Bathroom scale it is, hold the fish and have a buddy balance me lol.
Here is a formula found in Fishing Hawaii Style written by Jim Rizzuto. 100lbs is our big boy mark here in Hawaii.
Length Girth Weight
52.5 inches 43inches 100.5 pounds
53 44 107.5
56 42 114.5
If length plus girth =96inches then weight = 100lbs or I guess 45kg
you guys can do the metric conversion
The disclaimer is without a scale you can't really prove it is 100lbs, of course.