Jedt, it shoudl work fine. The same forumal is used for water body records with TPW to verify weights for fish. It gives you a ball park estimate and surpisngly is pretty close (within a few ounces, I've found) I'll see if I can't find the link on their site and post it

 

you can use your bathroom scale if you have one. weigh yourself then yourself with the fish it should be close to actual weight.

 

Thanks hillcountry I can find the TPW. Hard to remeber their exact addres because it has a lot of dot this and dot that.

 

Found it but it is a little complicated to remember for me. Here goes:
How can I estimate the weight of a fish that was caught and released?
You will need to have the length and girth of the fish to calculate an estimate. This formula works for all fish, although the example is for largemouth bass. To get weight in pounds, square the girth in inches (girth multiplied by itself) then multiply this times the length in inches. Divide this product by 800.
For example, we can use one of the recent ShareLunker bass. The girth was 20.25 inches and the length was 26 inches. First, take 20.25 x 20.25 to get 410.06. Multiply this times 26 to get 10,661.6. Divide this by 800 to get 13.33 pounds. The actual weight of this particular bass was 13.05 pounds, so that is a reasonable estimation.