The tip size is figured this way. Take the first number and double it. That is the width of the pattern when spraying from 12" away. The last two numbers are the size in inches. 619 = 12" wide pattern when spraying from 12" and it has a .019" orifice. A 515 = 10" wide pattern when spraying from 12" away and it has a .015" orifice. The paint manufacturer determines these sizes and list them on the back of the paint can under the application section. For more help call us at 1-800-328-8251.