Choose a slab, wall or round column, enter the size, and get cubic yards of concrete plus the number of bags to buy. Add prices to compare ready-mix with bags.
Extra for waste and cost
Bag yields are typical for premixed concrete. Check the yield printed on your bag.
How to use the concrete calculator
- Pick what you’re pouring: a slab (or footing pad), a wall, or a round column.
- Enter the length and width (or height) in feet and the thickness in inches. For a column, enter the diameter and height.
- If you’re pouring several identical pieces, enter how many.
- Choose your bag size, and open “Extra for waste and cost” to add prices.
Concrete formula
- Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet ÷ the yield of one bag
Example: a 10 ft × 10 ft patio slab, 4 inches thick.
- 10 × 10 × 4 ÷ 12 = 33.3 cubic feet
- 33.3 ÷ 27 = 1.23 cubic yards
- With 10 percent extra: 36.7 cubic feet, or 1.36 cubic yards
- 36.7 ÷ 0.60 = 62 bags of 80 lb, or 82 bags of 60 lb
How many bags of concrete make a cubic yard?
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
These are typical yields for premixed concrete. Each brand prints its own yield on the bag, so check it if you’re using a different mix.
How thick should a concrete slab be?
| Project | Typical thickness | 1 cubic yard covers |
|---|---|---|
| Patio, walkway, shed base | 4 in | 81 sq ft |
| Driveway for cars | 4–5 in | 65–81 sq ft |
| Driveway for heavier vehicles | 6 in | 54 sq ft |
These are common starting points. Footings, foundations and anything structural should follow your plans and local building code.
Bags or ready-mix?
Bags are handy for small jobs like a few posts, a step or a small pad. For anything over about a cubic yard, mixing by hand gets hard: one cubic yard is 45 bags of 80 lb, all mixed and placed before the first ones start to set. Ready-mix delivery is usually the better choice there, although many suppliers charge extra for small loads. Enter both prices in the calculator to compare.
Tips for a good pour
- Lay and compact a gravel base first. The crushed stone calculator works out how much you need.
- Add reinforcement for slabs that will carry weight. The rebar calculator plans the grid.
- Check the forms are level and braced, so they don’t bow and use extra concrete.
- Avoid pouring in freezing weather or very hot sun, which both weaken the finished slab.
Frequently asked questions
How many bags of concrete do I need?
Work out the volume in cubic feet, then divide by the yield of your bag: 0.60 cubic feet for an 80 lb bag or 0.45 for a 60 lb bag. The calculator does this and rounds up.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 ft × 10 ft slab needs 1.23 cubic yards, which is about 56 bags of 80 lb before any extra. With 10 percent extra, plan on 62 bags.
How much does a cubic yard of concrete weigh?
About 4,000 pounds, or roughly 2 tons. That’s why trailers and pickup trucks can usually carry only part of a yard.
Is it cheaper to use bags or ready-mix concrete?
For small jobs, bags are usually cheaper because there’s no delivery fee. For larger slabs, ready-mix is often cheaper per cubic yard and far less work. Enter both prices above to compare for your job.
How long before I can walk on new concrete?
Usually about a day or two for foot traffic and about a week for cars. Concrete keeps gaining strength for around four weeks, so avoid heavy loads until then.
Results are estimates for planning. Bag yields are typical for premixed concrete. See How We Calculate for the formulas and values we use.