Concrete volume
Cubic yards = length × width × thickness (ft) ÷ 27
Bags = cubic feet ÷ bag yield
A 10 × 10 ft slab 4 inches thick is 1.23 cubic yards, or 1.36 with 10% extra. That is 62 bags of 80 lb mix, so for a slab this size, ready-mix is usually the better choice.
Bag yields
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Slab thickness
Walkways 4 inches, patios 4 inches, car driveways 4 to 5 inches, garage floors and truck driveways 5 to 6 inches.
The gravel base under a slab is covered by the gravel calculator.
Slab sizes: yards and 80 lb bags
Common slab footprints at the two usual thicknesses. Bags are rounded up and assume 0.60 cubic feet per 80 lb bag. Add 5 to 10% on top for uneven ground.
| Slab | 4 in: cubic yards | 4 in: bags | 6 in: cubic yards | 6 in: bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.20 | 9 | 0.30 | 14 |
| 6 × 6 ft | 0.44 | 20 | 0.67 | 30 |
| 10 × 10 ft | 1.23 | 56 | 1.85 | 84 |
| 10 × 12 ft | 1.48 | 67 | 2.22 | 100 |
| 12 × 12 ft | 1.78 | 80 | 2.67 | 120 |
| 20 × 20 ft | 4.94 | 223 | 7.41 | 334 |
What changes the amount
- Thickness: going from 4 to 6 inches adds half again as much concrete, so confirm the thickness before you order.
- Ground: a base that is lower in the middle takes more concrete than the flat figure.
- Forms: measure inside the forms, because the outside of a board adds area you do not pour.
- Cost: see the concrete cost calculator to compare bags with ready-mix, and the post hole calculator for fence posts.
How many 60 lb bags of concrete are in a yard?
60 bags. A 60 lb bag yields about 0.45 cubic feet and a cubic yard is 27 cubic feet. With 80 lb bags you need 45 per yard, and with 40 lb bags, 90.
How much concrete do I need for a fence post?
For a 4 × 4 post in a 10-inch hole 3 feet deep, plan about 1.4 cubic feet of concrete, which is two to three 80 lb bags. The hole holds about 1.6 cubic feet and the post takes up about 0.25 of that.
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
45 bags.
How much concrete for a 10x10 slab?
1.23 cubic yards at 4 inches thick, or about 62 bags of 80 lb mix with 10% extra.
How thick should a concrete patio be?
4 inches over 4 inches of compacted gravel.
How much concrete do I need for a 10x10 slab?
At 4 inches thick it is 1.23 cubic yards, or 56 bags of 80 lb mix. At 6 inches it is 1.85 cubic yards, or 84 bags.
When is ready-mix better than bags?
Many people switch to a truck around 1 cubic yard, which is 45 bags of 80 lb mix. Ask the plant about its minimum load and short-load fee.
Reference charts
More landscape material calculators
Gravel Calculator
Tons, yards and bags by stone type.
Asphalt Calculator
Tons of hot mix for driveways and lots.
Cubic Yard Calculator
Any dimensions to cubic yards.
Mulch Calculator
Yards and bags for beds and trees.
Topsoil Calculator
Soil for lawns, beds and fill.
Dirt Calculator
Fill dirt in yards, tons and truckloads.