How Much Concrete Do I Need?
Estimate concrete in cubic yards and bags, and decide between bagged mix and ready-mix delivery.
Concrete is estimated by volume in cubic yards, then converted to bags or ready-mix. Here's the process, the bag yields you need, and how to decide between hauling bags and ordering a truck.
Step by step
- Measure length, width, and thickness Measure in feet, and the slab thickness in inches. A typical patio or walkway slab is 4 inches thick.
- Convert to cubic feet Multiply length × width × (thickness ÷ 12). A 10 × 10 ft slab at 4 inches is about 33.3 cubic feet.
- Divide by 27 for cubic yards There are 27 cubic feet in a cubic yard, so 33.3 ÷ 27 ≈ 1.23 cubic yards.
- Choose bags or ready-mix Small jobs suit bags; anything over about half a cubic yard is usually cheaper and faster as ready-mix delivery.
- Add 5–10% for waste Uneven ground and spillage mean you should order a little extra.
Concrete bag yields
| Bag size | Yield (cubic feet) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
That last column is the key takeaway: it takes about 45 eighty-pound bags to make a single cubic yard. Once you're mixing dozens of bags by hand, ready-mix delivery is almost always the better call.
Frequently asked questions
How many 80-lb bags of concrete are in a cubic yard?
About 45. An 80-lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
Should I use bags or ready-mix?
Bags are convenient for small pours. Above about half a cubic yard, ready-mix delivery usually saves money, time, and effort.
How much extra concrete should I order?
Add 5–10% for waste from spillage, uneven subgrade, and over-excavation.