Fish weight calculator
Estimate how much a fish weighs from its length — no scale needed. Add a girth measurement and the estimate gets a lot more accurate. Everything is calculated in your browser.
How the estimate works
There are two standard ways anglers estimate weight without a scale. The first uses length alone: weight rises with the cube of length, divided by a shape factor that differs by species (a slender pike weighs far less than a deep-bodied bass of the same length). The second, more accurate method also uses girth — the distance around the fish at its fattest point — which captures how heavy-set an individual fish actually is.
When you enter a girth, this calculator uses the girth-based formula (length × girth² ÷ 800) and shows it as the primary estimate, because two fish of the same length can differ in weight by a pound or more depending on condition.
How accurate is a length-to-weight estimate?
Treat it as a good ballpark, not a certified weight. Length-only estimates are typically within about 10–15% of true weight for a fish in average condition, and girth-based estimates are usually closer still. A fish full of eggs before the spawn, or one that just gorged on baitfish, will weigh more than any formula predicts. For anything official — a tournament or a potential record — you need a certified scale.
How do I measure girth on a fish?
Lay a soft tape measure around the deepest part of the body, just in front of the dorsal fin, and read the distance all the way around. If you do not have a flexible tape, loop a piece of line around the fish, mark it, then measure the marked length against a rule. Measure quickly and get the fish back in the water — see our catch and release guide.
Why do different species use different formulas?
Body shape. A muskie or pike is long and cylindrical, so it weighs much less per inch than a stocky largemouth bass or a common carp. The shape factor in the length-only formula accounts for that difference, which is why picking the right species matters when you do not have a girth measurement.
Estimates use widely published angler formulas and are provided for interest only. Actual weight varies with condition, season and sex. Always use a certified scale for records or competition, and follow your local regulations on handling and retaining fish.