I used the one from the table that ewest attached. I graphed this table then used a power trend-line in excel and displayed the equation on the graph. Here is the formula's that will match the table. I use this formula in my fish tracking sheets. You enter the fish length and weight and it calculates the Standard weight and the Relative weight.
BASS
y=0.0003345869 * x^3.17775
Redear Sunfish
y=0.0005318159 * x^3.12
Bluegill
y=0.000338346 * x^3.405
y= weight in lbs
x= length in inches
Backup
NR-1193, New Nov 2000. Russell A. Wright, Extension Specialist, Fisheries and Allied Aquaculture, Auburn University
Fisheries managers often need to know if fish are growing poorly or even losing weight. Lack of food, poor water quality, poor water temperatures (too hot or too cold), or disease can cause stress that results in poor growth. While growth may be difficult to measure, condition or plumpness of the fish is easy to measure and indicates if the fish are under stress.
One measure of the condition of a fish is its relative weight. Relative weight is the ratio of the actual weight of a fish to what a rapidly growing healthy fish of the same length should weigh, called standard weight. Fish with high relative weights are fat while those with low relative weights are thin. Of course ponds should be managed to produce healthy, fast-growing fish.
To calculate the relative weight for a fish, one simply divides the weight of the fish in pounds by the standard weight for a fish of the same length. Standard weights can be found in Table 1 for largemouth bass, bluegill, and redear sunfish (also called shellcrackers). Fish should be measured from the tip of the nose with the mouth closed to the end of the tail. Scales for weighing fish can be purchased at most sporting goods stores. Fish that have a relative weight less than 0.80 or 80 percent of the standard are considered severely thin, indicating a lack of food for that animal. Relative weights between .8 and 1, while not ideal, are well within the range found in healthy populations.
