Fish Weight Calculator - Estimated Weight in Pounds or Kilograms

A fish weight calculator turns a fish's total length, girth, and species group into an estimated weight in pounds or kilograms, using the standard length-by-girth formula. Anglers, tournament directors, and catch-and-release programs rely on the result when a scale is not available, when a fish must be released quickly, or when a tournament bracket needs a recorded weight.

• • Catch-and-release weigh-ins: Estimate a fish's weight from a quick length and girth measurement when a scale would stress the fish or delay release.
• • Tournament weight brackets: Compute the estimated weight for a measured fish so the angler can record a number without weighing the fish in front of a judge.
• • Trophy or personal-best tracking: Log the estimated weight from each measured catch so a personal best can be compared to the species-group trophy threshold.

The result is an estimate within 5 to 10 percent for most freshwater sport fish in good condition, and the calculator is a planning tool, not a substitute for a scale.

The fish weight estimate starts with a length measurement, so the Length Converter handles the inch-to-centimeter conversion that lets an angler match the calculator's input to the markings on a measuring board or to a metric reference.

The fish weight calculator looks up the species-group K constant, then plugs the fish's length and girth into the standard length-by-girth formula. For pike, muskie, and gar, it switches to the length-cubed form.

• length (L): Total length of the fish in inches, measured from the tip of the snout to the most posterior part of the tail.
• girth (G): Maximum circumference around the fish's body in inches. Leave the field at zero to estimate girth as 0.58 x length.
• K constant: Species-group constant. Bass and striper use 800, trout use 900, panfish and walleye use 1500, salmon use 750, and pike use 2700 in the length-cubed form.
• size class: Light, Average, Heavy, or Trophy label based on species-group weight thresholds.

When centimeters are selected, the calculator converts length and girth to inches using 1 inch = 2.54 cm before running the formula, and returns the weight in kilograms using 1 lb = 0.45359237 kg. The girth, length, and weight secondary results are always shown on both sides of the unit toggle.

According to In-Fisherman — Fish Length to Weight, the standard length-by-girth formula for freshwater sport fish uses species-group constants K=800 for bass and striper, K=900 for trout, K=1500 for panfish, walleye, and catfish, K=750 for salmon and sturgeon, and a separate length-cubed form L^3 / 2700 for pike, muskie, and gar.

When a recipe, weigh-in slip, or trip log calls for ounces or grams instead of the primary unit, the Weight Converter turns the same number into the unit the angler writes down, so the catch log and the kitchen scale share the same weight.

Four ideas explain why the calculator uses two formula forms, why a default girth is built in, and why the size-class label is calibrated to each species group.

These four ideas work together: the formula form is set by the species group, the girth is set by the measurement (or the 0.58 x length default), and the size-class label sits on top of the resulting weight.

Fishing trips and day hikes share the same kind of route-and-load planning, so the Hiking Time Calculator sits next to the fish weight estimate on the same outdoor trip sheet when an angler is also planning the shore hike, portage, or drive time around the launch.

Use the form below to estimate a fish's weight in about thirty seconds, then re-run it any time you change the species, the length, or the girth measurement.

1. 1 Measure total length: Lay the fish on a flat measuring board and read total length from snout to the most posterior part of the tail.
2. 2 Pick a length unit: Choose inches for an answer in pounds, or centimeters for an answer in kilograms. The result panel shows both units.
3. 3 Measure girth (or leave it at zero): Wrap a soft tape around the widest part of the fish and enter the result. Leave the field at zero to estimate girth as 0.58 x length.
4. 4 Pick the species group: Match the fish to the closest group: bass, panfish, catfish, pike, trout, or salmon.
5. 5 Read the estimated weight: The first row shows the estimated weight in the selected unit, with a metric or imperial secondary beside it.

A 20 inch trout with no girth tape produces 2.99 lb, a 27 inch bass produces 8.28 lb, and a 30 inch pike produces 10 lb in the length-cubed form. Switching the length to centimeters converts all three results to kilograms without re-running the math.

Trolling speed is logged in knots on most fish-finder units and GPS plotters, so the Knots to Kph converter pairs a 2.5-knot trolling pass with the fish weight estimate on the same trip sheet.

A length, a girth, and a species group return a single number that fits a tournament, a personal best, or a catch-and-release log.

• • Auditable estimate without a scale: Returns a fish weight in pounds or kilograms from a length and a girth, with the species-group K constant shown in the formula row.
• • Fast catch-and-release workflow: Estimates the weight from a quick length measurement and the built-in 0.58 x length girth default.
• • Tournament-ready brackets: Maps the result to a Light, Average, Heavy, or Trophy label calibrated to the species group.
• • Metric and imperial at the same time: Switches between inches and centimeters with the length unit, and reports the weight on both sides of the unit toggle.
• • Length-cubed fallback for long species: Switches to the length^3 / 2700 form for pike, muskie, and gar so the result is closer to the real weight for long slender species.

The K constant, the girth used, and the formula are all visible in the result panel, so the same inputs always return the same number across anglers, tournaments, and studies.

If the catch is kept for the table, the Cooking Measurement Converter translates the same weight into the ounces and grams a recipe calls for, so the catch log and the kitchen prep list share the same weight.

Most of the variability in the result comes from the species group, the girth measurement, and the body condition of the fish.

• • The estimate is within 5 to 10 percent for fish in good condition but can fall outside that band for spawning, spent, or starved fish, so the calculator is a planning tool rather than a substitute for a scale.
• • The default girth of 0.58 x length is the empirical mean for freshwater sport fish and does not fit saltwater species, juvenile fish, or fish outside the validated 1 to 80 inch length range.
• • The K constants are group averages and do not adjust for regional body shape, so a wild river trout and a stocked lake trout of the same length can return slightly different results.

Picking the right species group, measuring girth with a soft tape, and using total length instead of fork or standard length bring the estimate into the documented 5 to 10 percent error band for almost every catch.

According to FishBase — LENGTH-WEIGHT table, length-weight relationships for thousands of species follow the W = a x L^b form with species-specific coefficients fitted from field measurements, the same form used here with simplified K constants per group.

According to In-Fisherman — Measuring Fish, total length measured from the snout to the tip of the longer lobe of the caudal fin is the standard input for length-by-girth and length-cubed formulas, while fork-length or standard-length measurements exclude part of the tail and understate the result.

The estimate comes back in pounds when the length unit is inches and in kilograms when the length unit is centimeters, so the Lbs to kg Converter verifies the 1 lb = 0.45359237 kg factor and catches any mix-up between the inch toggle and the kilogram readout.