The use of "pound" for either "mass" or "force" is terribly confusing, and actually reflects two different systems of units. To reduce confusion, some have suggested lbm for the mass unit, and lbf for the force unit.
Here's one reference on this topic:
A unit of weight in the British engineering system (O'Hanian 1985, pp. 14 and 96) equal to 4.448 newtons that is commonly used in the United States. Unfortunately, there is a great deal of confusion about the definition of the pound, with many authors using the unit to denote the mass-equivalent of 4.448 newtons, namely 453.592 grams. The use of the unqualified term "pound" is therefore discouraged, with the more descriptive terms pound-mass (abbreviated lbm) and pound-force (lbf) being preferred. The confusion between pound-mass and pound-force alone is a good reason to use metric units wherever possible (in the MKS system, the base unit of mass is the kilogram, while the unit of force is the newton).
In the foot-pound-second system of units, the pound is defined as the weight of a pound-mass at a point on the Earth's surface where the acceleration due to gravity is 32.174 ft s, while the slug is a derived unit of mass equal to 1/32.174 lbm, or 14.5939 kg.
14 pounds equal one stone, 94 pounds equal one bag.
Source:
http://scienceworld.wolfram.com/physics/Pound.html
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I have never heard or seen "stone" or "bag" used as a unit by a US-based speaker or writer. In fact, I had never seen the term "bag" used as a unit until I saw the above reference.
To further complicate matters, there is a unit of force called the "poundal" which I have heard or seen used:
The poundal is defined as the force necessary to accelerate 1 pound-mass at 1 foot per second per second. 1 pdl = 0.138254954376 N exactly.
Source:
https://en.wikipedia.org/wiki/Poundal
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I believe using the MKS system exclusively is the only sure path to sanity.