How the calculation works
- Volume in cubic feet = length × width × thickness in inches ÷ 12.
- Cubic yards = cubic feet ÷ 27 — the unit ready-mix suppliers sell by.
- Bags = cubic feet ÷ the yield of one bag, rounded up. Yields are from the Quikrete Concrete Mix data sheet: 40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 and 90 lb 0.675 cu ft.
- An optional extra percentage adds a margin for spillage and uneven ground. It is 0% unless you set it.
Example
Length: 10 ft · Width: 10 ft · Thickness: 4 in · Bag size: 80 lb (0.6 cu ft) · Extra for spillage: 0 %
Result: 56 bags.
Tips before you buy
- Measure thickness in several places; a slightly deeper base adds up quickly.
- Yields differ between products — check the yield printed on the bag you buy.
- For large pours, compare the bag count with ordering ready-mix by the cubic yard.
- Always confirm coverage and yield with the manufacturer's label — products differ.
- Material quantity only — not reinforcement or slab design.
FAQ
How many 80 lb bags make a cubic yard?
27 cu ft ÷ 0.60 cu ft per bag = 45 bags of 80 lb mix per cubic yard.
Does this cover rebar or slab design?
No. This calculator estimates material quantity only. Reinforcement, base preparation and slab design are outside its scope.