Steelhead Weight Our original fish weight estimator was based on measurements of length, girth, and weight of multiple fish species. We concluded that the best overall formula for estimating fish weight is: weight (lbs) = length (inches from nose to fork of tail) x girth (inches) x girth (inches) / 690 The 690 divisor was a surprise to many people because it is so different from the well-known "traditional" and Sturdies formulas (which use divisors of 800 and 750 respectively).  However, the Fly Fishing Research 690 formula is based on hard data, which shows clearly that higher divisors like 800 and 750 significantly underestimate fish weight on average (see our article in Fly Fisherman Magazine in 2008). This section of our website reports the results of an in-depth and extended analysis for steelhead in particular. It is not based on hearsay or opinion, but rather on hard data of weight, length, girth, and gender of 115 of actual steelhead from multiple rivers in the Pacific Northwest. Our conclusions is as follows: Based on analysis of the data, the 690 divisor is the best divisor to use for steelhead when using a formula of the above form. Each individual fish has its own divisor, which is about equally likely to be above or below 690. The actual weight of a steelhead is within +/- 10% of the 690-based estimate about 90% of the time. The +/- 10% variability is primarily due to naturally occurring shape differences among individual fish (i.e. individual fish have different shapes just as individual humans do). A small portion of this variability may be due to gender. A small portion of this variability may be due to river. A signficant portion of this variabililty is due to differences in girth taper between fat and thin steelhead. We have developed an improved formula for steelhead based on accounting for girth taper.  This improved formula is more accurate than the simpler 690 formula above. The improved formula is validated not only by the data for fat vs thin steelhead but also by our conical fish model, which proves to our satisfaction that differences in divisor between fat and thin steelhead are a fact of nature -- not just an artifact of the data. Please click on the following links to learn more about these sources of variability and how much difference they make: variability by individual, variability by gender, and variability by river. Any such analysis is only as good as the data on which it is based.  Fly Fishing Research is fortunate to have acquired data from highly credible sources.  If you have additional data that includes length, girth, weight, gender and river of actual steelhead, please send it to tom@flyfishingresearch.net. We will update our records and analyses from time to time and credit you for your contributions accordingly.