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Calculate premixed concrete bags for slabs, pads and rectangular pours. Compare common bag sizes, include waste, estimate pallet loads and calculate material cost.
Concrete bag formulas
The calculator converts every dimension to feet, then reports cubic feet, cubic yards and cubic meters. Whole bags are required, so rounding happens after allowance is added.
Worked example
A 10 ft × 10 ft slab, 4 inches thick, contains 33.33 ft³. With 10% allowance, the order volume is 36.67 ft³. At 0.60 ft³ per 80 lb bag, buy 62 bags.
Visual guide
Break complex pours into rectangles and calculate each section separately. Use the total bag count only after all sections and allowances are combined.
Bag-yield reference
- 40 lb bag: approximately 0.30 ft³.
- 50 lb bag: approximately 0.375 ft³.
- 60 lb bag: approximately 0.45 ft³.
- 80 lb bag: approximately 0.60 ft³.
These are planning values. High-strength, fast-setting and specialty mixes can have different yields and water requirements.
Ordering checklist
- Confirm package yield and required compressive strength.
- Check shelf condition and protect bags from moisture.
- Plan mixing capacity, water supply and placement time.
- Verify pallet deposits, delivery fees and return policy.
Frequently asked questions
How many 80 lb bags make one cubic yard?
At 0.60 ft³ per bag, one cubic yard needs 45 bags before allowance.
Should I use bags or ready-mix?
Bags suit smaller pours and limited access. Larger pours may be faster and more consistent with ready-mix.
Why add an allowance?
Spillage, uneven subgrade, form variation and incomplete bag yield can increase demand.
Does the weight include water?
The dry mix result uses nominal bag weight only. Mixed concrete weight is reported separately as an estimate.
Common mistakes
- Using inches as feet for slab depth.
- Dividing before adding allowance.
- Rounding down partial bags.
- Assuming every bag size has the same yield.