Why is the allowance separate from net volume?
Keeping it separate preserves the measured basis and makes the adjustment reviewable. The contractor and supplier should select an appropriate allowance for the actual design and placement conditions.
FREE TOOL / QUANTITY MATH
Concrete quantities.
One transparent calculation.
Measure a rectangular slab, then see exactly what your selected allowance adds.
RECTANGULAR SLAB
From dimensions
to volume.
Calculate one flat rectangular slab at a uniform thickness.
feet
feet
inches
%
The 5% allowance is an editable example, not a recommended ordering factor. Confirm dimensions, mix, and allowance with your contractor and supplier.
Understand the full concrete scopeYOUR QUANTITIES
Volume including selected allowance31.11 CYNet concrete volume29.63 CY
Added allowance (5%)1.48 CY
Plan area2,400 SF
Perimeter200 LF
NET VOLUME FORMULA
Length × width × (inches ÷ 12) ÷ 27Excludes footings, thickened edges, reinforcement, openings, slopes, and pricing.
What this volume does—and does not—include
The calculation converts thickness from inches to feet, multiplies by length and width, and divides cubic feet by 27 to obtain cubic yards. Results retain full precision until display; rounding is not an ordering recommendation.
It assumes a flat rectangular slab of uniform thickness. It does not include thickened edges, foundations, piers, slopes, openings, reinforcement, soil conditions, placement, finishing, curing, or material prices. Each of those may require separate measurement and professional review.
Use the concrete estimating guide to define the full package. The demonstration worksheet uses the same example dimensions so you can follow the quantity basis.
A FEW USEFUL ANSWERS
Questions, clarified.
Can I use it for a footing or sloped slab?
It can perform rectangular volume arithmetic, but the visible tool is specifically a uniform slab model. Footings, steps, thickened edges, and slopes require additional geometry and interpretation of the structural documents.
LET’S DEFINE THE SCOPE
Your drawings. A clearer next step.
Start with your project location, document set, and bid deadline.