The best way to measure:  take one of your existing shirts, lay it flat, and then measure the width and length. Or, know the persons chest and height measurements.

DOUBLE the Width to get the Chest size 

I wear a Size 14 top (XL) in the stores.  These Gildan tee shirts are a ladies cut - wider and shorter than a man's tee shirt.  I normally make myself a large and it fits fine.  There is no shrinkage so I don't have to worry about that.

For some reason, when I cut open a sweatshirt to make it into a jacket, it ends up wider than the pullover of the same size.  Choosing a size for a jacket depends on how you want it to fit.  I prefer a more "fitted" look so I make myself a medium jacket and a large t-shirt.  The jackets are not designed to close in the front.  However, many women who try them on want a more roomy, relaxed look so they opt for the size they would normally order.

Regardless of what you choose to order, if the size isn't right and the garment hasn't been worn, washed or altered, it can be returned for a different size.

Jerzee Sizing Descriptions
*50/50 poly cotton Jerzee NuBlend fleece, 8 ounce fleece
*Cover seamed neck and armholes
*Ribbed Spandex in cuffs, collar & waistband
Finished Jerzee Garment Measurements
Adult Sizes

I have very few Jerzees brand left so chances are if you order a pullover sweatshirt it will be in the Gildan brand.  One exception:  Gildan doesn't make the Texas Orange color that looks so good with the fall fabrics.  I'll be usingJerzees for those.

If this helps at all:  I wear 14 top (XL in the stores).  In the Jerzee sweatshirts I normally make myself a large if I'm going to keep the elasticized bottom on.   In the tunic style, I've found the medium to be more fitted, not quite as loose and "blousy" as a sweatshirt can be sometimes.  I have very few Jerzees sweatshirts left as I'm switching my brand to Gildan. 

 New Ladies t-shirts brand  -  LAT

 I had to go to a different brand from Gildan to get some different colors. Following is the sizing chart LAT puts out for their clothes.  These LAT tees are a ring-spun cotton - very soft.  They are slightly more narrow through the body than the Gildan, and just an inch or so shorter.  I can still wear a Large if I'm going to tuck it in, but I might prefer an XL for that loose comfortable fit if I'm just going to wear it out over jeans or capris.   Regardless, if you order a size and it doesn't fit, it can be exchanged as long as it hasn't been worn, washed or altered.