How this calculator works
The calculator multiplies length × width × thickness to get the slab volume in cubic feet, adds your waste allowance, then converts to cubic yards (27 cubic feet per yard). Bags are found by dividing the volume by the yield of the bag size you picked and rounding up.
Assumptions and tips
- Bag yields follow the manufacturer’s published figures: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 50 lb ≈ 0.375 cu ft, 40 lb ≈ 0.30 cu ft.
- Bag counts are always rounded up to whole bags.
- The waste allowance covers spillage, uneven subgrade and over-excavation. 5–10% is common; use more for irregular forms.
- One cubic yard takes about 45 bags of 80 lb mix (27 ÷ 0.60).
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Length | 10 ft |
| Width | 10 ft |
| Thickness | 4 in |
| Waste allowance | 10 % |
| Bag size | 80 lb bag (0.60 cu ft) |
| Price per bag (optional) | $0 |
| Ready-mix price per cubic yard (optional) | $0 |
| Result | Value |
|---|---|
| Concrete needed | 1.36 cubic yards |
| 80 lb bags to buy | 62 bags |
| Volume (with waste) | 36.67 cu ft |
| Volume (with waste) | 1.04 m³ |
| Slab area | 100 sq ft |
| Total bag weight | 4,960 lb |
Step by step:
- Volume = 10 ft × 10 ft × (4 in ÷ 12) = 33.33 cu ft
- Add 10% waste: 33.33 × 1.1 = 36.67 cu ft
- Cubic yards: 36.67 ÷ 27 = 1.358 yd³
- Bags: 36.67 ÷ 0.6 cu ft per 80 lb bag = 61.11 → round up to 62 bags
How many bags of concrete for common slab sizes?
Calculated with this tool at 10% waste, using 80 lb premix bags.
| Slab size & thickness | Cubic yards | 80 lb bags |
|---|---|---|
| 4 × 4 ft pad, 4 in | 0.22 cubic yards | 10 bags |
| 8 × 8 ft, 4 in | 0.87 cubic yards | 40 bags |
| 10 × 10 ft, 4 in | 1.36 cubic yards | 62 bags |
| 10 × 12 ft shed base, 4 in | 1.63 cubic yards | 74 bags |
| 12 × 12 ft patio, 4 in | 1.96 cubic yards | 88 bags |
| 20 × 20 ft, 4 in | 5.43 cubic yards | 245 bags |
| 12 × 24 ft driveway pad, 5 in | 4.89 cubic yards | 220 bags |
Common mistakes to avoid
- Measuring thickness in inches but entering it as feet — always divide inches by 12.
- Forgetting waste. Uneven subgrade alone can add 5–10% to the concrete you need.
- Mixing up 60 lb and 80 lb bags at the store: you need about a third more 60 lb bags for the same volume.
- Planning to hand-mix a full cubic yard alone. That is 45 heavy bags; concrete starts setting before you finish.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags.
How thick should a concrete patio be?
Most patios and walkways are poured 4 inches thick. Areas that carry vehicles are usually thicker. Follow your plans and local building requirements.
Should I use bags or order a truck?
Bags suit small pads and post holes. Once you need roughly a cubic yard or more, ready-mix delivery is often faster and gives a more consistent pour. Enter both prices above to compare.