Not sure there is any number for that, here are some definitions
GVW: Gross Vehicle Weight
- the actual weight
of a vehicle.
GVWR: Gross Vehicle Weight
Rating: The maximum allowed GVW for a vehicle.
GCW: Gross Combined Weight: The total weight of everything - tow vehicle, trailer, etc.
GCWR: The maximum allowed GCW.
GAW (front or rear): The Gross Axle
Weight - the amount of weight on that particular axle
GAWR (front or rear): The Gross Axle
Weight Rating: The maximum allowed GAW.
UVW: Unloaded Vehicle Weight, also know as "Dry Weight". This is the weight of a trailer with no liquids or options. The GVW of the trailer will usually be 500-1500 pounds more than this number.
Tow Rating: The rated towing capacity of a tow vehicle - generally based upon curb weight plus one 150-lb driver. This number is NOT designed as an indicator of what you can tow under normal conditions, but it is useful for comparisons.
Actual towing capacity is about 1,000 pounds less that the tow rating, or, to be more accurate, it's GCWR-GVW, provided that the number you get is less than the tow rating, which it may not be for certain vehicles (it's pretty unusual - mostly heavier duty trucks).
A note on GVWR and GCWR: Sometimes odd things happen with GVWR and GCWR on larger trucks especially. The thing to remember is that neither GVWR nor GCWR can be exceeded safely.
Tongue weight is normally 15% of the GVW
I hope that helps!