The main standard we refer to for the tolerances that can be applied to door leaves and doorsets is BS 4787: Part 1:1996. This standard is written specifically for metric sizes but, in the absence of an equivalent standard for imperial sizes it is assumed that the information in the standard is applicable to both.
Tolerances are used to provide the acceptable deviation from a given dimension. They are used to describe a range of dimensions that would be considered not to be detrimental to the performance of the item being produced. It is usual for the manufacturing process to become more expensive as the tolerance becomes tighter. That is the range of acceptable dimensions becomes narrower. In woodworking a range of processes would be sawing, moulding or planning and sanding. Each step becoming more accurate than the last but adding cost to the finished item.
The Standard provides the following tolerances for internal doorsets, leaves and frames:
BS 4787: Part 1:1980 does not give a tolerance for the opening width of a door frame because it is assumed in the Standard that the opening width is determined from the sum of the width of the door leaf and the clearances on each side.
All sizes specified in the Standard are at the time of handing over the doorset, door or frame to the purchaser and are at the moisture content agreed between the manufacturer and the purchaser at the time of order.
The tolerances given above do not apply to fire resisting doorsets except for the tolerance applied to the clearance between the door leaf and frame and then, when smoke seals are to be fitted, other clearance sizes and related permissible deviations may be specified. These should be determined by reference to the guidance provided by the door leaf manufacturer which usually provides a maximum clearance and the smoke seal manufacturer which usually provides a minimum clearance.
It should be noted that when a 762 (2'6) door leaf, which is hung on standard hinges, swings open the diagonal measurement of the leaf becomes effective and will reduce the clearance on the lock side by between 1 and 1.5 mm depending on the width and thickness of the door leaf. There is a European Standard (BS EN 1529:1999) that also gives tolerances for door leaves, but as this refers to four classes, the class would have to be agreed before a contract is placed.