Have you ever purchased a cheap tarp from a local retailer and opened it up and measured it against the size that is shown on the outside of the packaging? If you do this you will find that the tarp is actually smaller than the size indicated. The reason for this is that most companies show the "cut size" of the tarps. This is generally about 4 - 6% larger than the actual size of the finished tarp.
The reason for this difference is that "cut size" refers to the total area of the material used to make the tarp. Let's say the raw material used to make the tarp is 3 sheets of 5' x 10'. If you ignore hems and seams this could make a tarp that is 15' x 10'. Unfortunately, you can't ignore the hems and the seams so when they actually make the tarp it ends up being something like 14' 3" x 9' 8". In fact, they can't even say how big the finished tarp will be because each one will be slightly different.
Usually, this difference in size is not critical to your application but if you happen to need a tarp that is a precise size it can be very annoying to discover that it is smaller than you expected.
At Heavy Duty Tarps, all of our custom tarps are sold using "Finished Sizes". This means, for example, if you order a 10' x 15' tarp from us the tarp will be 10' x 15' when you open it up (give or take a 1/4 inch manufacturing tolerance).
Most other tarp websites use Cut Sizes so when comparing prices make sure you are comparing apples-to-apples. If you need a tarp that has to be an exact odd size you don't pay any extra with us since we just charge according to how much material is required.