Concrete Calculator: Cubic Yards, Bags and What to Order
Concrete is sold by the cubic yard and poured by the inch, and that unit mismatch is where most short orders come from. Put the dimensions in and you get yards, the quarter yard you should actually order, and the bag count if the pour is small enough to mix.
When the figures look right, the concrete material list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Length20 ft
- Width12 ft
- Thickness4 in
- Waste allowance10 %
Concrete needed3.26 cubic yards
| Volume | 80 cu ft |
|---|---|
| Cubic yards before waste | 2.96 |
| Cubic yards with 10% waste | 3.26order to this number |
| Order in quarter yards | 3.5 cu yd |
| 60 lb bags, if bagging it | 196 |
| 80 lb bags, if bagging it | 147 |
The material list this produces
| Ready mix concrete, cubic yards | 3.5 |
|---|---|
| Rebar or mesh, sq ft of slab | 240 |
| Form lumber, linear ft | 64 |
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.
Yards, feet and the mistake in between
One cubic yard is 27 cubic feet, a 3 by 3 by 3 box. Thickness is almost always given in inches, so it has to be divided by 12 before anything else happens. A 4 inch slab is 0.333 feet thick.
A 20 by 12 patio at 4 inches is 20 x 12 x 0.333 = 80 cubic feet, which is 2.96 cubic yards. Forget the division and you get 960 cubic feet, and you have just ordered thirty five yards of concrete for a patio.
Bags or ready mix
A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.60. That means 45 of the 80 lb bags to a cubic yard, which is 3,600 pounds you carry, open and mix by hand.
Past roughly one cubic yard, ready mix is cheaper once you count your own time, and far more consistent. Below that, most plants charge a short load fee that wipes out the saving. The break even sits around three quarters of a yard for most people.
Order in quarter yard steps. Plants batch that way and you cannot order 2.96.
Add waste, and add more on grade
Ten percent is the usual allowance. It covers spillage, the bit left in the chute, and the fact that nobody grades a subbase perfectly flat.
On an uneven subgrade, a slab poured on a dip is thicker than the dip. Over a 20 by 12 area, half an inch of extra average depth is 0.37 of a yard, which is more than the whole 10 percent allowance. If the base is rough, grade it or add 15.
Questions people ask
How many 80 lb bags are in a cubic yard of concrete?
About 45, since an 80 lb bag yields roughly 0.60 cubic feet and a yard is 27 cubic feet. For 60 lb bags, at about 0.45 cubic feet a bag, it is about 60 per yard.
How much concrete do I need for a 20x20 slab?
At 4 inches thick, 400 square feet is 133 cubic feet, which is 4.94 cubic yards before waste. With 10 percent waste, order 5.5 yards.
Does this include rebar or mesh?
The material list includes the slab area so you can size mesh or rebar to it, but it does not design the reinforcement.
Concrete estimate template and Concrete invoice template, free, no signup, PDF Word and Excel.
Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.
More sizes, answered
Each row is this calculator run with those numbers when the page was built. Type the same numbers in above to see every line of the working.
| Example | What it assumes | Concrete needed |
|---|---|---|
| 4x4 pad | A 4 by 4 foot pad at four inches, with ten percent on top for waste. | 0.22 cubic yards |
| 4x6 pad | A 4 by 6 foot pad at four inches, with ten percent on top for waste. | 0.33 cubic yards |
| 4x8 pad | A 4 by 8 foot pad at four inches, with ten percent on top for waste. | 0.43 cubic yards |
| 5x5 pad | A 5 by 5 foot pad at four inches, with ten percent on top for waste. | 0.34 cubic yards |
| 6x6 pad | A 6 by 6 foot pad at four inches, with ten percent on top for waste. | 0.49 cubic yards |
| 6x8 shed pad | A 6 by 8 foot shed pad at four inches, with ten percent on top for waste. | 0.65 cubic yards |
| 7x7 hot tub pad | A 7 by 7 foot pad at four inches, with ten percent on top for waste. | 0.67 cubic yards |
| 8x8 slab | An 8 by 8 foot slab at four inches, with ten percent on top for waste. | 0.87 cubic yards |
| 8x10 slab | An 8 by 10 foot slab at four inches, with ten percent on top for waste. | 1.09 cubic yards |
| 10x12 slab | A 10 by 12 foot slab at four inches, with ten percent on top for waste. | 1.63 cubic yards |
| 10x20 slab | A 10 by 20 foot slab at four inches, with ten percent on top for waste. | 2.72 cubic yards |
| 12x16 slab | A 12 by 16 foot slab at four inches, with ten percent on top for waste. | 2.61 cubic yards |
| 12x20 slab | A 12 by 20 foot slab at four inches, with ten percent on top for waste. | 3.26 cubic yards |
| 16x20 slab | A 16 by 20 foot slab at four inches, with ten percent on top for waste. | 4.35 cubic yards |
| 20x30 slab | A 20 by 30 foot slab at four inches, with ten percent on top for waste. | 8.15 cubic yards |
| 24x24 garage, 6 in | A 24 by 24 foot garage slab at six inches, with ten percent on top for waste. | 11.73 cubic yards |
| 30x30 slab | A 30 by 30 foot slab at four inches, with ten percent on top for waste. | 12.22 cubic yards |
| 30x40 shop, 6 in | A 30 by 40 foot shop slab at six inches, with ten percent on top for waste. | 24.44 cubic yards |
| 40x60 barn, 6 in | A 40 by 60 foot slab at six inches, with ten percent on top for waste. | 48.89 cubic yards |