Concrete Calculator
Enter your slab dimensions to get the volume in cubic yards and the number of 40, 60 or 80 lb bags — including a spillage allowance.
How the concrete calculator works
Concrete is ordered by volume, not by weight, so every estimate starts by turning your project into cubic feet and then into cubic yards:
- Volume in cubic feet = length (ft) × width (ft) × thickness (ft). Convert thickness from inches by dividing by 12.
- Volume in cubic yards = cubic feet ÷ 27.
- Bags needed = cubic feet ÷ the yield of one bag, then rounded up.
Bag yields are set by the manufacturer and printed on the bag. The three standard sizes are 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb, and 0.30 cu ft for 40 lb. Those are the figures this calculator uses.
Worked example
A 10 ft × 12 ft patio slab at 4 inches thick:
- 10 × 12 = 120 sq ft
- 4 in ÷ 12 = 0.333 ft
- 120 × 0.333 = 40 cubic feet
- 40 ÷ 27 = 1.48 cubic yards
- 40 ÷ 0.60 = 66.7 → 67 bags of 80 lb mix
Add 5% for spillage, an uneven sub-base and the inch you always lose at the edges, and you land on about 70 bags.
Bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard | Weight per yard |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 3,600 lb |
| 60 lb | 0.45 cu ft | 60 | 3,600 lb |
| 80 lb | 0.60 cu ft | 45 | 3,600 lb |
All three sizes add up to roughly the same weight per yard, because they are the same mix in different packaging. Buy the size you can lift and mix before it starts to set — for most people that is 60 lb, not 80 lb.
Slab thickness by project
| Project | Typical thickness | Reinforcement |
|---|---|---|
| Garden path, stepping pads | 3–4 in | Not required |
| Patio, shed floor | 4 in | Mesh optional |
| Hot tub pad | 5–6 in | Mesh or rebar |
| Driveway (cars) | 5 in | Rebar or mesh |
| Driveway (trucks, RV) | 6 in | #4 rebar grid |
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
45 bags. An 80 lb bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45. That is 3,600 lb of mix per yard, which is why bagged concrete stops making sense for anything larger than a small slab.
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick the slab is 33.3 cubic feet, which is 56 bags of 80 lb mix. Add 5% and buy 59. At 6 inches thick the same slab needs 50 cubic feet, or 84 bags.
How thick should a concrete slab be?
Four inches for patios, walkways and shed floors. Five inches for driveways carrying cars, and six inches with a rebar grid where trucks or RVs will park. Thickness matters more than mix strength for avoiding cracks.
Do I need rebar or mesh?
For slabs under 4 inches, welded wire mesh is enough to hold cracks together. For driveways and anything structural, use #4 rebar on a 12–18 inch grid, placed in the middle third of the slab — not resting on the ground.
Is ready-mix cheaper than bags?
Above roughly one cubic yard, usually yes — but most suppliers charge a short-load fee under 3 yards, and some have a minimum. Get a delivered price for your exact yardage and compare it to bags × price per bag before deciding.
How long before I can walk on it?
Usually 24–48 hours for foot traffic, 7 days before parking a car, and around 28 days for full design strength. Cold weather slows all of these down considerably.