Piw means pounds per inch of belt width and is widely used in the United States of America.
The piw figure is indicating the working tension a conveyor belt is intended to be used for, irrespective of the breaking strength of the splice. Usually, the belt's breaking strength is not given.
Outside of the USA, the breaking strength of the conveyor belt is used, rated in N/mm, i.e. Newton per millimeter of belt width. The belt breaking strength is determined precisely, depending on the operating conditions, the splice type etc. The safety factor is related to the weakest point of the conveyor belt, which is the splice.
Without consideration of a safety factor, piw are converted into N/mm by multiplying by 0.175.
Example: 2284 piw x 0.175 = 400 N/mm.
N/mm converted into piw are multiplied by 5.71. Example: 400 N/mm x 5.71 = 2284 piw.
If a safety factor of 10 is assumed for both terms, then a 400 piw belt would equal to a 700 N/mm belt. However, this is not academic.
The above applies to textile conveyor belts. For steel cable conveyor belts, the rating in N/mm breaking strength is used in the USA, too.