Concrete Sqft Calculator
Enter your slab area and thickness to get the concrete volume in cubic feet, cubic yards and bags.
To calculate concrete for a slab, multiply the square footage by the thickness in feet, then divide by 27 for cubic yards. A 400 sq ft patio at 4 inches thick is 400 × 0.333 = 133.3 cubic feet, which is 133.3 ÷ 27 = 4.94 cubic yards.
Order 5–10% extra for spillage and an uneven subgrade. Ready-mix suppliers usually have a minimum order of around 1 cubic yard. Bag counts assume 0.45 ft³ per 60 lb bag and 0.6 ft³ per 80 lb bag.
How thick should the slab be?
Thickness is driven by what the slab has to carry:
- 4 inches — patios, walkways, shed bases, garden paths
- 5 to 6 inches — driveways carrying cars and light trucks
- 6 inches or more — heavy vehicles, RV pads, workshop floors
Most residential work is 4 inches over a compacted gravel base of 4 to 6 inches. Going thicker without reinforcement is not usually the right answer — rebar or mesh does more for crack resistance than extra depth.
Slab thickness, subgrade preparation and joint spacing are covered in detail by the American Concrete Institute, which publishes the guides most residential slab work follows. Local building codes take precedence where they differ.
Working out the volume
Three steps:
- Area: length × width. A 20 ft × 20 ft patio is 400 sq ft.
- Volume in cubic feet: multiply by thickness in feet. At 4 inches (0.333 ft): 400 × 0.333 = 133.3 ft³.
- Cubic yards: divide by 27. 133.3 ÷ 27 = 4.94 yd³.
Then add 5–10%. On this job you would order 5.5 yards.
Bags or ready-mix?
The crossover point is around 1 cubic yard, or about 40 to 45 of the 80 lb bags.
- Under 1 yard — bags are practical. An 80 lb bag yields about 0.6 ft³, a 60 lb bag about 0.45 ft³.
- Over 1 yard — ready-mix delivery is cheaper, far faster, and gives a consistent mix throughout the pour.
Mixing 45 bags by hand for a patio is a long, hard day and risks inconsistent strength between batches. Most suppliers deliver a minimum of 1 yard, sometimes with a short-load fee.
Concrete needed by slab size
At the standard 4 inch thickness, before adding waste.
| Slab size | Area | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|---|
| 8 × 10 ft | 80 sq ft | 26.7 ft³ | 0.99 yd³ | 45 bags |
| 10 × 10 ft | 100 sq ft | 33.3 ft³ | 1.23 yd³ | 56 bags |
| 12 × 12 ft | 144 sq ft | 48 ft³ | 1.78 yd³ | 80 bags |
| 16 × 20 ft | 320 sq ft | 106.7 ft³ | 3.95 yd³ | 178 bags |
| 20 × 20 ft | 400 sq ft | 133.3 ft³ | 4.94 yd³ | 222 bags |
| 24 × 24 ft | 576 sq ft | 192 ft³ | 7.11 yd³ | 320 bags |
Frequently asked questions
How much concrete do I need per square foot?
At 4 inches thick, each square foot needs 0.333 cubic feet of concrete, which is 0.0123 cubic yards. So 100 square feet needs about 1.23 cubic yards.
How many cubic yards for a 20x20 patio?
A 20 by 20 foot patio is 400 square feet. At 4 inches thick that is 133.3 cubic feet, or 4.94 cubic yards. Order about 5.5 yards to allow for spillage and an uneven base.
How thick should a concrete patio be?
Four inches is standard for a patio or walkway. Increase to 5 or 6 inches for a driveway, and 6 inches or more for heavy vehicles.
How many bags of concrete in a cubic yard?
About 45 of the 80 lb bags, or 60 of the 60 lb bags. At that point ready-mix delivery is usually cheaper and much faster.
Should I order extra concrete?
Yes — add 5 to 10%. Subgrade is never perfectly level, some concrete is lost to spillage, and running short mid-pour creates a cold joint that weakens the slab.