There are many differences between custom wigs and pre-made wigs 

Custom wigs are made a variety of ways depending how custom you want to go. But typically it is made with custom cap based off your head measurements. A consultation is required and the size is not just by saying you're a small or medium size but instead by numbers. 

For example, if we do your measurements and  on my chart you measure a solid 21inch circumference  we would create you a wig based on the 6 measurements we take during that appointment to insure the best fit.

However...

For pre-made wigs they are made without knowing your measurements and based on a range of size. For example a size small would be 21-21.75 which means if you measure anywhere in between that range you can order a small. It may be slightly bigger or smaller because it is a range size wig selection not exact.

Also the cap used for a pre-made wig is different from the caps we use for custom unit. 

It is overall a different level of experience at a higher price point for custom wigs versus a pre-made wig. 

Here's a copy of our preferred measuring chart:

Let us know in the comments below if you prefer custom wigs over pre-made wigs!