How this calculation works
The Concrete Calculator computes the total volume in cubic yards and cubic meters needed for concrete slabs, footings, and pathways, plus the exact quantity of 60 lb and 80 lb pre-mix bags required.
Mathematical formula and logic
Volume (Cubic Feet) = Length (ft) × Width (ft) × (Thickness (in) / 12). Cubic Yards = Volume (cu ft) / 27 × (1 + Waste % / 100).
Worked example
A 20 ft by 10 ft patio slab at 4 inches thickness with a 10% waste factor equals 20 × 10 × (4/12) = 66.67 cu ft. Dividing by 27 and adding 10% yields 2.72 Cubic Yards (approx 123 eighty-pound bags).
Calculation assumptions
- 1 Cubic Yard of concrete yields 27 Cubic Feet.
- One 80 lb bag yields 0.60 cu ft (45 bags per cu yd); one 60 lb bag yields 0.45 cu ft (60 bags per cu yd).
Frequently asked questions
How thick should a concrete patio or driveway be?
Residential walkways and patios typically require a 4-inch concrete slab. Driveways supporting passenger vehicles require 4 to 5 inches, while heavy trucks require 6 inches with rebar reinforcement.
How many 80 lb bags of concrete make 1 cubic yard?
It takes 45 eighty-pound (80 lb) bags of pre-mix concrete to make exactly 1 cubic yard of poured concrete.
How many 60 lb bags of concrete make 1 cubic yard?
It takes 60 sixty-pound (60 lb) bags of concrete to equal 1 cubic yard.
When should I order ready-mix truck concrete vs bags?
For projects requiring more than 1 to 1.5 cubic yards (over 50-70 bags), ordering ready-mix delivered by a concrete truck is significantly faster and more cost-effective than mixing by hand.
Why should I add a 10% waste margin?
Excavation ground irregularities, sub-base compaction dips, and spillage during pouring frequently consume 5% to 10% more concrete than theoretical geometric dimensions.
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