I am putting braid on my baitcaster. It says 40lb, but it also says "10lb Test Diameter"...So which is it? I've read 30-40lbs is a good starting point for baitcasters, but the 10lb Test Diameter is throwing me off.  How do I know what the diameter is in inches?

Theres no standard for lb test diameters, so each manufacture will say what they believe is true. They mean that the braid is rated to 40lb but has a diameter of a 10lb mono. As far as that line is concerned, its perfect, spool it up.

This ^^^

What difference does it make? That line has a breaking point of 40# test yet has a diameter of traditional 10# test mono. Test strength to diameter proportion varies from product to product. Braid companies may have a chart on their website with actual dimensions if you need or want to know. 

Most 40lb braid is going to be around .013 inches in diameter. 

The reason both poundages are listed is to help you select a braid that is similar in diameter to the mono you usually use. Almost all braid manufactures give the diameter in inches & mm. Baid is listed as x pounds with x pound mono equivalent dia. The diameter in inches/mm allows you to compare different braid manufactures by stated poundage as well as mono equivalent diameter. Not all 50lb braid is the same diameter. 

What I do is I jump on the TW website and pull up the line im looking at.If you click on product description and scroll down it will show you line diameter and mono equivalent from 10-65#.

Thanks guys, I appreciate it