To find how much concrete a slab needs, multiply length × width × thickness, with all three in feet, to get cubic feet. Divide by 27 to get cubic yards, which is how ready-mix concrete is sold. Then add 5–10% for waste.
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
Worked example: a 10 × 12 ft patio, 4 inches thick
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 10 × 12 × 0.333 = 40 cubic feet.
- Cubic yards: 40 ÷ 27 = 1.48 yd³.
- Add 10% waste: 40 × 1.1 = 44 ft³, or about 1.63 yd³.
- In 80 lb bags (about 0.60 ft³ each): 44 ÷ 0.60 = 73.3, so 74 bags.
The concrete calculator does this for slabs and round post holes, with 40, 60 and 80 lb bag counts.
How many bags make a cubic yard?
Bag yields are printed on the bag. For standard concrete mix they’re about:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Mixing dozens of bags by hand is heavy work. For bigger pours, compare the price of a ready-mix delivery (most suppliers have a minimum order) or a rented mixer.
How thick should the slab be?
4 inches is the usual thickness for patios, walkways and shed bases. Driveways and slabs that carry heavier loads are commonly 5 to 6 inches. Your local building code or the project plans have the final say. Each extra inch on a 10 × 12 ft slab adds 10 ft³, about 17 more 80 lb bags.
Why add extra?
- Ground is rarely perfectly flat, and low spots take more concrete.
- Forms bow slightly under the weight of wet concrete.
- Some is always lost to spillage and what’s left in the mixer.
Running short mid-pour is far worse than having a few bags left over, so 5–10% extra is standard.
Irregular shapes
Split the area into rectangles, work out each one and add them up. The square footage calculator adds up to four areas, and you can then multiply the total square feet by the thickness in feet. To switch between cubic yards and cubic feet, use the cubic yards to cubic feet converter.
Before you pour
- Pour to the full depth in one go. Partial layers can leave weak joints.
- Cut control joints about 1 inch deep every 6 to 8 feet to keep cracks in straight lines.
- Keep the slab damp, or use a curing compound, for 5 to 7 days, and protect it from freezing for the first 48 hours.