Tool
Fish weight calculator
Catch-and-release a fish without a scale? Get a credible weight estimate from length (and girth, when it matters).
Fish weight estimator
Estimate weight from length
Enter a length to see an estimate.
Long-bodied; girth-based estimate is far more accurate on big fish.
How the estimate works
Length-weight relationships in fish follow predictable curves — weight scales roughly with the cube of length. Each species has its own coefficient because body shape differs: a deep-bodied tambaqui weighs much more per cm than a slim alligator gar.
For most Thai sport fish the length-only estimate gets within ±15%. For deep-bodied species (giant Siamese carp, GT, tambaqui, barramundi over 80 cm), adding girth dramatically improves accuracy — the formula switches to girth² × length / k, which captures actual mass much better.
How to measure
- Length: Snout to fork of tail. For sailfish and other billfish, body only — exclude the bill.
- Girth: Tape around the deepest part of the body, just behind the pectoral fins. Measure with the fish wet and not squeezed.
Use it for catch-and-release
Big trophy fish should not be hauled out of the water for a scale weigh-in. Measure length (and girth if the fish allows), get the estimate, photograph for evidence, and release. For IGFA-record fish you do need a verified scale weight — but for personal records and trip logs, length-based estimates are the responsible choice.
For more on photographing and handling trophy fish, see the catch-photography guide.