slab | 320 |
| Form lumber, linear ft | 72 |
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 | 4.35 cubic yards |
| 5 in | 5.43 cubic yards |
| 6 in | 6.52 cubic yards |
| 8 in | 8.69 cubic yards |
What else matters here
Three hundred and twenty square feet is a single car garage floor, a generous patio or a one bay workshop. At four inches it is four and a half yards, which is the point where the pour stops being a job you can think your way through as it happens.
Four and a half yards arrives as one truck and comes off the chute in about twenty minutes if the driver is in a hurry, and drivers are usually in a hurry. Before it arrives, have the forms braced, the mesh chaired up off the base, a screed board cut to length, and one person whose only job is running the screed.
At this size the base matters more than the concrete. Four inches of compacted crushed stone under the slab does more for its life than an extra inch of concrete on top, because almost every slab failure at this scale is the ground moving rather than the concrete breaking.
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 16x20 slab?
3.95 cubic yards at four inches, 4.35 with a ten percent waste allowance, so order 4.5. At six inches the same slab is about 6.5 yards.
How thick should a garage floor be?
Four inches is the minimum for cars. Go to six for a truck, an RV, a lift or heavy machinery, and use rebar rather than mesh if anything with a real axle load will sit on it.
Do I need rebar in a 16x20 slab?
Yes, or at least wire mesh. Anything over about ten feet in a direction will crack somewhere, and reinforcement is what keeps the two sides of that crack in the same plane.
The concrete calculator takes any dimensions you like and hands back the same list.