How to Measure for Drawer Boxes when using any of the FULL EXTENSION - SIDE MOUNT drawer slide kits that we sell. 
standard slides AND slides with "soft-close dampers" are included in these instructions.
1) Measure the width, height and depth (to the back wall of the cabinet) of the drawer box opening that the New drawer box will fit into.
2) Deduct 1 1/16" from the width of the opening. The slides are approx. 1/2" thick on each side of the drawer, but if you give yourself that extra 1/16", the drawer will never be too tight, and if it is too loose, you can stick a small washer behind one of the slides where it connects to the cabinet!
3) Pick a height that is at least 3/8" LESS the height of the drawer box opening.
4) Use the deepest (front to back) drawer box that will fit in the cabinet. If the cabinet measures 22 7/8" from the front of the face on the cabinet to the back wall on the cabinet, then order a 21" deep drawer box. Do Not order an 18" deep (front to back) drawer box simply because your old drawer box was that depth. Not only will you be giving up valuable space, but when you order an 18: deep drawer box, you will be getting 18" long drawer slides, which will not reach to the backwall of the cabinet!
Each set of these Side Mount Drawer Slides Include Four pieces (Members)
For Each Drawer Slide KIT, we also include:
Self Adjusting Steel Rear Mounting Brackets
REMEMBER, AN OPENING SIZE OF 1” (PLUS 1/16” TO PREVENT ANY TIGHTNESS) WIDER THAN THE DRAWER BOX ITSELF IS REQUIRED WITH SIDE MOUNT SLIDES.
IF YOU FIND THAT YOU DO NOT HAVE THE REQUIRED 1/2”+ OF SPACE ON EACH SIDE OF THE DRAWER BOX FOR THE SIDE MOUNT DRAWER SLIDES, YOU CAN EASILY CORRECT THE PROBLEM BY “NOTCHING” THE FACE FRAME A BIT TO GET THE SPACE NEEDED, AS SEEN IN THE PICTURE BELOW.