Proper Estimate

Concrete calculator

Pick the shape and enter its size. You get the cubic yards to order, the number of bags for every bag size, and what it costs in bags or as ready-mix.

Shape

What are you pouring?

4 in for patios, walkways and most driveways

Identical pieces

For uneven ground and spills

Prices

Sample price

Sample, delivered

Concrete to order

1.36 cu yd

36.67 cu ft, with 10% extra

Bags, if you mix it yourself

  • 40 lb bags123
  • 50 lb bags98
  • 60 lb bags82
  • 80 lb bags62
  • 90 lb bags55

Cost

  • 62 bags of 80 lb$418.50
  • Ready-mix, 1.5 cu yd$262.50
  • Weight once poured5,000 lb

Over about a cubic yard, ready-mix is usually easier and cheaper than mixing bags by hand.

How to calculate concrete

  1. Multiply length × width × thickness, with every measurement in feet. 4 in is 4 ÷ 12 = 0.333 ft.
  2. A 10 × 10 ft slab 4 in thick: 10 × 10 × 0.333 = 33.33 cu ft.
  3. Divide by 27 for cubic yards: 33.33 ÷ 27 = 1.23 cu yd.
  4. Add about 10% for uneven ground and spills: 1.36 cu yd, or 36.67 cu ft.
  5. For bags, divide the cubic feet by what one bag makes: 36.67 ÷ 0.6 = 61.11, so 62 bags of 80 lb.

A round hole or tube form holds π × radius² × depth, and steps are worked out step by step, each one a tread shorter than the one below it.

Concrete for common slab sizes

Slab, 4 in thick Concrete To order 80 lb bags
10 × 10 ft 1.23 cu yd 1.36 cu yd 62
12 × 12 ft 1.78 cu yd 1.96 cu yd 88
10 × 20 ft 2.47 cu yd 2.72 cu yd 123
16 × 16 ft 3.16 cu yd 3.48 cu yd 157
20 × 20 ft 4.94 cu yd 5.43 cu yd 245
24 × 24 ft 7.11 cu yd 7.82 cu yd 352

To order and bags include 10% extra.

Bags per cubic yard

Bag size Makes about Bags per cubic yard
40 lb 0.3 cu ft 90
50 lb 0.38 cu ft 72
60 lb 0.45 cu ft 60
80 lb 0.6 cu ft 45
90 lb 0.68 cu ft 40

Yields from the Quikrete Concrete Mix data sheet. Other brands are close.

Post holes and tube forms

Diameter, 48 in deep Concrete 80 lb bags 50 lb bags
8 in 1.4 cu ft 3 4
10 in 2.18 cu ft 4 6
12 in 3.14 cu ft 6 9
16 in 5.59 cu ft 10 15
18 in 7.07 cu ft 12 19
24 in 12.57 cu ft 21 34

Per hole, with no extra. A post in the hole takes up a little of the space. For deck footings sized by load, use the deck footing calculator.

Bags or ready-mix?

Bags make sense for post holes, small pads and repairs. Above about a cubic yard, that's 45 bags of 80 lb, more than 3,600 lb to lift and mix, so ready-mix from a truck is usually easier, more even and often cheaper. Plants sell it by the cubic yard, often in quarter yards, and many charge extra for small loads. Order a little more than you need: running short in the middle of a pour leaves a weak joint.

How thick to pour

  • Patios, walkways and shed floors: usually 4 in.
  • Driveways: 4 in for cars; 5 to 6 in where heavy trucks or RVs park.
  • House floor slabs: at least 3 1/2 in under the IRC (Section R506.1).

Thickness matters as much as length and width: going from 4 to 5 in adds a quarter more concrete. Slabs sit on a compacted gravel base, which isn't included here.

Questions

How many bags of concrete do I need for a 10 × 10 slab?

At 4 in thick, a 10 × 10 ft slab is 33.33 cu ft (1.23 cu yd). With 10% extra, buy 62 bags of 80 lb or 82 bags of 60 lb. That's a lot to mix by hand; 1.5 cu yd of ready-mix is usually easier.

How much concrete do I need for a 12 × 12 slab 4 inches thick?

48 cu ft, which is 1.78 cubic yards. Order about 1.96 cu yd with 10% extra for uneven ground and spillage, so 2 cu yd of ready-mix or 88 bags of 80 lb.

How many 80 lb bags of concrete make a yard?

45. An 80 lb bag of premixed concrete makes about 0.6 cu ft, and a cubic yard is 27 cu ft. With 60 lb bags it's 60, and with 40 lb bags 90.

How much does a yard of concrete cost?

It varies a lot by region. The calculator uses a sample $175.00 per cubic yard of ready-mix, delivered; replace it with your local supplier's price. Small orders often carry a short-load fee, and a pump or extra time on site costs more.

How thick should a concrete slab be?

Patios, walkways and most driveways are poured 4 in thick; driveways that carry heavy trucks or RVs are usually 5 to 6 in. The concrete floor slab of a house must be at least 3 1/2 in under the IRC (Section R506.1).

How much does concrete weigh?

About 150 lb per cubic foot once it's set, so a cubic yard weighs about 4,050 lb, roughly 2 tons. A 10 × 10 ft slab 4 in thick weighs about 5,000 lb.