Calculating yards of concrete is vital for a successful job. It is one of the most fundamental skills a concrete contractor must have. Fortunately, it is also a relatively straight forward process for most projects.
Running out of concrete part of the way through can spell disaster. If you make a mistake calculating yards of concrete, it will force you to stop working sooner. The delay in getting a new load ordered and delivered may force you to resume work the next day or later. It can also cause unplanned cold joints, visual inconsistencies, and all sorts of problems. At the end of the day, failing to make this calculation correctly will cost you time and money.
On the surface, calculating yards of concrete is pretty simple. First, determine the total area of the surface. This is a simple length by width multiplication for square or rectangular slabs. Irregular shaped concrete slabs will require more complicated calculations to get surface area. Fortunately, odd shapes are usually able to be converted into rectangles for the purposes of ordering concrete. Once the surface area is determined, determine the appropriate thickness of the slab. Convert that thickness from inches to decimals of a foot. The multiply that number by the surface area to find cubic feet. Finally, multiply that number by 0.037 to convert it into cubic yards. There are many handy and useful concrete calculators on-line. These can reduce the likelihood for error and help you get the right yardage each time.
Always make sure to order extra concrete. The rule of thumb is to at least 10% more concrete than you expect. It is also common practice to overestimate slab thickness by a small fraction to account for uneven grading. For example, if you are calculating yards of concrete for a 20' x 30' patio at 4" thickness, you will get 7.41 yards of concrete. Increasing the thickness measurement by 0.25" will yield 7.87 yards. Adding a safety buffer of 10% will result in a final order of 8.66 yards of concrete.
By what if you are going to stamp concrete? How does that affect yardage? Fortunately, concrete texturing has almost no impact on the total amount of concrete needed. Concrete stamps displace very little concrete and should not affect the amount of material needed. Generally, most forms of decorative concrete finishing will not affect the amount of concrete you must order. However, if you are going to be using textured concrete or another decorative finish, you may also be coloring your concrete. This will not necessarily affect how much concrete you order, but it will affect the kind of concrete you order. Integral colors, for example, limit the kinds of additives, aggregates, and mix designs usable. Likewise, planning around some topical coloring products like concrete acid stain may also limit what you can use in your mix design.
But what happens if you make a mistake calculating yards of concrete? Ordering too much concrete wastes money, but is otherwise not much of a problem. Not getting enough, however, can cause some serious problems. The best option is usually to adjust the location of your control joints. If you get lucky, your ready mix producer may be able to rush another order of concrete to you on the same day. In any case, you risk delaying your project and/or having differing shades of concrete. If the concrete looks different, using a topical coloring product like Tinta' Seal, Refresh, or Cem-Coat can help even out the final appearance. This is especially handy if you are pouring integrally colored concrete as color variations are more noticeable and less acceptable.