49 CFR 375.509 - How must I determine the weight of a shipment?
(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.
(b) The following three conditions must exist for both the tare and gross weighings:
(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.