How to use this calculator
Select your project type — slab, footing/wall, or column/post — then enter your dimensions. The calculator instantly shows how much concrete you need in cubic yards, plus the equivalent number of 80 lb and 60 lb bags.
The 10% waste factor is enabled by default. This accounts for uneven ground, form imperfections, and spillage during pouring. For very precise formwork on flat ground, you can disable it, but it's generally safer to leave it on.
Concrete coverage by bag size
| Bag Size | Coverage (cu ft) | Bags per Cu Yd | Typical Price |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags | $3–$4 |
| 50 lb | 0.375 cu ft | 72 bags | $4–$5 |
| 60 lb | 0.45 cu ft | 60 bags | $4–$6 |
| 80 lb | 0.60 cu ft | 45 bags | $5–$7 |
Ready-mix vs. bags: which should you choose?
Bags (under 1 cubic yard): Best for small projects like fence posts, mailbox bases, small repair patches, and stepping stones. You control the pace and don't need to coordinate delivery. Downside: mixing bags by hand is physically demanding and slow.
Ready-mix (1+ cubic yards): A concrete truck delivers pre-mixed concrete directly to your site. It's faster, ensures consistent mix quality, and is more cost-effective for larger pours. Most suppliers have a minimum order of 1 cubic yard and may charge short-load fees for small orders.
Rule of thumb: If your project needs more than 45 bags (about 1 cubic yard), ready-mix is almost always the better choice in terms of cost, time, and quality.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 foot slab at 4 inches thick requires about 1.23 cubic yards of concrete, which equals approximately 56 bags of 80 lb mix or 74 bags of 60 lb mix. Add 10% for waste, bringing the total to about 62 bags of 80 lb mix.
How thick should a concrete slab be?
Standard concrete slab thickness is 4 inches for patios, walkways, and shed floors. Driveways should be 5–6 inches thick. Garage floors are typically 6 inches. For heavy equipment or commercial use, 8 inches or more may be required.
Should I use bags or ready-mix concrete?
For projects under 1/2 cubic yard (about 14 square feet at 4 inches thick), bags are practical. For anything over 1 cubic yard, ready-mix delivery is more cost-effective and ensures consistent quality. Ready-mix typically costs $125–$150 per cubic yard delivered.
How much does concrete cost?
Ready-mix concrete costs $125–$150 per cubic yard delivered. An 80 lb bag costs $5–$7 at most home improvement stores and yields about 0.6 cubic feet. For a typical 10x10 patio (4" thick), expect to spend $150–$185 for ready-mix or $280–$430 for bags.