Figuring out the weight of a fish without using fish scales is easy, you just need to do a little math and have a tape measure. The best tape measure I have found for this is a nylon sewing tape (I keep one in my tackle box). You can get one of these at Walmart or anywhere that sells sewing goods.
Here is the formula for calculating fish weight:
(Girth X Girth) X Length ÷ 800 = weight in pounds
To get your measurements follow these steps:
1. Measure from the tip of the nose to the fork of the tail to get your Length measurement.
2. Measure the circumference of the fish in front of the Pectoral Fins to get your Girth measurement.
3. Square your Girth measurement.
4. Take the number you came up with in step 3 and multiply that by the Length.
5. After multiplying the Girth times the Length divide the sum total by 800. This will give you your est. weight.
6. Convert the numbers after the decimal into ounces by multiplying by 16.
If the Girth of the fish is 16 inches and the Length of the fish is 31 inches the formula would look like this.
16(Girth) X 16(Girth) = 256 X 31 (Length) = 7,936
7,936 ÷ 800 = 9.92 lbs
Then convert the .92 into ounces by multiplying by 16, that would look like this.
.92 X 16 = 14.72 round this up to 15 ounces. So this fish would weigh 9 lbs 15 oz
This method is not as accurate as using a scale but it will give you a very good idea of what your fish weighs.