A 20 x 10 ft driveway slab at 4 in thick needs 2.47 cubic yards of concrete, or 2.59 with a 5% allowance - which is 117 eighty-pound bags if you mix on site.
0.016990107955200003 m3/bag — Manufacturer datasheet yield for a dry ready-mix sack (2026-09-09)
How the number is worked out
Volume first, then packaging. A slab is length x width x thickness, a strip
footing the same multiplied by how many runs, and a round column is pi x r² x
height. Everything is converted to metric internally and back out again, so a
figure entered in feet and inches and the same figure entered in metres give the
same answer to the millimetre.
The waste allowance is applied to the volume, not to the bag count. Adding it
afterwards would round twice and quietly overstate what you need.
Cubic yards or bags
Bags are quoted by yield, not by weight: what matters is how much wet
concrete comes out of the sack, and the manufacturer prints that on the bag.
An 80 lb sack yields 0.6 cubic feet. Divide the volume you need by the yield and
round up - always up, because a pour that runs short sets before the next trip
to the store, and a cold joint in a driveway is a repair, not a blemish.
Above roughly half a cubic yard, mixing by hand stops being a saving and starts
being a weekend. That is the point at which the short-load fee is worth paying.
What this calculator does not do
It sizes the pour, not the structure. Slab thickness, reinforcement and bearing
capacity are design decisions that depend on loads and on the ground underneath,
and no calculator that has not seen your site should be making them for you.
Worked example
Driveway slab, 20 × 10 ft at 4 in
Concrete needed
2.47 yd³
Including waste
2.59 yd³
Bags
117
Weight
10,487 lb
Calculated by materiacalc using the same engine that runs the tool above.
In this market
Ready-mix in the United States is sold by the cubic yard and a truck carries about ten. Suppliers set their own minimum, commonly 3 to 5 yards, and add a short-load fee of roughly $50 to $150 below it. That threshold, not the arithmetic, is what usually decides between a truck and a pallet of sacks on a small residential pour - and at 2.59 yards this example sits right on the line.
ACI CODE-332-20, Code Requirements for Residential Concrete — plus the state and local codes that actually govern the permit.
Questions
How many 80 lb bags make a cubic yard?
An 80 lb sack yields 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 45 bags to the yard. At 60 lb it is 60 bags, and at 40 lb it is 90.
How much waste should I allow?
Five percent covers spillage and a slightly uneven subgrade on a formed slab. Ten is more realistic when you are pouring onto dug ground, where the excavation is never quite the depth you marked.
Does the calculator include rebar or mesh?
No. Steel displaces so little volume that ignoring it is well inside the waste allowance, and adding it would imply a precision the tape measure does not support.