I think I've come up with a pretty decent formula to determine weight using measurements. From what I can determine the tolerance is +/- 3lbs. With accurate measurements it should be very close. A fish we caught the other night weighed 23lbs and the formula calculated 22.9 If anyone is interested I put together an Excel chart. Conversion Chart

good info, my fish weighed 83# and was 54 inches i never measured girth, do u think u can estimate it as many have asked his girth?

Awesome Fish! I read about it in the Catfish In-Sider. Using the formula, your fish had a girth of 36".

It may work on some. But you can take 50 fish that are the same length. And most will not be close to the formula. Or they do not here on the Tn river. for 30+ years they will be different lengths. But look the same. Pete

Did you look at the chart? You cannot determine weight of a flathead solely on length (anywhere you catch them). This chart calculates the weight based on a known Length and Girth. I am pretty confident in the numbers and quite frankly would like to see the tolerance of +/- 3 lbs challenged. With accurate measurements the tolerance is probably closer to 2 lbs or better.

Dtro, I don't doubt your conversion chart, I know this has been debated before....but for me the simplest thing I can do is invest in a good scale and a camera and leave the conversion charts to those who log their catches in a more scientific manner than I do. I amire your effort for sampling enough fish to realize that the chart is accurate within a couple of pounds, It is probably a good thing to have beings so many people classify their fish in different ways.....some people state the size of their catch in inches some state the size of their catch in pounds and so on...now if we all would remember to measure the girth we would always have a good rule of thumb to go by.

I will try to record any fish I catch, And check them with your chart. And I am sure there's a lot of hard work in the chart. Thanks for the work. If I can find a few I will save the LxG for you. Pete

DTro: I'd really like to see this chart and print a copy of it for my records but I clicked on your link and never could get it to show up! Don't know what I did wrong. Can you help or direct me to another way I can view it?

I have tested the traditional LxGxG/800 for blue cats and it holds pretty well (always within 10%), although it tends to underestimate weights on big fish. I have also tested it on a few flatheads and it has pretty consistently overestimated their wieghts by about 15% (I'm guessing based on the shape of flatheads with a disproportionally bigger stomach). I like the idea of your newly adjusted formula a lot, and it seems to be fairly accurate (I tested it on a couple of fish of mine with measurements and weights that I could recall), but I have a question: It looks like you built it around LengthXLengthXGirth/1275. It would seem to make more sense to use LengthXGirthXGirth, because the fish are more circular in cross section than they are in length, and the girth squared is proportional to the cross sectional area (since the area of a circle is based on the radius squared). Then you'd just be coming up with a new adjustment factor (instead of the 800) that fit flatheads better.

That pretty cool. The flathead in my pic was 54" long and had a 43" girth. He weighted 67lbs. I'll see how it adds up to the flats I catch. I got a 31.5 Thurday afternoon but I did not measure him.

I think the conversion chart is a great idea, and in our part (Minnesota) of the country, seams to be pretty accurate. Part of the benefit of using a conversion chart is to lessen the stress on a flathead once it hits the boat. Get a couple quick measurements and photos and let them go! I've seen too many big fish flop off of a scale onto the boat floor. Ouch! In the general fishing world, specifically northern pike & muskie fishing, vertical holds on big fish are very damaging to their internal organs. I don't think there has ever (or maybe ever will) be a study on flathead cats with respect to hooking mortality and handling mortality but I believe a vertical hold has to affect them somewhat, and certainly hanging a 50, 60lb fish on a scale hook can't be any better. When you do hang a 40lb fish on a digital scale, doesn't the weight typically fluctuate 3-4lbs. I know it does. So your final reading might be 42lb if you read it high or 38 if you read it low. If I can get a similar weight estimate from a conversion formula (even if there is a 2-3lb margin of error) and lessen the stress on a flathead in the boat, I'm all for it! What we are finding in Minnesota here is that most flathead anglers are in it for the sport, with a 90% plus release rate on flatheads caught. Our state record is 75lbs or so but we catch lots of flatheads in the 40-50lb class. I think this is a true testimate to C&R of these great fish. MN fishing regulations are also responsible for the trophy nature of our fishery, we can only angle with 1 line, and the daily flathead limit is 2 fish, with only 1 over 24".