How fish weight formulas work
Fisheries biologists model fish weight from length using species-specific formulas built from thousands of measured fish. The formulas this calculator uses, with their sources (lengths in inches, weights in pounds):
- Bass (length + girth): weight ≈ length² × girth ÷ 1,200 — the Wisconsin DNR bass formula. (Florida FWC publishes an alternative, length × girth² ÷ 800.) Girth captures whether your fish is a football or a snake, so use it when you have it.
- Trout (length + girth): weight ≈ length × girth² ÷ 800 — Wisconsin DNR.
- Length only: weight ≈ length³ ÷ C — Wisconsin DNR publishes walleye ÷2,700 and northern pike ÷3,500. Constants for other species (catfish, crappie, musky, striper) are general heuristics, not agency-published formulas — the calculator labels them as such in the result.
Where an agency girth formula exists the calculator uses it when you provide girth; otherwise it uses the length-only constant, and every result shows exactly which formula and source produced it.
Quick length-to-weight reference (length-only formulas)
| Species | 16" | 20" | 24" | 30" |
|---|---|---|---|---|
| Largemouth bass | ~2.6 lb | ~5.0 lb | ~8.6 lb | — |
| Walleye | ~1.5 lb | ~3.0 lb | ~5.1 lb | ~10 lb |
| Channel catfish | ~1.7 lb | ~3.3 lb | ~5.8 lb | ~11.3 lb |
| Northern pike | — | ~2.3 lb | ~4.0 lb | ~7.7 lb |
Values computed exactly from the length-only constants above; adding girth gives a better estimate for bass and trout. Individual fish vary.
Frequently asked questions
Why is my estimate different from the scale? Condition. A pre-spawn female full of eggs outweighs the formula; a skinny post-spawn fish comes in under. Girth-based estimates track condition much better than length-only.
Do these work for saltwater fish? Similar length-and-girth formulas exist for many saltwater species, but the constants differ; our species list is tuned for freshwater favorites.
Can I claim a record with a formula estimate? No — records require certified scales. This is for bragging rights and healthy releases.
Sources
- Wisconsin DNR — Estimating Fish Weight (length/girth formulas)
- Florida FWC — bass weight estimation
- Standard fisheries length–weight references