A trowel and bricks.
Brick sizes are standardized sizes used by brick manufacturers to ensure products remain consistent and reliable. Bricklayers can theoretically determine the amount of coverage needed along with the desired look and then order a certain number of bricks in a given size for the job. However, brick sizing is immensely complicated, and while it should be standardized, it really isn’t.
The first thing to keep in mind when looking at brick sizes is that there are different standards for facing bricks and paving bricks. Paver brick is the brick used for walkways and patios, while facing brick is used to make walls. Sometimes the same term will be used to talk about two different sizes of paver and facing brick. For example, many nations have a so-called “standard” size, which is quite different depending on whether it is brick or cobblestone.
The next thing to keep in mind is that brick sizes vary considerably from country to country. The size of a “standard” facing brick, for example, is far from standard, which can become a serious problem when people order 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 mixing these bricks will not be uniform.
As if that were not enough, the names of the sizes of the bricks are also not standardized, even in the same country. The “jumbo” brick of a company can be the “economy” of another company, for example. This means that the standard assumptions that apply to brick sizes are meaningless, because one cannot use standard terms and assume that someone will know what size is being discussed.
Finally, the bricks are scaled in nominal and actual size. The actual size refers, as you can imagine, to the actual measured size of the brick, and it is given in three dimensions, length, width and height, so that people understand the size and shape of the brick. The nominal size, also given in length, height and width, is the size calculated with the joints, but as joint sizes vary, the nominal sizes are also not uniform in nature. For this reason, companies often list both brick sizes so people know exactly what they are buying.
As brick sizes are not standardized, it is preferable to ask brick manufacturers to simply provide the dimensions of their products when placing an order. Most companies also indicate the area a given number of bricks can cover by estimating the 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 overages in case the bricks break or the measurements are slightly off is a very good idea.