Concrete Calculator

Work out how much concrete you need in cubic yards or m³ for slabs, footings, columns, post holes and steps, plus how many 40–80 lb or 20–30 kg bags.

Concrete is ordered by volume: cubic yards in the US, cubic metres in the UK and Australia. The volume of a slab is its length times its width times its thickness, so a 10 × 10 ft patio 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. That's 56 of the common 80 lb premixed bags, or 62 once you add 10% for spillage and an uneven base. This calculator does the same sum for slabs of any shape, strip footings, round columns and tube forms, post holes with the post taken off, and solid steps. It shows the answer in cubic yards, cubic feet and cubic metres, the number of 40, 50, 60 and 80 lb or 20, 25 and 30 kg bags, the weight of the mixed concrete, and every step of the working.

Units
Bag sizes
Pour
Area
ft
ft
in
Feet and inches work too: type 12'6 or 12.5.
%
Concrete
1.36 yd³

36.67 ft³ · 1.038 m³

Bags
62

80 lb bag (0.60 ft³)

Weight when mixed
5,494 lb

2.75 tons

Premixed bags needed by size
80 lb bag (0.60 ft³)62
60 lb bag (0.45 ft³)82
50 lb bag (0.375 ft³)98
40 lb bag (0.30 ft³)123
20 kg bag113
25 kg bag89
30 kg bag74
That's 62 × 80 lb bags. At this size many people order ready-mix instead: ask suppliers for 1.36 cubic yards, and check their minimum load and how they round.
How it was worked out
  1. Area: 10 ft × 10 ft = 100 ft²; × 4 in (0.333 ft) deep = 33.33 ft³
  2. 33.33 ft³ ÷ 27 = 1.23 yd³
  3. Plus 10% extra = 36.67 ft³ (1.36 yd³)
  4. 80 lb bag (0.60 ft³): 36.67 ft³ ÷ 0.6 ft³ made per bag = 61.11, rounded up to 62 bags

Bag yields are typical figures: 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags, and about 108 × 20 kg bags per m³. Check the yield printed on the bag you buy. Mixed concrete is taken as 2,400 kg/m³ (about 4,050 lb per cubic yard).

Common use cases

  • Order ready-mix for a patio, driveway or shed base without paying for a second truck
  • Count premixed bags for a small slab, steps or a set of deck footings
  • Price a job by the bag or by the cubic yard or m³
  • Check a contractor's quote for the amount of concrete

How to use

  1. Pick what you're pouring: a slab, footing, round column, post holes or steps.
  2. Choose feet and inches or metres and millimetres, and US, UK or Australian bag sizes.
  3. Enter the sizes. For a slab you can add several areas, including circles and triangles.
  4. Keep the 10% extra or change it, then read the volume, bags and weight.
  5. Optionally enter a price per bag or per cubic yard or m³ for a cost estimate.

FAQ

How many bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet. Typical premixed bags make 0.60 ft³ (80 lb), 0.45 ft³ (60 lb), 0.375 ft³ (50 lb) and 0.30 ft³ (40 lb), so a cubic yard takes about 45 × 80 lb, 60 × 60 lb, 72 × 50 lb or 90 × 40 lb bags, before any extra.

How many 20 kg bags make a cubic metre?

About 108 bags of a typical 20 kg concrete mix make 1 m³. Yields vary by brand, so check the bag: most print how much each bag makes.

How thick should a concrete slab be?

Paths, patios and shed bases are commonly 4 inches (100 mm). Driveways and slabs that take vehicles are usually 5 to 6 inches (125 to 150 mm). Your local building code, the ground and the load decide the real figure, so check before you pour anything structural.

How much extra concrete should I order?

Most people add 5 to 10%. An uneven or soft base, spillage and forms that bow out all eat concrete, and running short halfway through a pour is far worse than a little left over. The calculator starts at 10%.

How much does concrete weigh?

Mixed concrete weighs about 2,400 kg per cubic metre, or roughly 4,050 lb per cubic yard. That matters if you're hauling bags or mixed concrete in a trailer: a cubic yard is about two tons.

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