Here is a Largemouth Bass Weight Estimator developed by Texas Parks and Wildlife! We modified the table to use a calculator format. Here are the details on how the Estimator was developed:
Estimating Weight of Largemouth Bass
This information is provided for those who wish to hold “paper” fishing events, releasing bass immediately after catch instead of holding them for weigh-in. Figures in the conversion table below represent average weights calculated from statistics on 3,178 Texas bass. By using this table, you can get a reasonable estimate of a fish’s weight.
This method is not accepted for official fish records and will never be as accurate as weighing on a certified scale, but it can be helpful at slot limit lakes, or for fishing clubs that wish to provide better care for bass during the hot summer months when fish are easily stressed.
Measure the bass to the nearest eighth of an inch.
Go down the first (left-hand) column and locate the row corresponding to the length of your fish in whole inches.
In the “Fractions” columns across the top of the table, find the nearest eighth of an inch that matches the length of your bass.
Read down the column to find the estimated weight in pounds.
For example, you want to find the weight for a bass that is 18¼ inches long. Find 18 inches in the first column, then move across the table to the 1/4-inch column. Read the estimated weight of 3.46 pounds (3 pounds, 7.4 ounces).
Retrieived from http://tpwd.texas.gov/fishboat/fish/recreational/catchrelease/bass_length_weight.phtml. Note: The table referenced in the details above is posted on TPWD’s Website.