1.When using standard under-mount runners (Blum, Grass, etc) the dimension from the inside of the drawer to the inside of the carcase should be 21mm. This results in the drawer width being 16mm less than the internal carcase width when the drawers are made with 13mm thick material or 12mm less than internal carcase width if 15mm material. For in-frame applications use the frame opening width for these calculations instead of the internal carcase dimension.
2.Side-fixed ball-bearing runners generally need 12.5mm between carcase side and drawer side, but you should check for whichever runners you are using, because sometimes they need more.
3. For internal drawers which are behind doors it is essential to ensure that the clearances mentioned above are maintained when the doors are in the open position. This is sometimes overlooked with the result that the drawers hit the back edge of the doors.
4.For drawer heights in a stack of drawers, as a general rule allow 15mm below the bottom drawer, 25mm between drawers and 10mm above the top drawer. The height of your separate drawer fronts may change this a little and specific calculations will probably be necessary. If the drawers are not having separate fronts, specific calculation will be necessary.
5. Drawer depths (front-to-back dimension) are determined from the actual runners used. The commonest under-mount runner lengths are 250mm, 270mm, 300mm, then in 50mm increments up to 750mm. There are intermediate sizes but these are not so readily available. The maximum runner length is determined by the space available in the carcase – allow a minimum of 5mm clearance beyond the runner to the back of the carcase.