How many bags of concrete do I need for a 4×8 slab?

At 4 inches thick, a 4×8 slab takes 20 bags of 80 lb concrete mix, 27 bags of 60 lb mix or 40 bags of 40 lb mix, including 10% extra.

How many yards of concrete for a 4×8 slab?

0.4 cubic yards at 4 inches thick, 0.49 at 5 inches and 0.59 at 6 inches, before adding extra for waste.

How much does a 4×8 concrete slab cost?

The concrete alone for a 4 inch 4×8 slab costs about $130 in 20 bags of 80 lb mix (at about $7 a bag) or about $83 for 0.5 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $192 to $384 for 32 sq ft.

How much does a 4×8 slab weigh?

About 0.8 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).

Concrete for a 4×8 slab

0.4 cubic yards (20 × 80 lb bags)

for a 4×8 ft slab 4 inches thick: 10.7 cu ft. Bag counts include 10% extra; at 6 inches you need 0.59 yards.

A 4×8 foot slab covers 32 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 10.7 cubic feet of concrete, or 0.4 cubic yards. That is a small enough pour for bagged mix: 20 bags of 80 lb at about $130.

Concrete for a 4×8 slab by thickness

Thickness	Cubic yards	Cubic feet	80 lb bags	60 lb bags	40 lb bags
3 in	0.3	8	15	20	30
4 in	0.4	10.7	20	27	40
5 in	0.49	13.3	25	33	49
6 in	0.59	16	30	40	59
8 in	0.79	21.3	40	53	79

Bag counts include 10% extra for spillage and uneven ground. Cubic yards and cubic feet are the exact volume.

How it’s worked out

Convert the thickness to feet: 4 in ÷ 12 = 0.333 ft.
Multiply length × width × thickness: 8 × 4 × 0.333 = 10.67 cu ft.
Divide by 27 for cubic yards: 10.67 ÷ 27 = 0.395 cu yd.
Add 10% and divide by the bag yield: 11.73 ÷ 0.6 = 20 bags of 80 lb mix (rounded up).

Under the slab, plan on 4 inches of compacted gravel: about 0.4 cubic yards. For driveways and anything that carries a vehicle, go to 5 or 6 inches and add rebar or wire mesh.

Change the numbers

Length (ft) Width (ft) Thickness (in) Extra for waste (%) Price per 80 lb bag ($) Ready-mix price per yard ($)

Concrete needed

0.4 cubic yards

10.7 cubic feet. With 10% extra, order 0.43 yards or 20 bags of 80 lb mix.

Area	32 sq ft
Volume	10.7 cu ft (0.4 cu yd)
80 lb bags	20 (about $130)
60 lb bags	27
40 lb bags	40
Ready-mix to order	0.5 yd (about $83 before delivery fees)
Weight of the slab	0.8 tons

Other slab sizes

Questions

How many bags of concrete do I need for a 4×8 slab?: At 4 inches thick, a 4×8 slab takes 20 bags of 80 lb concrete mix, 27 bags of 60 lb mix or 40 bags of 40 lb mix, including 10% extra.
How many yards of concrete for a 4×8 slab?: 0.4 cubic yards at 4 inches thick, 0.49 at 5 inches and 0.59 at 6 inches, before adding extra for waste.
How much does a 4×8 concrete slab cost?: The concrete alone for a 4 inch 4×8 slab costs about $130 in 20 bags of 80 lb mix (at about $7 a bag) or about $83 for 0.5 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $192 to $384 for 32 sq ft.
How much does a 4×8 slab weigh?: About 0.8 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).