How Much Concrete for a 20×40 Slab?

Answer
10.86 yd³ at 4 in · 489 × 80 lb bags

A 20 × 40 ft slab (800 sq ft) at 4 inches is 266.7 cubic feet. With 10% extra for waste that is 10.86 cubic yards, or 489 bags of 80 lb premix. At this size, ready-mix delivery is usually cheaper than bags.

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Shape
ft
ft
in
Patios & walkways: 4 in. Driveways: 5–6 in.
in
ft
pcs
%
5–10% is typical.
Concrete needed
–
How we calculated this

    For estimating and educational purposes only. Results are approximate and are not a substitute for a licensed professional, the manufacturer's instructions or your local code. Disclaimer

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    Concrete by slab thickness (10% waste included)

    ThicknessReady-mix80 lb bags60 lb bags
    4 in10.86 yd³489652
    5 in13.58 yd³612815
    6 in16.30 yd³734978

    Use 4 in for patios and walkways, 5–6 in for driveways and garage floors that carry vehicles.

    Frequently asked questions

    How many 80 lb bags for a 20x40 slab?

    489 bags at 4 in thick, including 10% extra.

    How thick should a slab be?

    4 inches for patios, sheds and walkways; 5–6 inches for driveways and heavier loads.

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