Estimate bags of 60 or 80 lb ready-mix concrete from slab area, thickness, and waste allowance.
Slab Area (sq ft) — Value for slab area in sq ft. Provide accurate numbers for a reliable Bags Needed. Example: 100 sq ft.
Slab Thickness (in) — Value for slab thickness in in. Provide accurate numbers for a reliable Bags Needed. Example: 4 in.
Waste Allowance (%) — Value for waste allowance in %. Provide accurate numbers for a reliable Bags Needed. Example: 10 %.
Bag Size — Value for bag size. Provide accurate numbers for a reliable Bags Needed. Example: 60.
Bags Needed
82
Bags Needed — Calculated outcome. Derived from your inputs via the displayed formula; use to plan.
Concrete Volume
33.33 cu ft
Concrete Volume — Calculated outcome cu ft. Derived from your inputs via the displayed formula; use to plan.
Yield Per Bag
0.45 cu ft
Yield Per Bag — Calculated outcome cu ft. Derived from your inputs via the displayed formula; use to plan.
What this means
Standard bag yields used here are 0.45 cu ft for a 60 lb bag and 0.60 cu ft for an 80 lb bag — typical published figures for common ready-mix brands.
Estimate how many bags of ready-mix concrete you need for a slab. Volume is area × thickness, converted to cubic feet, with an extra allowance for waste.
Formula
volume (cu ft) = area × (thickness ÷ 12) × (1 + waste ÷ 100) | bags = ceil(volume ÷ bag yield)
Worked examples
FAQ
Why add a waste allowance?
Mixers spill, forms shift, and small repairs pop up. 5–10% is a common buffer.
Are bag yields the same for every brand?
Most 60lb and 80lb bags are near 0.45 and 0.60 cu ft, but always check the bag label.