How the slab volume is calculated
Concrete is sold by volume, so the first job is turning your slab dimensions into a single cubic measurement. Length and width are entered in feet, while thickness is usually quoted in inches, so the calculator divides the thickness by twelve to put every dimension in the same unit before multiplying the three together.
That product is the volume in cubic feet. Because ready-mix is ordered by the cubic yard, the figure is then divided by 27 (the number of cubic feet in a cubic yard). The bag counts come from dividing the cubic-foot volume by the published yield of a single bag of dry mix.
Reading the bag estimates
A 60 lb bag of standard mix yields roughly 0.45 cubic feet once water is added, and an 80 lb bag yields about 0.60 cubic feet. The calculator rounds the bag counts up to whole bags, since you cannot buy a fraction of one.
Use bags for small jobs such as a fence post, a step, or a pad under an air-conditioning unit. For anything larger than about a cubic yard, mixing by hand becomes impractical and ordering ready-mix delivered by truck is usually cheaper and faster.
Ordering tips and common mistakes
The single most common mistake is ordering exactly the calculated amount. Subgrade is never perfectly level, forms bow slightly, and some concrete is always lost to spillage, so add a buffer before you place the order.
- Order 5 to 10 percent extra to cover spillage and an uneven base.
- Double-check that thickness is entered in inches, not feet — a four-inch slab is 0.33 ft, not 4 ft.
- For footings and irregular shapes, split the pour into simple rectangles and add the volumes.
- Have a plan for placing the whole load quickly; concrete begins to set within an hour or two.
What this estimate does not include
This is a volume estimate only. It does not size reinforcing steel or wire mesh, account for a gravel sub-base, or include the cost of forms, finishing, or delivery fees. For structural slabs, footings, or anything load-bearing, confirm the thickness and reinforcement with local building codes or an engineer.
Formula
volume(ft³) = length(ft) × width(ft) × (depth(in) / 12); cubic yards = volume / 27Frequently asked questions
- Why order in cubic yards?
- Ready-mix concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet.
- Should I order extra?
- Yes. Most suppliers suggest ordering 5–10% more than the calculated volume to allow for spillage and uneven subgrade.