49 CFR 375.509 - How must I determine the weight of a shipment?
(a) You must weigh the shipment by using one of the following two methods:
(1) First method - origin weigh. You determine the difference between the tare weight of the vehicle before loading at the origin of the shipment and the gross weight of the same vehicle after loading the shipment.
(2) Second method - back weigh. You determine the difference between the gross weight of the vehicle with the shipment loaded and the tare weight of the same vehicle after you unload the shipment.
(b) The following three conditions must exist for both the tare and gross weighings:
(1) The vehicle must have installed or loaded all pads, dollies, hand trucks, ramps, and other equipment required in the transportation of the shipment.
(2) The driver and other persons must be off the vehicle at the time of either weighing.
(3) The fuel tanks on the vehicle must be full at the time of each weighing, or, in the alternative, when you use the first method - origin weigh, in paragraph (a)(1) of this section, where the tare weighing is the first weighing performed, you must refrain from adding fuel between the two weighings.
(c) You may detach the trailer of a tractor-trailer vehicle combination from the tractor and have the trailer weighed separately at each weighing provided the length of the scale platform is adequate to accommodate and support the entire trailer at one time.
(d) You must use the net weight of shipments transported in containers. You must calculate the difference between the tare weight of the container (including all pads, blocking and bracing used in the transportation of the shipment) and the gross weight of the container with the shipment loaded in the container.