Brick sizes are standardized sizes used by brick manufacturers to ensure that products remain consistent and reliable. Masons who work with brick can theoretically determine the amount of coverage needed along with the look and feel desired and then order a set number of bricks in a set size for the job. However, brick sizing is immensely complicated, and despite the fact that it is supposed to be standardized, it really isn't.
The first thing to be aware of when discussing brick sizes is that there are different standards for face brick and paving brick. Paving brick is brick used for things like walkways and patios, while face brick is used to make walls. Sometimes the same term will be used to talk about two different sizes of paving and face brick. For example, many nations have a size called “standard” which is quite different, depending on whether one is talking about face brick or paving brick.
The next thing to be aware of is that brick sizes vary considerably by nation. The size of a “standard” face brick, for example, is far from standard, which can become a serious problem when people are ordering bricks from different countries. An Italian standard brick and an American standard brick, for example, are not the same size, which means that a course which mixes such bricks will not be even.
As if this wasn't enough, the names used for brick sizes are not standardized either, even in the same country. One company's “jumbo” brick may be another company's “economy,” for example. This means that the supposed standards which apply to brick sizes are effectively meaningless, because one cannot use standardized terms and assume that someone will know what size is being discussed.
Finally, bricks are sized in nominal and actual size. Actual size refers, as one might imagine, to the actual measured size of the brick, and is given in three dimensions, length, width, and height, so that people understand the size and shape of the brick. Nominal size, also given in length, height, and width, is the size calculated with joints, but because joint sizes vary, nominal sizes are also not uniform in nature. For this reason, companies usually list both brick sizes so that people know precisely what they are buying.
Because brick sizes are not standardized, it is preferable to ask brick manufacturers to simply give the dimensions of their products when placing an order. Most companies also indicate how much area a set number of bricks can cover estimating an average joint size between the bricks, and people can also use a brick calculator to determine how many bricks they need to order for a project. As always, ordering overage in case bricks are broken or the measurements are slightly off is a very good idea.