It seems like there would be a straight forward answer to this question but unfortunately there isn’t. The same pillow can actually have two different measurements. First there is the measurement of the pillow, BEFORE being stuffed. Second there is the size of the pillow once it has an insert in it. The trick is knowing when to use what nomenclature so that no one gets confused.
In order to set the standard straight, here is how the pillow industry measures pillows. This goes for hospitality, retail, residential, commercial and anything else in between. The phrase '18"x 18" pillow' refers to the size of the pillow cover, BEFORE it is stuffed. For example when ordering or specifying a 18" x 18" pillow you are specifying that the pillow cover be made to measure 18” x 18”. This seems logical and straight forward but the confusion comes when you actually receive the pillow. Will it, WITH an insert in it, be 18" x 18"? No it will NOT.
Why? Simply put, the 18" x 18" pillow cover now gets a 19" x 19" insert stuffed into it (the pillow insert is generally about an 1" bigger than the cover). As the pillow insert is stuffed into the fabric case the pillow is ‘plumped’ out, the corners are stretched and the sides are pulled in. Ultimately resulting in a pillow that is actually 14" - 16” from side to side.
Even in these 2D pictures above you can see the change in the pillow. It gains depth in the picture, the sides come in and the corners stretch out. Makes sense doesn't it? It really is as easy as that. So next time you are ordering or specifying pillows, just remember the size you choose is referring to the size of the pillow case BEFORE being stuffed with an insert.
Check out the following graphic which shows the progression of building a pillow with respective sizes of each element. We hope that this has been helpful in clearing up any confusion. Just remember 'the size' you call a pillow, refers to the size of the cover!