Concrete Bag Calculator
Quick answer: measure your pour, and this calculator turns it into cubic yards and the number of 40, 60 or 80 lb premix bags to buy — including a waste allowance so you don't run short. A 10×10 ft slab at 4 inches thick, for example, needs 1.36 cubic yards — 62 bags of 80 lb mix with 10% waste.
Bag counts include your waste allowance and are rounded up to whole bags. Volumes are shown with the allowance included.
How it works — the formula
Every concrete estimate is a volume problem. For a rectangular slab or footing:
Volume (ft³) = Length (ft) × Width (ft) × Thickness (in) ÷ 12For a round post hole, the volume is a cylinder:
Volume (ft³) = π × (Diameter (in) ÷ 24)² × Depth (in) ÷ 12 × Number of holesConvert and divide by how much finished concrete one bag actually makes:
Cubic yards = ft³ ÷ 27 · Bags = ⌈ ft³ × (1 + waste %) ÷ bag yield ⌉| Bag size | Yield per bag | Bags per cubic yard (no waste) |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Yields are the standard figures published on manufacturer data sheets for premix concrete (for example QUIKRETE Concrete Mix). Real-world yield varies slightly with water and mixing, which is one more reason the waste allowance matters.
Worked examples
| Project | Volume (with 10% waste) | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| Patio slab 10 × 10 ft, 4 in thick | 36.7 ft³ · 1.36 yd³ | 62 | 82 | 123 |
| Driveway section 12 × 20 ft, 6 in thick | 132 ft³ · 4.89 yd³ | 220 | 294 | 440 |
| 3 fence post holes, 12 in Ø × 36 in deep | 7.78 ft³ · 0.29 yd³ | 13 | 18 | 26 |
Check the first row by hand: 10 × 10 × (4 ÷ 12) = 33.3 ft³. Add 10% → 36.7 ft³. Divide by the 0.60 ft³ an 80 lb bag yields → 61.1, rounded up to 62 bags. At this size a ready-mix delivery usually beats mixing 62 bags by hand — the calculator doesn't sell you anything, so we'll say so.
Frequently asked questions
How many 80 lb bags of concrete do I need for a 10x10 slab?
A 10×10 ft slab at 4 in thick is 33.3 ft³ (1.23 yd³): 56 bags exactly, or 62 bags with a 10% waste allowance. At 6 in thick it is 84 bags, or 92 with waste.
How many bags of concrete make one cubic yard?
45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb — before waste.
Should I add a waste allowance?
Yes — 5–10% covers spillage, uneven ground and formwork losses. One unopened spare bag is cheaper than a second trip mid-pour.
When is ready-mix cheaper than bags?
Usually once you pass about 1 cubic yard (≈45 × 80 lb bags). Below that, bags are simpler and avoid short-load fees.
How much concrete does a fence post hole need?
A 12 in Ø × 36 in deep hole is about 2.36 ft³ before subtracting the post — roughly 4 × 80 lb bags per hole with a small allowance. Try the Round mode above with your real measurements.
Related tools
Fence Post Concrete Calculator — bags per hole by post size and depth.
See the full list on the Home & DIY Calculators page.
Estimates only. This calculator gives planning quantities for typical residential pours; it is not a structural or engineering calculation. Check local code for required slab thickness, footings and frost depth, and confirm quantities with your supplier before ordering.