I have alot of confusion when It comes to this. I have alot of fishing buddies and see alot of flatties caught each year. We all use the Berkly and Rapala 50lb. digital scales, While weighing fish in a decoy bag. This is what I come up with 37" Depending on girth weighs about 20 pounds 38" Depending on girth weighs about 23 pounds 39" Depending on girth weighs about 26 pounds 40"&41" weighs from 29 pounds to 33 pounds 42" weighs about 34 pounds depending on girth 43" to 43 3/4" weighs 35 to 39 lbs. which runs about 27 1/2"girth to 28 3/4" girth Havent seen enough of the bigger flatheads to be able to say anymore. It could be the different rivers and lakes making the inch to pound ratio different. Over the past 5 years I have records, Over 240 flatheads been caught and released by me and my family and some buddies. Over 40 were over 3 feet long. and I have the weights for the majority of them. And thats what I come up with. It might be the small river im fishing or peoples fishing scales or some might be lies, But theres been about 10 40'' caught in our river and none weighed over 35lbs. Theres no way in our river a 40" flathead is weighing 40lbs Unfortunately Tell me if your inch to pound is different.