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Calculating Fish Weight – The Formula

Posted on August 18 2015

Calculating fish weight
How much does this one weigh? Photo: Mike Sanders.

The silver salmon fishing is as good as ever here at Alaska West and Rapids Camp Lodge, and not only are we catching a lot of silvers this year, we’re also catching a lot of bigger fish.

That has our anglers are asking, “how big is that hog?”, which has us scratching our heads.. “Yeah, exactly how big is it?”

It’s easy to weigh a fish you take for the smoker or for the barbecue, but for those catch and release fish, we need another way to keep score.

Therefore, we use “The Formula” for calculating the weight of rainbows and dollies that we catch and release. Length multiplied by the Girth Squared, divided by 750 is the age old, tried and trued formula that has been passed down over the ages for measuring rainbows with a tape. We don’t physically weigh them with a scale, simply because we release all of them on our river.

However, we decided to see how “The Formula” measures up with our silver salmon.

We took a small sample size of 10 random coho destined for the vacuum packer and numbered them 1 through 10. Then, we measured the length and the girth with a non stretch vinyl tape. Next, we weighed them on our scale.

We compared the results the old math gave us to the actual weight of the fish and calculated an average accuracy percentage.

It turns out that the trout factor of 750 actually turned out to be a little too heavy when it came to silver salmon. 850 was too light, but 790 got us to .9995% accurate for the 10 fish we measured and we think that’s pretty darned accurate.

So, the verdict? For rainbows and dollies, we’ll keep using the age old formula of:

  • Weight = (Length) x ((Girth)(Girth)) / 750 

However, when it comes to silver salmon, we’re going to use our new formula of:

  • Weight = (Length) x ((Girth)(Girth)) / 790

Needless to say, math is way more fun when fishing is involved..

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