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Concrete Calculator

Enter your dimensions to get cubic yards of concrete, a ready-mix order and the number of premix bags for a slab, footing, column or steps.

Shape
Units
Identical pours, such as 4 tube forms.
For spillage, an uneven base and forms that bow.

You need

Ready-mix order 1.75 cu yd
  • Exact, with waste1.63 cu yd
  • Cubic feet44 cu ft
  • Cubic meters1.25 m³
  • Before waste40 cu ft

Premix bags

  • 80 lb (0.60 cu ft)74
  • 60 lb (0.45 cu ft)98
  • 50 lb (0.375 cu ft)118
  • 40 lb (0.30 cu ft)147

74 bags of 80 lb is 5,920 lb of dry mix.

Ready-mix suppliers often have a minimum order or a short-load fee for small loads. Ask before you order.

How to use the concrete calculator

  1. Pick the shape: a slab or patio, a footing, a round column or tube form, or a set of steps.
  2. Choose feet and inches or meters and centimeters, then enter the dimensions. Switching units converts the numbers you already typed.
  3. Set the quantity if you have several identical pours, such as 6 deck footings.
  4. Keep the 10 percent waste allowance or change it for your site.
  5. Read the ready-mix order in cubic yards, or the bag count for the bag size you plan to buy.

Concrete volume formulas

Convert every dimension to feet first (inches divided by 12), work out cubic feet, then divide by 27 to get cubic yards. 

Slab or patio:  length × width × (thickness ÷ 12)
Footing:        length × (width ÷ 12) × (depth ÷ 12)
Column:         3.1416 × (diameter ÷ 24)² × height
Steps:          width × (run ÷ 12) × (rise ÷ 12) × n(n + 1) ÷ 2

Cubic yards   = cubic feet × quantity × (1 + waste % ÷ 100) ÷ 27
Cubic meters  = cubic yards × 0.7646
Bags          = cubic feet with waste ÷ bag yield, rounded up

The steps formula treats each step as a solid block one tread deep and as tall as all the risers up to it: the bottom step is 1 rise tall, the next 2 rises, and so on. Adding the blocks gives n(n + 1) ÷ 2 rises in total.

Worked example: a 10 × 12 ft patio slab

A patio 10 ft long, 12 ft wide and 4 in thick, with 10% waste:

  • 10 × 12 × (4 ÷ 12) = 40 cubic feet
  • 40 × 1.10 = 44 cubic feet with waste
  • 44 ÷ 27 = 1.63 cubic yards, so order 1.75 cubic yards
  • 1.63 × 0.7646 = 1.25 cubic meters
  • Bags: 44 ÷ 0.60 = 73.3, so 74 bags of 80 lb, or 98 bags of 60 lb

For steps: three 7 in risers with 11 in treads, 4 ft wide, is 4 × (11 ÷ 12) × (7 ÷ 12) × 6 = 12.8 cubic feet before waste.

Bag yields and bags per cubic yard

Bag counts use the yields printed on the Quikrete Concrete Mix data sheet. The Sakrete High-Strength Concrete Mix data sheet lists the same yields for 40, 60 and 80 lb bags. Lightweight high-yield mixes yield more per bag, so check the label of the mix you buy.

Bag size Yield per bag Bags per cubic yard
40 lb 0.30 cu ft (8.5 L) 90
50 lb 0.375 cu ft (10.6 L) 72
60 lb 0.45 cu ft (12.7 L) 60
80 lb 0.60 cu ft (17 L) 45

Slab coverage by thickness

How far a cubic yard goes, and what 100 square feet of slab takes, before any waste allowance.

Thickness Sq ft per cubic yard Cubic yards per 100 sq ft 80 lb bags per 100 sq ft
3.5 in 92.6 1.08 49
4 in 81 1.23 56
5 in 64.8 1.54 70
6 in 54 1.85 84

Bags or ready-mix

Compare the bag count with the work it means. One cubic yard is 45 bags of 80 lb, which is 3,600 lb of dry mix to carry, mix and place before it starts to set. Ready-mix arrives mixed, but suppliers often have a minimum order or charge a short-load fee for small loads. Ask your supplier for both before you decide.

Tips before you pour

Concrete calculator FAQ

How many bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet. At the yields on the Quikrete Concrete Mix data sheet, that is 45 bags of 80 lb (0.60 cu ft each), 60 bags of 60 lb (0.45 cu ft), 72 bags of 50 lb (0.375 cu ft) or 90 bags of 40 lb (0.30 cu ft).

How many 80 lb bags of concrete do I need for a 10 x 10 slab?

A 10 x 10 ft slab 4 inches thick is 33.3 cubic feet. That is 56 bags of 80 lb with no waste, or 62 bags with 10 percent waste added.

How much area does a yard of concrete cover?

One cubic yard covers 81 square feet at 4 inches thick, about 65 square feet at 5 inches and 54 square feet at 6 inches.

How do I calculate concrete for a round column or tube form?

Multiply 3.1416 by the radius squared and by the height, all in feet. A 12 inch tube form 4 feet tall holds 3.14 cubic feet, which is 6 bags of 80 lb.

How is concrete for steps calculated?

Each step is a block one tread deep and as tall as all the risers up to it. Adding the blocks gives width x run x rise x n(n+1)/2 for n steps. Three 7 inch risers with 11 inch treads, 4 feet wide, is 12.8 cubic feet. A landing at the top is extra; add it as a slab.

How much extra concrete should I order?

This calculator adds 10 percent by default to cover spillage, an uneven base and forms that bow under the load. Use less for formed work on a flat, compacted base and more for rough ground. The ready-mix order is then rounded up to the next quarter yard.

When should I use bags instead of ready-mix concrete?

Compare the bag count with the work. One cubic yard is 45 bags of 80 lb, or 3,600 lb of dry mix to carry and mix. Ready-mix suppliers often have a minimum order or charge a short-load fee for small loads, so ask your supplier before you decide.