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FFM Fish Weight Article


Fish Weight

Anglers have long wanted to estimate the weight of a fish from measurements of its length and girth.  The most commonly used formula is: weight (lbs) = length(inches) * girth(inches)^2 / 800. Other anglers and guides have used this same formula, but with a denominator of 750.  Based on our data, both formulas systematically underestimate the weight of your fish.

We have derived our formula based on data, not on opinion and hearsay.  It was derived for steelhead, but works too for other species.  We based it on physical measurement of length (from tip of nose to fork of tail), girth (at largest point), and weight of 85 steelhead caught and released during 2001-2006.  72 of these were from the Babine River in northern British Columbia.  These fish were measured and weighed by guides Mark McAneely, Billy Labonte, and Jeremy Dufton.  Their careful measurements were recorded in the log books of the Silver Hilton Lodge.  Each steelhead was weighed in a dunk bag hung on an IFGA-certified scale. The best denominator for 72 Babine steelhead, as determined by our regression analysis, is 690. In 2004, we made our table based on this analysis available to the Babine guides.  Since then, they have confirmed its accuracy and used it as their "bible" for steelhead weight. 

12 other steelhead from other rivers (including the Kispiox River, northern British Columbia, and the Sandy River, Alaska) were similarly measured and weighed by Tom Keelin or friend Tom Wallingford.  They showed no visible deviation from the Babine steelhead pattern.

We also measured and weighed several species of Pacific salmon: chinook, chum, and sockeye(fresh, before spawning distortions).  Further, we measured and weighed several salt water Pacific albacore tuna.  The salmon and albacore were all weighed on an IFGA-certified scale. The salmon and albacore also fit the Babine steelhead pattern with no visible deviation.

In total, we measured and weighed 97 fish ranging from 3 to 33 lbs.  Results for the various species are graphed below.  The center trend line, which is the best fit for steelhead (denominator 690), appears to work well too for the other species.  This extrapolation to other species, however, it highly tentative since it is based on so few samples from other species. Note that 80% of the samples for all species combined fall within the high and low trend lines, which correspond to +/-8% of weight.

A fish-weight table based on this analysis as well as this graph are available on the Weight Estimator. Or calculate the weight yourself using our on-line fish weight calculator.  See also frequently asked questions for further analysis and information.