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.
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.
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.