Concrete Bag Calculator
Find out how many bags of premix concrete a slab, footing or post hole takes — pick your bag size (40, 60 or 80 lb) and get an exact bag count with a waste allowance.
Result
| Volume | |
| Bags (exact) |
Units & conversions used
Volume is computed in cubic feet (1 yd³ = 27 ft³; 1 ft³ = 0.02832 m³). Bag yields are the manufacturers' published figures for standard premix: an 80 lb bag makes 0.60 ft³ of concrete, a 60 lb bag 0.45 ft³, a 40 lb bag 0.30 ft³.
Formula & source
Bags = Volume(ft³) ÷ yield per bag × (1 + waste%)
Source: rectangular-prism geometry; bag yields as printed on standard premix bags (e.g. Quikrete/Sakrete 40/60/80 lb). The result is always rounded up — you cannot buy a fraction of a bag, and running one bag short mid-pour is far worse than having one left over.
Worked example (you can verify by hand)
Input: 10 ft × 12 ft slab, 4 in thick, 80 lb bags, 0% waste.
- Volume: 10 × 12 × (4 ÷ 12) = 40.00 ft³ (= 1.48 yd³)
- Bags exact: 40 ÷ 0.60 = 66.67
- Rounded up: 67 bags
Expected output: 67 × 80 lb bags for 40.0 ft³ (1.48 yd³).
Limitations — what this cannot do
- Assumes a rectangular area of uniform depth. For round post holes, use the column calculator for volume, then divide by your bag yield.
- Above roughly 1 yd³ (about 45 × 80 lb bags), ready-mix delivery is usually cheaper and far less labor than hand-mixing bags. This calculator tells you the volume either way — compare before you commit to mixing.
- Bag yield assumes the manufacturer's stated water ratio. Over-watering weakens concrete and stretches yield on paper only.
FAQ
How many 80 lb bags of concrete are in a cubic yard?
One cubic yard is 27 ft³, and an 80 lb bag yields 0.60 ft³, so 27 ÷ 0.60 = 45 bags per cubic yard. For 60 lb bags it is 60, and for 40 lb bags it is 90.
How many bags do I need for a 10×10 slab at 4 inches?
Volume is 10 × 10 × (4÷12) = 33.3 ft³. In 80 lb bags: 33.3 ÷ 0.60 = 55.6, so 56 bags before waste — buy about 59 with a 5% allowance.
Is it cheaper to buy bags or order ready-mix?
For small jobs (a few post holes, a small pad) bags win: no delivery minimums. Past roughly 1 yd³, ready-mix usually costs less per yard and saves hours of mixing. Get the yardage from our slab calculator and price both.
How much does an 80 lb bag of concrete cost?
Standard 80 lb premix bags commonly sell in the $5–$8 range at US home centers (2025–2026 shelf pricing), with high-strength mixes at the top of the range. Pallet pricing lowers the per-bag cost — for 60+ bags, ask about pallet discounts and delivery instead of hand-loading singles.