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Concrete calculator

Find how much concrete a slab, patio or pad needs, in cubic yards and in bags of mix. Enter the size and thickness; the estimate updates as you type.

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LengthDepthWidth

How to measure

Measure the inside of your forms and the depth from the base to the top of the form.

Slab size

Measure the depth of your forms.

Mix

Concrete needed

1.36 cubic yards

Buy 62 bags of 80 lb mix, or order 1.36 cubic yards of ready-mix

Slab area100 sq ft
Volume33.33 cu ft
Volume with 10% extra36.67 cu ft
80 lb bags (0.60 cu ft each)62 bags
60 lb bags (0.45 cu ft each)82 bags
40 lb bags (0.30 cu ft each)123 bags

Bag yields are the figures printed by the manufacturer. A cubic yard is 27 cubic feet.

How much concrete do I need?

Multiply length × width × thickness to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab 4 in thick is 33.3 cu ft; with 10% extra that is 1.36 cubic yards, or 62 bags of 80 lb mix, since each 80 lb bag makes about 0.60 cubic feet.

Worked example: a 10 × 10 ft patio slab

  1. Convert thickness to feet: 4 in ÷ 12 = 0.333 ft.
  2. Volume: 10 × 10 × 0.333 = 33.33 cu ft.
  3. Add 10% extra: 33.33 × 1.10 = 36.67 cu ft.
  4. Cubic yards: 36.67 ÷ 27 = 1.36 cu yd.
  5. Bags: 36.67 ÷ 0.60 = 61.1, rounded up to 62 bags of 80 lb mix.

Concrete needed by slab size

Including 10% extra for spills and an uneven base.

Cubic yards and bags of concrete mix by slab size
Slab Cubic yards 80 lb bags 60 lb bags
4 × 4 ft, 4 in 0.22 10 14
8 × 8 ft, 4 in 0.87 40 53
10 × 10 ft, 4 in 1.36 62 82
12 × 12 ft, 4 in 1.96 88 118
10 × 20 ft, 4 in 2.72 123 163
20 × 20 ft, 4 in 5.43 245 326
10 × 10 ft, 6 in 2.04 92 123

How much concrete does one bag make?

Quikrete lists the yield of its standard concrete mix on its specification sheet. A cubic yard is 27 cubic feet, which sets the number of bags per yard.

Bag yields for standard concrete mix
Bag size Yield per bag Bags per cubic yard
80 lb 0.60 cu ft 45
60 lb 0.45 cu ft 60
40 lb 0.30 cu ft 90

How the calculator works

Volume (cu ft) = length × width × thickness (in) ÷ 12

Cubic yards = volume × (1 + extra %) ÷ 27

Bags = volume × (1 + extra %) ÷ bag yield, rounded up

The extra allowance covers concrete lost to spills, form bulges and low spots in the base. Quikrete notes that real yields vary with waste and uneven subgrade, so a margin of around 10% is a sensible default.

For larger pours, compare the bag count with ordering ready-mixed concrete by the cubic yard. Mixing dozens of bags by hand takes time, and a slab poured in one go cures more evenly.

Frequently asked questions

How many bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet. With Quikrete yields of 0.60, 0.45 and 0.30 cubic feet per bag, that is 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix.

How much concrete do I need for a 10 × 10 slab?

A 10 × 10 ft slab 4 in thick is 33.3 cubic feet. With 10% extra, plan on 1.36 cubic yards, which is 62 bags of 80 lb mix or 82 bags of 60 lb mix.

How many square feet does an 80 lb bag of concrete cover?

At 4 inches thick, one 80 lb bag covers about 1.8 square feet, because 0.60 cubic feet spread 0.333 feet deep covers 1.8 square feet. At 6 inches thick it covers about 1.2 square feet.

Why add extra concrete?

Forms bow, the base is never perfectly flat and some concrete is always lost in mixing and placing. Running short in the middle of a pour leaves a cold joint, so it is safer to have a little left over.

Sources

  1. Quikrete, Concrete Mix specification sheet
  2. Lowe's (answer from Quikrete), How many bags for a 16 × 5 × 4 in slab?