Concrete is ordered by volume, so a patio estimate starts with area and ends with a thickness. This guide walks the square-feet-to-cubic-yards math, shows how many bags equal a yard, and covers the thickness and base that keep a slab from cracking.
What’s in this guide
The concrete volume formula
Cubic yards per 100 sq ft
| Slab thickness | Cubic feet / 100 sq ft | Cubic yards / 100 sq ft |
|---|---|---|
| 4 inches | 33.3 | 1.23 yd³ |
| 5 inches | 41.7 | 1.54 yd³ |
| 6 inches | 50.0 | 1.85 yd³ |
| 8 inches | 66.7 | 2.47 yd³ |
Worked example: a 12×12 patio
A 12 ft × 12 ft patio poured 4 inches thick:
| Step | Value |
|---|---|
| Area | 12 × 12 = 144 sq ft |
| Thickness | 4 in = 0.333 ft |
| Cubic feet | 144 × 0.333 = 48 cu ft |
| Cubic yards | 48 ÷ 27 = 1.78 yd³ |
| + 10% order | 1.96 → order 2 yd³ |
A pour must be continuous. Ordering 5–10% extra avoids a cold joint if the subgrade is deeper than planned in spots.
Bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard | Bags per 0.5 yd³ |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 45 |
| 60 lb | 0.45 cu ft | 60 | 30 |
| 80 lb | 0.60 cu ft | 45 | 23 |
Mixing 45+ eighty-pound bags by hand for a single patio is impractical. A short-load or ready-mix truck is usually cheaper and stronger once you pass a cubic yard.
Thickness & base guidelines
| Use | Slab thickness | Base |
|---|---|---|
| Walkway / light patio | 4 inches | 4 in compacted gravel |
| Standard patio | 4 inches | 4–6 in gravel |
| Hot tub / heavy load | 5–6 inches | 6 in gravel + rebar |
| Driveway | 5–6 inches | 6–8 in gravel + reinforcement |
Common mistakes
Thickness must be in feet in the formula. Four inches is 0.333 ft, not 4.
Subgrade is never perfectly flat. Order 5–10% extra.
Concrete cracks from movement, not just thin pours. Compacted gravel and control joints matter as much as volume.
Key takeaways
- Cubic yards = L × W × thickness(ft) ÷ 27.
- A 4-inch slab needs about 1.23 yd³ per 100 sq ft.
- One cubic yard ≈ 45 × 80 lb bags — use ready-mix beyond a yard.
- Order 5–10% extra and pour over a compacted base.
Related calculators & guides
Frequently asked questions
- How do I calculate concrete for a patio?
- Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27 for cubic yards. A 12×12 ft patio at 4 inches (0.333 ft) thick = 48 cubic feet = 1.78 cubic yards.
- How many bags of concrete are in a cubic yard?
- It depends on bag size: about 45 bags of 60 lb or 60 bags of 80 lb per cubic yard. Bagged concrete is practical only for small slabs; order ready-mix for anything over roughly a cubic yard.
- How thick should a concrete patio be?
- Four inches is standard for a residential patio. Go to 5–6 inches if it will bear heavy loads such as vehicles or hot tubs, and always over a compacted gravel base.
- How much extra concrete should I order?
- Add 5–10% to allow for uneven subgrade, spillage and over-excavation. Running short mid-pour creates a cold joint, so it is better to have slightly too much.