How many bags of concrete do I need for a 3×4 slab?

At 4 inches thick, a 3×4 slab takes 8 bags of 80 lb concrete mix, 10 bags of 60 lb mix or 15 bags of 40 lb mix, including 10% extra.

How many yards of concrete for a 3×4 slab?

0.15 cubic yards at 4 inches thick, 0.19 at 5 inches and 0.22 at 6 inches, before adding extra for waste.

How much does a 3×4 concrete slab cost?

The concrete alone for a 4 inch 3×4 slab costs about $52 in 8 bags of 80 lb mix (at about $7 a bag) or about $41 for 0.25 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $72 to $144 for 12 sq ft.

How much does a 3×4 slab weigh?

About 0.3 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).

Concrete for a 3×4 slab

0.15 cubic yards (8 × 80 lb bags)

for a 3×4 ft slab 4 inches thick: 4 cu ft. Bag counts include 10% extra; at 6 inches you need 0.22 yards.

A 3×4 foot slab covers 12 square feet. At the standard 4 inch thickness for a patio, walkway or shed floor, that is 4 cubic feet of concrete, or 0.15 cubic yards. That is a small enough pour for bagged mix: 8 bags of 80 lb at about $52.

Concrete for a 3×4 slab by thickness

Thickness	Cubic yards	Cubic feet	80 lb bags	60 lb bags	40 lb bags
3 in	0.11	3	6	8	11
4 in	0.15	4	8	10	15
5 in	0.19	5	10	13	19
6 in	0.22	6	11	15	22
8 in	0.3	8	15	20	30

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: 4 × 3 × 0.333 = 4 cu ft.
Divide by 27 for cubic yards: 4 ÷ 27 = 0.148 cu yd.
Add 10% and divide by the bag yield: 4.4 ÷ 0.6 = 8 bags of 80 lb mix (rounded up).

Under the slab, plan on 4 inches of compacted gravel: about 0.15 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

0.15 cubic yards

4 cubic feet. With 10% extra, order 0.16 yards or 8 bags of 80 lb mix.

Area	12 sq ft
Volume	4 cu ft (0.15 cu yd)
80 lb bags	8 (about $52)
60 lb bags	10
40 lb bags	15
Ready-mix to order	0.25 yd (about $41 before delivery fees)
Weight of the slab	0.3 tons

Other slab sizes

Questions

How many bags of concrete do I need for a 3×4 slab?: At 4 inches thick, a 3×4 slab takes 8 bags of 80 lb concrete mix, 10 bags of 60 lb mix or 15 bags of 40 lb mix, including 10% extra.
How many yards of concrete for a 3×4 slab?: 0.15 cubic yards at 4 inches thick, 0.19 at 5 inches and 0.22 at 6 inches, before adding extra for waste.
How much does a 3×4 concrete slab cost?: The concrete alone for a 4 inch 3×4 slab costs about $52 in 8 bags of 80 lb mix (at about $7 a bag) or about $41 for 0.25 yards of ready-mix (at about $165 a yard, before delivery fees). Professionally poured slabs usually run $6 to $12 per square foot, about $72 to $144 for 12 sq ft.
How much does a 3×4 slab weigh?: About 0.3 tons at 4 inches thick (concrete weighs about 145 lb per cubic foot).