Concrete calculator formula: Cubic Yards (with waste %) & 80-lb bag count

Never divide by 9 instead of 27—1 Cubic Yard equals 27 Cubic Feet!

Contractors measure slab footprints in feet (20 ft × 15 ft), slab depth in inches (4 in), and order ready-mix concrete in Cubic Yards (3 ft × 3 ft × 3 ft = 27 cu ft). Dividing Thickness_in / 12 converts depth to feet first: NetCuFt = Length_ft × Width_ft × (Thickness_in / 12) = 20 × 15 × (4/12) = 100.0 cu ft -> NetCuYd = 100 / 27 = 3.70 cu yd!

The example adds a chosen 10% allowance, producing about 4.0741 cubic yards, displayed as 4.07. The allowance is not a universal requirement; select it for the project and supplier. With an assumed 0.60 cubic-foot yield per bag, the unrounded volume requires 184 whole bags.

This is a volume estimate for a rectangular prism, not structural slab or footing design. Bag yield varies by product. Display rounding to two decimals can round down; use the unrounded volume and supplier order increments when purchasing.

Ready-mix Cubic Yards (with waste %) and 80-lb pre-mix bag count formulas

Divides C2 by 12 (inches to feet) and by 27 (cubic feet per cubic yard).

=IF(OR(COUNT(A2:E2)<>5,MIN(A2:C2)<=0,D2<0,E2<=0),"Invalid volume inputs",ROUND(((A2 * B2 * (C2 / 12)) / 27) * (1 + D2), 2))

Whole bags for the supplied per-bag yield

E2 supplies the product yield in cubic feet per bag. Check the product data sheet;0.60 is the illustrative value, not a rule for every 80-lb product.

=IF(OR(COUNT(A2:E2)<>5,MIN(A2:C2)<=0,D2<0,E2<=0),"Invalid volume inputs",ROUNDUP((A2 * B2 * (C2 / 12) * (1 + D2)) / E2, 0))

Further reading

• Microsoft Support — ROUNDUP function
• QUIKRETE — Concrete mix product sheet