The unit of board measure is the board foot, which is the contents of a board 1 foot square and 1 inch thick. The number of board feet which can be sawed from logs of different diameters and lengths is shown in log rules.

Logs are usually measured at the small end inside the bark, because the removal of the slabs reduces the logs to the dimensions of the small end. This is the custom in measuring short logs by all the rules which are used, except in certain cases. Some of the rules, for example the Doyle and the Partridge rules, were intended by their originators to be used for an average diameter, but most persons who use them take the diameter at the small end, except in case of long timber. In measuring long logs which are to be cut into short logs before being sawed into boards, the diameter is usually not taken at the small end alone. Thus in using the Maine Rule, long logs are sealed as two logs. The diameter at the small end inside the bark is measured and is taken as the diameter of the uppermost log. The diameter at the small end of the lower log is estimated by the log-scaler. Another method of measuring long logs, often used with the Doyle Rule, is to take the diameters at both ends inside the bark, average them, and use this average as the diameter of the log. Still another method in use is to take the diameter inside the bark, one-third the distance from the small end of the log.

Logs are usually cut from 2 to 6 inches longer than the standard lengths of boards, to allow for bruising in handling. This additional length is disregarded in scaling.

Log rules give the number of board feet in logs which are straight and sound. If logs are unsound or otherwise defective, a certain allowance must be made by the scaler. The determination of the amount in board feet which should be deducted for unsoundness or defects in a given log requires great skill on the part of the scaler, and, as it is a matter of judgment in each case, no definite directions can be given.