The formula that is widely used for steelhead weight estimation is:

length x girth squared /775

or 31 x (14.5 x 14.5) /775

which turns out to be quite a bit smaller than 12-15 lbs. Using your measurements, it was about 8 1/2 lbs. But remember that this formula is only as accurate as the measurements you take. A couple inch error here or there can yield widely different results. I think the measurement of girth is taken around the body in front of the dorsal fin, and the length is from tip of snout to the center of the tail "fork". I've used this formula on hatchery fish ( I don't typically want to handle a Nate enough to get measurements) and it comes out within a half pound + or -. Of course there are those fish with an unusual physique that don't fit the mold, but it's usually pretty close. In any case, congrats on your catch, and hopefully you can put this formula to good use.

Fish on..........
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