You’re on lunch break from work and decide to toss a couple of casts into the park pond you pass every day. Suddenly, something big hits! Fortunately, the pond doesn’t contain too many obstacles and you manage to land one of the biggest bass you’ve ever seen. But you have no scale . . . All you can do is measure the bass length and girth with the tape measure you keep in your toolbox before releasing it: 27 inches long and 18 inches around. But the real question is, how much are you going to tell the guys back at the bass club it weighed?
Luckily for curious anglers, the relationship between a fish’s length and its weight is a fairly consistent one. The graph below, charting the weight and length of many individual bass, reveals that a nice, consistent relationship exists between the two. This is illustrated by the clustering together of the data points (dots) and the smooth curve that they make as a group. This graph displays information from bass surveyed from the L-67A Canal in Water Conservation Area 3.
This graph compares length and weight for 3,600 largemouth bass sampled from the L-67A Canal. Each dot represents one fish; most of the dots are so tightly clustered together that they are not individually visible.
One thing to keep in mind is that many fish change shape as they get larger. For example, bass anglers know that a big “hawg” has a much chunkier profile than a young “bank runner”. You can also see this in the above graph. Instead of a straight line formed by the grouping of dots (which would occur if weight was exactly proportional to length for bass of all sizes), there is an upward curve. In other words, as a bass gets longer it tends to weigh more per inch.
Does all this graphing and science have any practical application for anglers? Of course! There are various equations that have been developed to estimate bass weight based on its length and girth. Fortunately, FWC’s Bass Weight Calculator website saves you from doing the math: Just plug in your numbers and hit return! The first weight estimate displayed is the one FWC considers to be most accurate, but estimates using several other formulas are also provided for comparison. Plugging in your numbers from your park pond bass, you can tell your fishin’ buddies that your bass weighed about 11 pounds! (And you’ll definitely take your scale next time!)
The FWC's Bass Weight Calculator website will provide an accurate bass weight estimate from a length and girth measurement.
So far, we’ve only talked about largemouth bass, but most fish will have a similar relationship between length and weight. Of course, the numbers (and ratio of weight to length) will be very different for a round bluegill than they will be for a long gar. For a biologist, length-weight data like this can reveal what health condition a fish is in, whether a fish’s shape changes as it grows, and other important information.
Note that for TrophyCatch submissions, you MUST provide a readable weight-on-scale photo and we recommend that you always carry a scale with you — you never know when a trophy will show up in Florida! However, if you ever get caught without a scale, a length and girth measure can help you estimate the weight of your catch.