How many bags of concrete for a 24x24 slab?

A 4 in thick 24x24 slab needs 336 bags of 80 lb concrete.

That is 192.0 cubic feet (7.11 cubic yards), including 5% waste. In other bag sizes: 448 × 60 lb, or 672 × 40 lb.

At this size, order ready-mix instead. 7.11 cubic yards means mixing 336 bags by hand — roughly 26,880 lb of material, and realistically a full day with a mixer and help. Most ready-mix suppliers deliver from about 1 cubic yard, and past that point it is usually cheaper per yard as well as vastly faster. Bagged concrete makes sense below roughly 1 cubic yard, which is about 81 sq ft at 4 in thick.

A slab this size also has to be placed and floated before it starts to set. That is the real argument against bags: 336 of them cannot be mixed fast enough to keep a wet edge, and cold joints are where slabs crack.

Where that number comes from

  1. 24 ft × 24 ft = 576 sq ft
  2. 4 in thick = 0.3333 ft, so 576 × 0.3333 = 192.0 cubic feet
  3. An 80 lb bag yields 0.6 cu ft, so 192.0 ÷ 0.6 = 320 bags
  4. Add 5% for spillage and uneven subgrade = 336 bags

Bag yields are the finished volume of mixed concrete, not the dry weight, and they differ by brand by a few percent. The 5% margin absorbs that along with the low spots every real excavation has.

Change the numbers

What goes under the slab

  • Compacted gravel base, 4 in: about 7.11 cubic yards for this slab.
  • Rebar: #3 or #4 on an 18 in grid for a slab this size, kept up off the ground on chairs so it sits in the middle of the pour, not at the bottom.
  • Control joints: cut them at roughly 2.5 times the slab thickness in feet — about every 10.0 ft at 4 in. Concrete cracks; joints decide where.

Getting it right

  • Forgetting the base. Concrete poured onto soft ground will crack no matter how many bags you buy. Compacted gravel first.
  • Sizing off nominal thickness. Forms settle and subgrade dips. A slab "4 inches thick" usually consumes 5-10% more than the arithmetic says.
  • Mixing too wet. Extra water makes it easier to place and markedly weaker. Follow the water on the bag.
  • Running out mid-pour. A cold joint between yesterday's concrete and today's is a permanent weak line. Have every bag on site before you start.

Other slab sizes

Concrete calculator for any size, or all calculators.