Concrete Bags Calculator
Convert a slab, post hole, or patch volume into the number of premix concrete bags needed. Supports 40, 60, and 80 lb bag yields.
Assumptions
- Bag yields follow typical manufacturer data for premixed concrete.
- Results round up to whole bags.
- Water added per manufacturer instructions; mix consistency is uniform.
- Does not account for aggregate-only mixes or custom high-strength designs.
- 10% waste is applied when enabled.
Volume (ft³) = length × width × thickness, or enter volume directly. Bags needed = ceil(volume with waste ÷ yield per bag). Standard yields: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³.Post hole 12 in × 12 in × 36 in deep
- Volume = 1 ft × 1 ft × 3 ft = 3 ft³. With 10% waste = 3.3 ft³. Using 80 lb bags (0.60 ft³ each): 3.3 ÷ 0.60 = 5.5 → 6 bags.
How to measure
For holes, measure diameter or square side and depth to the bottom of the excavation. For slabs, measure the formed area and thickness. When in doubt, measure each hole separately and sum volumes.
Common mistakes
- Using the bag weight as volume (80 lb is weight, not 80 ft³).
- Ignoring waste on multiple small pours where spillage adds up.
- Mixing bag sizes within one calculation — pick one bag weight per order.
Frequently asked questions
40 lb vs 80 lb bags — which is better?
80 lb bags cover more volume per bag and often cost less per cubic foot, but 40 lb bags are easier to handle alone.
Can I enter volume in cubic yards?
Yes. Switch the volume unit to cubic yards and the calculator converts internally.
How much water per bag?
Follow the label — typically about 3 qt for 80 lb. Too much water weakens the mix.
Is this enough for a sonotube?
Measure the tube inside diameter and depth. Cylindrical holes can be entered as equivalent square dimensions or direct volume.