Measuring
If you are simply replacing doors, you can just measure the ones you have and order new ones with those measurements <>

Single Cabinet Doors:
Opening + (left overlay + right overlay) = Door Width
Width Example: Horizontal cabinet opening is 15 inches wide. If you want a ½ inch overlay on both sides, then you’d add 15 for the opening, ½ inch for the left, ½ inch for the right. This equals 16 inches for the total width of the door.
Opening + (top overlay + bottom overlay) = Door Height 
Height Example: Cabinet opening is 29 inches tall. If you want a ½ inch overlay on top and bottom, then you’d add 29 for the opening, ½ inch for the top, ½ inch for the bottom. This equals 30 inches for the total height of the door.

Double Cabinet Doors: Measure cabinet opening, divide by two, and add the desired overlay. Gap between doors can be achieved by adjusting the hinges.

(opening/2) + overlay = door width
Example: Cabinet opening is 23 inches wide. 23 divided by 2 equals 11 ½ inches. If you want a ½ inch overlay, you need to order two 12 inch wide doors. This is the measurement for each door. 
Measure cabinet opening, divide by two, and add the desired overlay. Gap between doors can be achieved by adjusting the hinges.

(opening/2)+overlay =door width
Example: Cabinet opening is 23 inches wide. 23 divided by 2 equals 11 ½ inches. If you want a ½ inch overlay, you need to order two 12 inch wide doors. This is the measurement for each door. Opening height-1 = DT drawer box height 
Height Example: cabinet opening is 7 inches tall. 7 minus 1 equals 6. The drawer box is 6 inches tall.

Plywood Drawer Boxes:
With side-mount glides, subtract 1 inch from the opening width. Also subtract 1 inch from opening height. Keep in mind that the side mounts come in even number measurements when measuring the drawer depth. If you order a box with an odd number (such as 17 inches) you won’t be able to see the very back of the drawer when pulled out completely. 
Opening width-1=PW drawer box width
Width Example: Cabinet opening is 11 inches wide. 11 minus 1 equals 10. The drawer box is 10 inches wide.
Height Example: Cabinet opening is 7 inches tall. 7 minus 1 equals 6. The drawer box is 6 inches tall.
With under-mount glides, subtract 5/8 inch from the width and subtract 1 inch from the height. The depth must be a multiple of 3 (example: 15, 18, 21, 24, etc.)

Opening width- 5/8 in=PW drawer box width
Width Example: Cabinet opening is 11 inches wide. 11 minus 5/8 equals 10 3/8. The drawer box is 10 3/8 inches wide. 
Opening height-1=PW drawer box height
Height Example: Cabinet opening is 7 inches tall. 7 minus 1 equals 6. The drawer box is 6 inches tall.

Drawer Fronts: 
Opening + (left overlay + right overlay) = Drawer front Width 
Width Example: Horizontal cabinet opening is 11 inches wide. If you want a ½ inch overlay on both sides, then you’d add 11 for the opening, ½ inch for the left, ½ inch for the right. This equals 12 inches for the total width of the drawer front.
Opening + (top overlay + bottom overlay) = Drawer front Height

Height Example: cabinet opening is 7 inches tall. If you want a ½ inch overlay on top and bottom, then you’d add 7 for the opening, ½ inch for the top, ½ inch for the bottom. This equals 8 inches for the total height of the drawer front. 

Dovetail Drawer Boxes: 
With side-mount glides, subtract 1 inch from the opening width. Also subtract 1 inch from opening height. Keep in mind that the side mounts come in even number measurements when measuring the drawer depth. If you order a box with an odd number depth (such as 17 inches) you won’t be able to see the very back of the drawer when pulled out completely. 

Opening width-1 = DT drawer box width
Width Example: Cabinet opening is 11 inches wide. 11 minus 1 equals 10. The drawer box is 10 inches wide. 
Opening height-1 = DT drawer box height Height Example: cabinet opening is 7 inches tall. 7 minus 1 equals 6. The drawer box is 6 inches tall.
With under-mount glides, subtract 3/8 inch from the width and subtract 1 inch from the height. The depth must be a multiple of 3 (example: 15, 18, 21, 24, etc.) 
Opening width- 3/8 in = DT drawer box width