Estimating Weight

So you catch a trophy size trout but you forgot your scale at home or have you ever wondered how much your “catch-n-release”  trophy trout weighed that you caught a few years ago.  Try this Formula to estimate its weight .

First thing you will need is a cloth or flexible measuring tape . Definitely avoid the metal “Stanley or construction ” style of measuring tapes. A measuring board is helpful but not required. We will need to have some sort of flat surface to accurately measure the length of the fish.

Find the length of the fish. Here’s where the board or a flat surface can make a difference in the overall length with the larger size trout . Lay the measuring tape under the fish and measure from the tip of the nose(Do not use the lower jaw on male trout with a kype )to the longest tip of the tail when pinched . With Mackinaw I measure to the V in the tail.  The reason why we measure under the fish on a flat surface is to avoid the natural curve of a trout which can add up to a inch in length. This will be your length number for the calculation.

To take the girth measurement. Find the widest part of the trout this will usually be in front of or behind the dorsal fin .Measure the fish’s girth by wrapping the measuring tape around the trout. This will be your girth number for the calculation. Of all the measurements the girth is the most important. If you’re Girth number is off by 1/2″on a 33″ trout your weight can be off by almost a pound. So take your time on this measurement .

Next up: some simple math. Multiply the square of the girth in inches by the length in inches and divide by 800

So you catch huge brown trout that is 30″ long with a girth of 18″. Well first of all congratulations you lucky guy.  18 times 18 is 324 Multiply that by 30 gives us 9,720. Now divide 9,720 by 800 and Just like that a 12.15 pound piggy brown trout that you just released unharmed without putting it on the scale . I would feel comfortable calling this a 12 pounder

Although no formula can ever replace an accurate scale,nor should it when it comes to lake,state or world records where weights are involved .This is an estimate only. Trout can vary greatly, just as people can . Many factors come to play such as age,sex, or muscle tone. However, the following calculator will give you  a good estimate that’s within about 10% of the actual weight

Brad Stout