How Many Bags of Concrete Do I Need?

The label on a concrete bag tells you its weight, but what matters for a slab is its yield: the volume of finished concrete that bag makes. Manufacturers print it, and it is the number the bag count depends on.

Yield by bag size

For QUIKRETE Concrete Mix, the data sheet gives approximate yields of 0.30, 0.375, 0.45 and 0.60 cu ft for 40, 50, 60 and 80 lb bags. Another brand or a fast-setting mix can differ, so use the number on your bag. A cubic yard is 27 cu ft, which makes the bags per yard simple arithmetic:

A 10 × 6 ft slab, 4 inches thick

That slab holds 20 cu ft. Here is what the planner returns for each bag size, with no extra allowance so the comparison is clean:

The calculation for 80 lb bags is 33.33 × 80 lb bags, and the list rounds that up to 34 × 80 lb bags. Heavier bags mean fewer bags to count and handle, but the total volume of concrete is the same whichever size you choose.

Adding the allowance

Bag yields are approximate and assume an even, level base. With the planner’s default 10% allowance, the same slab in 80 lb bags becomes 37 × 80 lb bags. The allowance is an editable planning default; it is not part of the manufacturer’s yield.

Bags or a cubic-yard order

The cubic-yard figure on the planner is the same volume expressed the way a ready-mix supplier sells it. If your project is large, compare the bag count against that volume. Suppliers set their own minimums and ordering rules, so ask them rather than assuming.