Concrete for a slab
Enter slab dimensions. Result rounds up with a 7 percent waste allowance.
ft
ft
in
m
m
cm
Cubic yards
-cu yd
80 lb bags
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60 lb bags
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40 lb bags
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cubic yards = length × width × (thickness / 12) ÷ 27, plus 7% waste
Cubic metres
-m³
25 kg bags
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20 kg bags
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cubic metres = length × width × (thickness / 100), plus 7% waste
Estimate only. Rounded up with a 7 percent waste allowance. Confirm the final quantity with your supplier before ordering.
How the concrete estimate works
Volume is length times width times thickness. A standard 80 lb bag of concrete mix yields about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. We add a 7 percent waste allowance and round up, because running short mid pour is far more costly than a little overage. For anything larger than a few bags, ordering ready mix by the yard is usually cheaper and easier than mixing by hand.
Mixing more than a few bags? A heavy duty mixing tub saves your back and keeps the batch consistent. For larger pours, price up ready mix delivery instead.
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Common questions
How much concrete do I need for a 10 by 10 slab at 4 inches?
A 10 by 10 foot slab at 4 inches thick is about 1.24 cubic yards, or roughly 56 of the 80 lb bags, before waste. Add about 7 percent and round up.
How many 80 lb bags are in a cubic yard?
About 45 bags of 80 lb concrete mix make one cubic yard, since each bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet.
Popular slab sizes
Pre-calculated concrete for common slab dimensions, in imperial and metric.
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