To figure out how much concrete you need, multiply length × width × thickness in feet, divide by 27 to get cubic yards, and add 10% for waste. A 10 × 10 ft patio at 4 in thick needs 1.23 yd³, or about 1.36 yd³ with waste. That's 62 bags of 80 lb mix, so a small truck load starts to make sense.
Concrete is sold by volume, so every estimate comes down to length × width × thickness. The catch is units. Length and width are usually in feet, thickness is in inches, and ready-mix is priced in cubic yards. So the full formula runs in three steps:
Then add waste. Our concrete calculator defaults to 10%, which takes that patio to 36.7 cu ft, or 1.36 yd³. That's the number to order, not the 1.23.
Why pad it? Forms bow a little, the ground is never perfectly flat, and some concrete ends up on your boots. Coming up short is the expensive mistake: a cold joint where fresh concrete meets a setting slab, or a second delivery.
Thickness does most of the work in any concrete estimate, and it scales in a straight line. Going from 4 in to 6 in adds 50% more concrete. This table shows 100 sq ft of slab at common thicknesses, with the same 10% waste and 80 lb bags the calculator uses:
| Thickness | Cubic feet | Cubic yards (no waste) | Cubic yards (+10%) | 80 lb bags (+10%) | Sq ft covered by 1 yd³ |
|---|---|---|---|---|---|
| 3 in | 25.0 | 0.93 | 1.02 | 46 | 108 |
| 3.5 in | 29.2 | 1.08 | 1.19 | 54 | 92.6 |
| 4 in | 33.3 | 1.23 | 1.36 | 62 | 81 |
| 5 in | 41.7 | 1.54 | 1.70 | 77 | 64.8 |
| 6 in | 50.0 | 1.85 | 2.04 | 92 | 54 |
| 8 in | 66.7 | 2.47 | 2.72 | 123 | 40.5 |
The last column is a good sanity check. One cubic yard covers 81 sq ft at 4 in and only 54 sq ft at 6 in, so a 400 sq ft driveway at 6 in needs about 7.4 yards before waste.
Bagged mix is sold by weight, but what matters is how much concrete it makes once you add water. The usual yields for standard concrete mix are:
Divide your cubic feet (with waste) by the bag yield and round up. Here's the calculator's math for common jobs, all at 10% waste. The cost column uses the tool's default ready-mix price of $150 per yard, which is a placeholder, so put in your local quote.
| Project | Size | Cubic yards (+10%) | 80 lb bags | 60 lb bags | Ready-mix at $150/yd |
|---|---|---|---|---|---|
| AC or generator pad | 4 × 4 ft, 4 in | 0.22 | 10 | 14 | $33 |
| Shed floor | 8 × 10 ft, 4 in | 1.09 | 49 | 66 | $163 |
| Patio | 10 × 10 ft, 4 in | 1.36 | 62 | 82 | $204 |
| Sidewalk | 3 × 40 ft, 4 in | 1.63 | 74 | 98 | $244 |
| Patio | 12 × 12 ft, 4 in | 1.96 | 89 | 118 | $293 |
| Driveway section | 10 × 20 ft, 6 in | 4.07 | 184 | 245 | $611 |
| Two-car parking pad | 20 × 20 ft, 4 in | 5.43 | 245 | 326 | $815 |
| Garage slab | 24 × 24 ft, 6 in | 11.73 | 528 | 704 | $1,760 |
Those bag counts get silly fast. Ten bags for an AC pad is an easy Saturday. Sixty-two bags for a 10 × 10 patio is about 5,000 lb of mix by hand.
The break-even point is roughly one cubic yard, which is 45 bags of 80 lb mix. Below that, bags almost always win. Above 2 yards, a truck almost always wins. In between, it depends on three things:
If you order a truck, have forms, reinforcement and helpers ready before it arrives. Ready-mix is usually expected in place within about 90 minutes of batching.
For most home projects, the choice is between 4 and 6 inches:
Base prep matters as much as thickness. A slab poured on soft, uncompacted soil will crack no matter how thick it is. Most slabs go on a compacted gravel base of about 4 in, and our gravel calculator works out the tons for that layer. If you're adding rebar or mesh, the rebar calculator handles the spacing and bar count. Neither one changes the concrete volume.
Your local building code sets the actual minimum, so check it before you set forms.
Footings use the same formula, but the width and depth are usually in inches, so convert both. A 20 ft wall footing that's 16 in wide and 8 in deep is 20 × (16 ÷ 12) × (8 ÷ 12) = 17.8 cu ft. With 10% waste, that's 0.72 yd³ or 33 bags of 80 lb mix. That's the calculator's footing default.
Round posts and tube forms use the cylinder formula: π × radius² × height. The radius is half the diameter, in feet. A 12 in tube that's 4 ft tall holds π × 0.5² × 4 = 3.14 cu ft, a bit over five 80 lb bags.
| Tube diameter | Concrete per 4 ft tube | 80 lb bags per tube (no waste) | Four tubes (+10%) | 80 lb bags, four tubes |
|---|---|---|---|---|
| 8 in | 1.40 cu ft | 2.3 | 0.23 yd³ | 11 |
| 10 in | 2.18 cu ft | 3.6 | 0.36 yd³ | 16 |
| 12 in | 3.14 cu ft | 5.2 | 0.51 yd³ | 24 |
| 16 in | 5.59 cu ft | 9.3 | 0.91 yd³ | 41 |
Notice how the diameter changes things. Going from a 12 in to a 16 in tube nearly doubles the concrete, because volume grows with the square of the radius. For deck piers, our Sonotube calculator runs the same math for any tube size and count, with waste and a ready-mix price.
A 10 × 10 ft slab at 4 in thick is 33.3 cu ft, or 1.23 cubic yards. With 10% extra for waste and uneven ground, order about 1.36 yards. In bags, that's 62 bags of 80 lb mix or 82 bags of 60 lb mix.
About 45. An 80 lb bag of standard concrete mix yields roughly 0.60 cu ft, and a cubic yard is 27 cu ft, so 27 ÷ 0.60 = 45 bags. With 60 lb bags (0.45 cu ft each) it takes 60, and with 40 lb bags it takes 90.
It depends on thickness. One cubic yard covers 81 sq ft at 4 in, 64.8 sq ft at 5 in and 54 sq ft at 6 in. Divide 324 by the thickness in inches to get the square footage for any depth.
For jobs under about 1 cubic yard, bags are usually cheaper and simpler. Above 2 yards, ready-mix almost always wins once you count labor. In between, compare your store's per-bag price against the plant's price per yard plus any short-load fee.
Order 5 to 10% more than the calculated volume. Use 5% for small pours with rigid forms and a flat base, and 10% for slabs on excavated soil, where depth varies. For larger slabs, round up to the supplier's next ordering increment.