How many bags of concrete for a 8x10 slab?
A 4 in thick 8x10 slab needs 47 bags of 80 lb concrete.
That is 26.7 cubic feet (0.99 cubic yards), including 5% waste. In other bag sizes: 63 × 60 lb, or 94 × 40 lb.
Bags are the right call here. At 0.99 cubic yards you are under the point where ready-mix delivery makes sense, and most suppliers have a minimum load plus a short-load fee. 47 bags is a manageable day with a wheelbarrow or a small mixer.
Where that number comes from
- 8 ft × 10 ft = 80 sq ft
- 4 in thick = 0.3333 ft, so 80 × 0.3333 = 26.7 cubic feet
- An 80 lb bag yields 0.6 cu ft, so 26.7 ÷ 0.6 = 45 bags
- Add 5% for spillage and uneven subgrade = 47 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 0.99 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.