Concrete Calculator
How much concrete a slab, a footing or a set of round columns takes, in cubic yards and in bags, with the 10% you should order on top.
Concrete
4.94 yd³
Order 5.43 yd³ · or 223 bags of 80 lb
- Area
- 400 ft²
- Concrete
- 4.94 yd³
- Cubic feet
- 133.33 ft³
- Order with 10% waste
- 5.43 yd³
- 80 lb bags0.60 ft³ of concrete each
- 223
- 60 lb bags0.45 ft³ each
- 297
Kalku is coming to iPhone and Android
The app has this calculator and 215 more, from rebar grids to a full basement take-off. It draws the pour in 3D as you type, works with no signal on site, and saves every job with a note to share as a PDF. Free, with no ads and no account.
How the volume is worked out
Concrete is sold by volume, so every pour comes down to the same multiplication: length × width × thickness. For a slab that is the whole calculation. A continuous footing is the same box, times the number of footings. A round column or a tube form is a cylinder: π × diameter² ÷ 4 × height. The calculator works in inches internally, exactly as the Kalku app does, and converts once at the end, so 20 feet and 240 inches give the same answer to the last decimal.
Ready-mix is ordered in cubic yards (27 cubic feet) or cubic metres. Premixed bags are sold by weight, and what matters is how much set concrete each one makes: about 0.60 cubic feet from an 80 lb bag and 0.45 cubic feet from a 60 lb bag. The bag counts are rounded up, since half a bag still means buying a bag.
Worked example: a 20 × 20 ft slab, 4 inches thick
This is the job the calculator opens with.
- Area: 20 ft × 20 ft = 400 ft².
- Volume: 400 ft² × 4/12 ft = 133.33 ft³.
- In cubic yards: 133.33 ÷ 27 = 4.94 yd³.
- Order with 10% waste: 4.94 × 1.1 = 5.43 yd³.
- In bags: 133.33 ÷ 0.60 = 222.2, so 223 bags of 80 lb, or 133.33 ÷ 0.45 = 296.3, so 297 bags of 60 lb.
At over 200 bags this pour is a ready-mix job. Where the line sits is up to you and your mixer, but the arithmetic makes the choice obvious long before you are lifting the bags.
Footings and rebar
In Footing mode the calculator also counts the reinforcing bar: the number of continuous runs × the footing length × the number of footings, with 10% added for laps and divided into 20 ft bars. A 20 ft footing 24 inches wide and 12 inches deep with two runs takes 1.48 yd³ of concrete and three 20 ft bars. Bar size and spacing are a design question; the count is not.
Questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag of premixed concrete yields about 0.60 cubic feet, so a yard takes 45 of them; a 60 lb bag yields about 0.45 cubic feet, so a yard takes 60. The calculator rounds the bag count up, but not on rounding noise: 4.2 cubic feet is exactly 7 bags of 80 lb, not 8.
How much extra concrete should I order?
The calculator adds 10% to the volume for the order figure. That covers spillage, forms that bow a little and a subgrade that is never perfectly flat. A subgrade dug deeper in places, or a long trench footing, can swallow more; for a small pour from bags, a spare bag or two does the same job.
How much area does a cubic yard of concrete cover?
At 4 inches thick, one cubic yard covers 81 square feet: 27 cubic feet divided by a third of a foot. At 6 inches it covers 54 square feet, and at 3 inches 108. In metric, one cubic metre covers 10 m² at 10 cm.
How do I work out concrete for sonotubes or round piers?
Choose Column / tube. The volume of one column is π × diameter² ÷ 4 × height, times the number of columns. A 12 inch tube 4 feet deep holds 3.14 cubic feet, so six 80 lb bags. The same mode works for a circular slab: put its thickness in as the height.
Can I use it in metres?
Yes. Switch to Metric and the length, width and thickness are typed in metres and centimetres, and the volume comes out in cubic metres. Whatever you have typed is converted when you switch, so you never lose the job. The bag counts stay in 80 lb and 60 lb bags, which are what the yields are measured for.
