As everyone knows, fishermen exaggerate the size and especially the weight of the fish they catch. We hear, "Yeah, we were catching 4, 5 and 6 pounders, got one that had to be over 8!" With Largemouth Bass, the weights described are almost always double the real weight of the fish. Why is that? We have measured and weighed many bass and here's what we've found:
Length: Weight Range:
13" .7-1 pounds
14" .9-1.3 pounds
15" 1.1-1.4 pounds
16" 1.25-1.9 pounds
17" 1.8-2.5 pounds
18" 2.2-3.4 pounds
19" 3.3-4.2 pounds
20" 4-5.5 pounds
21" 4.25-5.75 pounds
21.5"-23" 5-9 pounds
There is a well-known formula that comes out to within 5% of actual weight.

(Length X (Girth Squared))/880=weight in pounds

Here are some actual sizes & weights compared to the calculated weight:
Length Girth Scale Weight Calc. weight
13 8.25 .8 1
14 9 1.2 1.28
15 9.5 1.3 1.53
16 9.5 1.5 1.64
16 10 1.75 1.82
16.5 10.75 2 2.16
18 12.25 3.1 3.07
19 12.75 3.6 3.5
21 14 4.6 4.7
Some bass have a barrel shaped body and some are sleeker, so the girth is the main factor, not the length.
The Length is relatively easy to accurately measure. Simply lay the fish on a flat surface. With the fish's mouth closed, measure from the tip of its lip to the tip of the tail. The Girth is trickier because A Largemouth that is free of battle scars has no place to grab with the small hook on some scales. An easy way is to put the scale hook in the fishing line's snap swivel before the hook or lure is removed. Sometimes the hooks have to be immediately and carefully removed if there is a chance of injury to the fish (or fisherman). That is where the measurements can be taken and the weight calculated using the formula. Some Digital scales have a nice clamp that grabs the fish's jaw to safely weigh it.