12 x 20 slab at 4 inches
Concrete needed, with a 7 percent waste allowance, rounded up.
Cubic yards
3.17cu yd
80 lb bags
143
60 lb bags
191
40 lb bags
286
cubic yards = 12 × 20 × (4 / 12) ÷ 27 × 1.07 = 3.17 cu yd
In metric, that is about 3.66 × 6.1 m at 10.2 cm, roughly 2.42 m³ of concrete.
Open the interactive Concrete CalculatorAbout this slab
A 12 by 20 foot slab at 4 inches works out to 80 cubic feet, or 2.96 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 12x20 slab at 4 inches?
A 12 by 20 foot slab at 4 inches thick is about 2.96 cubic yards of concrete before waste, or 3.17 cubic yards with a 7 percent allowance. That is roughly 143 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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