40 x 40 slab at 4 inches
Concrete needed, with a 7 percent waste allowance, rounded up.
Cubic yards
21.14cu yd
80 lb bags
952
60 lb bags
1269
40 lb bags
1903
cubic yards = 40 × 40 × (4 / 12) ÷ 27 × 1.07 = 21.14 cu yd
In metric, that is about 12.19 × 12.19 m at 10.2 cm, roughly 16.16 m³ of concrete.
Open the interactive Concrete CalculatorAbout this slab
A 40 by 40 foot slab at 4 inches works out to 533.3 cubic feet, or 19.75 cubic yards before waste. We add 7 percent for spillage and uneven subgrade and round up to whole bags. An 80 lb bag yields about 0.60 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.30.
Always confirm the order with your supplier. Subgrade that is not level, over excavation and thickened edges all add volume.
Mixing more than a few bags? A heavy duty mixing tub keeps the batch consistent and saves your back. For larger pours, price up ready mix delivery instead.
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Common questions
How much concrete for a 40x40 slab at 4 inches?
A 40 by 40 foot slab at 4 inches thick is about 19.75 cubic yards of concrete before waste, or 21.14 cubic yards with a 7 percent allowance. That is roughly 952 bags of 80 lb mix.
Should I order bags or ready mix?
For a slab this size, compare both. Bagged mix suits small jobs, but once you pass roughly one cubic yard, ready mix delivered by the truck is usually cheaper and far less work than mixing bag after bag.
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