How it works
Weight (lb) = (length × girth²) ÷ 800 for bass-shaped fish, or ÷ 900 for slimmer fish such as trout, walleye and pike. Measure length from nose to fork of tail and girth around the fattest part, both in inches.
Frequently asked questions
How accurate is this?
It is an estimate. With an accurate girth it is usually within 5–10% of the true weight, but body shape, season and species all affect it.
Should I keep the fish out of water to measure it?
No — measure quickly, keep the fish wet, and release it promptly if you are practising catch and release.
What if I only have length?
Length alone cannot give a reliable weight, because two fish of the same length can have very different girths. Measure girth when you can.