How to use the concrete calculator
- Choose Slab / footing for rectangular pours (patios, sidewalks, driveways, shed bases) or Round post for fence posts, deck footings and columns.
- Enter the measurements. Slab length and width are in feet; thickness is in inches.
- Enter how many identical pours you need — for example, 8 fence post holes.
- Keep 5%–10% extra for spillage, uneven ground and over-excavation.
Concrete volume formulas
Example: a 10 × 10 ft patio, 4 inches thick: 10 × 10 × 4 ÷ 12 = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. With 10% extra, order about 1.36 yd³ — or buy 62 bags of 80 lb premix.
How many bags of concrete do I need?
| Bag size | Yield per bag | 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 |
Yields are typical for standard premixed concrete; check the bag, as some mixes differ slightly.
Recommended thickness
- Sidewalks, patios, shed floors: 4 inches.
- Driveways for cars: 4–5 inches; 6 inches for trucks, RVs or poor soil.
- Fence posts: a hole about three times the post width, one-third to one-half of the post's above-ground height deep, and below the frost line in cold climates.
Bags or ready-mix?
Bags make sense for small jobs such as posts, steps and pads under about one cubic yard. For larger slabs, ready-mix concrete from a truck is usually cheaper per yard, more consistent and far less work — many suppliers have a minimum order, and short loads may carry an extra fee.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 inches thick needs 33.3 cubic feet, or about 1.23 cubic yards. With 10% extra that is about 1.36 cubic yards, or 62 bags of 80 lb concrete.
How many 80 lb bags of concrete make a yard?
About 45 bags. Each 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How do I calculate cubic yards of concrete?
Multiply length × width × depth, all in feet, to get cubic feet, then divide by 27. For depth in inches, divide it by 12 first.
How much concrete do I need for a fence post?
A 10-inch-wide hole 3 feet deep holds about 1.6 cubic feet — roughly three 80 lb bags, minus the volume of the post itself.
How much extra concrete should I order?
Add 5% to 10% for spillage, uneven subgrade and forms that bow out. Running short in the middle of a pour is much worse than having a little left over.
Results are provided for convenience and rounded for display. Sales tax rules, tipping customs and date conventions differ by location, so check local rules when it matters.
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