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Concrete Volume Calculator (Bags & Tubes)

Calculate concrete volume for slabs, footings, and Sonotubes. Includes waste estimation and pre-mixed bag counters (60lb/80lb).

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About

Ordering concrete requires balancing structural sufficiency against financial waste. Shortages during a pour result in cold joints. This compromises structural integrity. Excess results in disposal fees. This tool addresses the specific needs of residential foundations and deck post installations.

We calculate volume based on standard geometric density. The density of cured concrete averages 145 lbs/ft3. Bag yields vary by manufacturer but follow reliable averages for 60lb and 80lb sacks. Waste factors must account for spillage and uneven subgrade excavation.

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Formulas

Volume V for rectangular slabs is the product of length l. width w. and thickness h.

Vslab = l × w × h

For cylindrical forms (Sonotubes). we use the cylinder volume formula with radius r.

Vtube = π × r2 × h

Total bags N required is calculated by dividing total volume (plus waste percentage W) by the specific bag yield Y.

N = V × (1 + W)Y

Reference Data

Bag Size (lbs)Yield (ft3)Bags per yd3Typical Use
400.3090Small Repairs, Setting Posts
500.3773Walkways, Steps
600.4560Slabs, Footings (DIY Standard)
800.6045Large Slabs, Structural Posts
900.6840Commercial Applications

Frequently Asked Questions

For rectangular forms on flat ground. 5% is standard. For excavation into uneven earth or post holes. use 10% to account for over-digging and spillage.
The concrete mix is identical in most cases. The difference is purely weight and yield. 80lb bags are more cost-effective for larger jobs but physically demanding to move and mix.
The break-even point is typically around 1 to 1.5 cubic yards. If your project requires more than 45 bags of 80lb mix. a ready-mix truck becomes more time-efficient and ensures a consistent cure.