This is merely a approximation using a formula devised from steelhead that were, weighed and measured. I carry a compact measuring tape in my vest. Common errors that occur when weighing fish Scale not calibrated correctly. Go to your tackle shop and weigh some known lead weights. Bouncing fish on scale moving slider bar to far for accurate reading.
Fish can also be live weighed if they are in a net.
Do not put a scale into fishes gill plate.