18 gauge versus 4d finishing nail

What is the difference between an 18 gauge brad and a 4d finishing nail in diameter? 15 or 16 gauge?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.