Carrying a scale all of time can be a pain and using a scale can do damage to the fish. An alternate way to weigh a fish is by measuring it and doing some calculations. You can either do the calculations in the field or just take the measurements and do the math at home. It is surprising how accurate this method is.
The correct way to measure any fish for International Game and Fish Association (I.G.F. A.) rules is from the middle of the tail in the fork to the tip of the nose. Use the length and width to figure out the weight of the fish.
Here is the math needed to weigh a fish without the need of a scale. Calculate the size of the fish with the formula: length times girth squared, divided by eight hundred e.g. 33” x 21” x 21” /800 = 18.191 pounds. Anglers who don’t have a measuring device can use dollars for size comparison with photographs for later calculations. A dollar is almost exactly six inches long; take a photograph with the dollar bill in the same frame to get a near exact idea of how big the fish is. You can also make markings on your fishing rod or take a measurement of your arm. What works great though is to go to a sewing/fabric shop and buy a flexible measuring tape. The roll up to fit in your pocket, they are waterproof and they are cheap.