How much concrete do you need for a 12×12 slab?

A 12×12 ft slab poured 4 inches thick needs 1.78 cubic yards (48 cu ft) of concrete. With 10% extra for waste, buy 88 bags of 80 lb premix or order 1.96 yards of ready-mix.

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Your results

Concrete to buy (incl. 10% extra)1.96 cu yd
80 lb bags88
60 lb bags118
40 lb bags176
Exact volume48 cu ft (1.78 cu yd)
How much concrete do you need for a 12×12 slab — diagram

Concrete for a 12×12 slab at different thicknesses

The slab covers 144 square feet. Every extra inch of thickness adds about 0.44 cubic yards. Quantities below include 10% for waste.

ThicknessCubic yards80 lb bags60 lb bags40 lb bags
3.5 in1.7177103154
4 in1.9688118176
5 in2.44110147220
6 in2.93132176264
8 in3.91176235352

Bags or ready-mix for this size?

At 1.96 cubic yards you are in the gray zone between bags and a truck. 88 bags of 80 lb weigh about 7,040 lb in total — get a ready-mix quote (ask about short-load fees) and compare it with the bag price before deciding.

How heavy is a 12×12 slab?

Normal-weight concrete weighs about 150 lb per cubic foot, so a 4-inch 12×12 slab weighs roughly 7,200 lb (3.6 tons) once cured. Make sure your base is compacted gravel on firm, undisturbed soil.

The math

12 ft × 12 ft × (4 in ÷ 12) = 48 cu ft 48 cu ft ÷ 27 = 1.778 cu yd +10% waste = 52.8 cu ft 52.8 ÷ 0.6 cu ft per 80 lb bag = 88 bags (rounded up)

Common questions

How many 80 lb bags of concrete for a 12x12 slab?

88 bags for a 4-inch slab including 10% extra (80 with no extra). At 6 inches thick you need 132 bags.

How many yards of concrete for a 12x12 slab 6 inches thick?

2.93 cubic yards including 10% extra, or 2.67 yards exactly.

How many 60 lb bags for a 12x12 slab?

118 bags of 60 lb concrete for a 4-inch slab with 10% extra.

Want the full method, tips and other sizes? See the concrete calculator guide.

Last reviewed November 1, 2026. Results are estimates — confirm with your supplier.

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