Concrete Calculator
Work out how much concrete you need for a slab, patio, footing or post holes in cubic yards and 40, 60 or 80 lb bags.
How to calculate concrete
Concrete is sold by volume, in cubic yards for ready-mix trucks and by weight in bags at hardware stores. Calculate the volume in cubic feet, then convert.
Cubic feet = Length (ft) × Width (ft) × Thickness (in) ÷ 12 Cubic yards = Cubic feet ÷ 27
A 10 × 10 ft patio, 4 inches thick: 10 × 10 × 4 ÷ 12 = 33.3 ft³, or 1.23 yd³. Adding 10% for waste gives 1.36 cubic yards, about 62 bags of 80 lb mix.
How much does a bag make?
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Typical thickness
- Walkways and patios: 4 inches.
- Driveways: 4–5 inches for cars, 6 inches for heavier vehicles.
- Garage floors: 4–6 inches.
- Fence posts: hole diameter about three times the post width, and one-third to one-half of the post length deep, below the frost line in cold climates.
Bags or ready-mix?
Bags suit small jobs such as posts, steps and small pads. Above roughly one cubic yard (45 bags of 80 lb, or 3,600 lb of mix), ready-mix delivery is usually cheaper and gives a more even pour, though many suppliers charge a short-load fee for small orders. Always order 5–10% extra: uneven ground and spillage use more than you expect.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
For a 4-inch slab, about 56 bags of 80 lb mix (or 75 bags of 60 lb), plus 5–10% extra.
How many 80 lb bags make a cubic yard?
About 45 bags, since each 80 lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.
How thick should a concrete patio be?
Four inches is standard for patios and walkways, on a compacted gravel base.
Last reviewed Oct 20, 2026. Results are estimates; check important decisions with a qualified professional.