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Calculators / Hardscape

How much concrete for your pad?

Put in the pad size and what sits on it. You'll get the yards for a truck, or the bag count if you're mixing it yourself.

These are example numbers. Put in yours.

What the pad is for

What sits on it

Size and thickness

ft
ft
in

4 in for a shed or walk. 6 in for a hot tub or a car.

Buy this much

40 bags of 80 lb

That's 0.87 cubic yards with 10% extra, or 53 bags of 60 lb.

Pad
8 × 8 ft64 sq ft
Concrete
0.87 yd
80 lb bags
40
60 lb bags
53

volume = 8 ft × 8 ft × 4 in ÷ 12 = 21.3 cu ft

with 10% for waste = 21.3 × 1.1 = 23.5 cu ft = 0.87 cu yd

80 lb bags = 23.5 ÷ 0.6 = 40, rounded up

60 lb bags = 23.5 ÷ 0.45 = 53, rounded up

Base

Put 4 in of compacted gravel under it. For this pad that's about 1.2 tons of crusher run.

Forms

Lumber runs short. A 2x4 is 3½ in tall and a 2x6 is 5½ in. For 4 in, use 2x6 forms with the top set 4 in above the gravel.

Mixing

Mix in a tub or a rented mixer, and pour in one go. A bag every couple of minutes keeps it from setting in layers.

See concrete mixing tubs on Amazon

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How we work it out

We take length times width times thickness, add 10% for spills and a base that isn't dead flat, and turn it into yards and bags.

cubic feet = length × width × thickness in inches ÷ 12 × 1.1

cubic yards = cubic feet ÷ 27

80 lb bags = cubic feet ÷ 0.6, 60 lb bags = cubic feet ÷ 0.45

A shed or walk needs 4 in. A hot tub or a car needs 6 in with rebar, unless your tub's manual says different. Code sets 3½ in as the floor for any slab.

Past about 45 bags of 80 lb, that's a yard, and a truck makes more sense than mixing. Ready-mix sells by the quarter yard. Put 4 in of compacted gravel under the pad either way.

Where the numbers come from

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