Fish Weight Calculator — Fish Weight Estimation Suite

The standard field formula is weight = girth × girth × length ÷ K. Measure length (in inches, from lower jaw to fork of tail) and girth (around the fattest part of the body), square the girth, multiply by length, and divide by a species shape constant K — most commonly 800. For example, a 20-inch largemouth bass with a 15-inch girth gives 15 × 15 × 20 ÷ 800 = 5.625 lb. This tool applies the right K per species automatically.

Treat every result as an estimate, not a certified weight. When girth is measured carefully, results are typically within about ±10% of the true weight; length-only estimates (girth guessed at 0.58 × length) are wider, about ±20%. Actual weight varies with condition, season, spawning, and whether the fish was recently fed. For tournaments or records, always use a certified scale.

800 is the most common "shape factor" K in the formula weight = girth² × length ÷ K (inches to pounds). It fits average deep-bodied fish such as bass and salmon. Slimmer or flatter fish need larger divisors (trout ~775, redfish/snook/catfish ~900, bluegill ~1200), while long skinny fish like walleye and pike are estimated from length alone with weight = length³ ÷ 3500 and ÷ 2700 respectively.

Use a soft measuring tape (not a stiff ruler) around the thickest part of the body, with fins folded flat. Measure length from the tip of the lower jaw to the fork of the tail — not the tail tips. Record measurements immediately, keep the fish in or over water, and handle it as little as possible. Small girth errors change the result a lot, because girth is squared.

Use the Length-only mode: it estimates girth as 0.58 × length (the default used by fishswami.com) and applies the species formula, showing a ±20% confidence band instead of a single number. For walleye and northern pike, dedicated length-cube formulas (L³ ÷ 3500 and L³ ÷ 2700) work without any girth input at all.

Relative weight is a fisheries-science condition index: Wr = (actual weight ÷ standard weight) × 100. The standard weight Ws comes from a published length–weight regression for the species — this tool uses the largemouth bass equation log10(Ws) = −5.316 + 3.191 × log10(length in mm) (Wege & Anderson, 1978), valid for bass 150 mm and longer. A Wr near 100 means the fish matches the expected weight for its length.

Fisheries biologists use roughly 95–105 as the benchmark for a fish in good condition. A Wr consistently below 95 suggests lean or thin fish — often a sign of poor forage, overcrowding, or habitat issues in that water. A single fish means little; the index is designed for comparing populations or tracking one pond over time. This is an informational estimate, not professional fisheries advice.

Enter the weight you are hoping for and your species; the tool inverts the formula to estimate the length that fish would need to be. If you measured girth, it solves length = weight × K ÷ girth²; otherwise it assumes the average girth ratio (0.58 × length) and solves the cubic. Handy for answering "how long would a 10 lb bass be?" (about 28¾ inches).

Because body shape differs. A 20-inch bass and a 20-inch pike weigh very different amounts, so each needs its own divisor. Even for one species, sources disagree slightly — for example, this tool uses /775 for trout (fishiohub) while fishswami groups trout at /900. The Compare formulas mode shows 2–3 published formulas side by side with an agreement percentage, so you can see the spread honestly instead of trusting one number.

Length in inches, centimeters, or millimeters; girth in the same three units; weight output in pounds, kilograms, or ounces. Formulas are unit-sensitive, so the tool converts your inputs to the formula's native units (inches→pounds for most species, centimeters→kilograms for the New Zealand species), computes, then converts the answer to your chosen output unit.

Wet your hands before touching the fish, keep air exposure as short as possible, support its body horizontally, and consider barbless hooks for faster release. Measure over water or a soft surface so a dropped fish is not injured. A quick length-plus-girth measurement with this calculator avoids hanging the fish on a scale at all.

Fishing licenses, size limits, bag limits, and seasons vary by location — sometimes down to the individual lake or river — and they change over time. Always check your local regulations before fishing. The estimates from this tool are not a substitute for knowing and following the rules where you fish.

New Zealand sources (fishingspotx.co.nz) publish metric-native formulas: kingfish and snapper use girth² × length ÷ D in centimeters to kilograms, while trevally, kahawai, and john dory use linear girth formulas. One data-quality note: the published kingfish divisor (/1140) contradicts that same page's worked example (a 50 cm kingfish with 25 cm girth stated as ~2.5 kg — the formula as written gives 27.4 kg). This tool defaults to the example-consistent value and flags the discrepancy in the methodology notes.

Yes — the Catch log mode stores species, length, weight, and date in your browser's localStorage, shows running stats (trip count, biggest fish), and exports everything to CSV. Nothing is uploaded anywhere: your log stays on your device, works offline, and you can delete individual entries or clear the whole log at any time.

Yes — this fish weight calculator works for saltwater fish. The species database includes redfish and snook (/900), tarpon and salmon (/800), plus New Zealand saltwater species — kingfish, snapper, trevally, kahawai, and john dory — with metric-native formulas. Saltwater fish vary a lot in shape, so treat results as estimates and compare formulas when it matters.

No. This is a free estimation tool for anglers, not professional fisheries, biological, or legal advice. Estimates only — actual weight varies by condition and season. Fishing licenses, size limits, and seasons vary by location; check your local regulations before fishing. For catch-and-release: wet your hands, minimize air exposure, and consider barbless hooks.