Fish Weight Calculator
Estimate a fish's weight from field measurements — so the catch can go back in the water instead of hanging on a scale.
Measure the fish while it is lying straight
Use total length from the tip of the snout to the end of the tail, and girth around the widest part of the body. Have the board and tape ready first so the fish can be returned promptly.
How the fish weight calculator works
A fish weight calculator estimates a fish's weight from its body measurements without using a scale. Enter the fish's length and girth to estimate its weight using a species-appropriate formula. Estimated fish weight varies with species, body shape, condition, and measurement accuracy.
The dimensions act as a proxy for body volume. Length-and-girth methods account for how thick the fish actually is, while length-only formulas apply a species-style divisor to the cube of length and assume typical proportions for that species.
Species or body shape decides which formula and divisor apply.
Length always; girth wherever the formula uses it.
Weight in pounds and kilograms, with the arithmetic shown.
Fish weight formulas
Fat-bodied fish
W = L × G² ÷ 800
Used for bass, salmon, and similar deep-bodied fish where the body carries real thickness.
Slender fish
W = L × G² ÷ 900
Used for trout and similarly streamlined fish, which carry less mass for the same girth reading.
Panfish
W = L × G² ÷ 1,200
Used for sunfish and bluegill, which are laterally deep but small in overall volume.
Length only
W = L³ ÷ 2,700
Used for pike, muskie, gar, and walleye-style estimates when girth was not taken.
Unit rule: these field formulas take length and girth in inches and return pounds. Metric entries are converted to inches before the calculation and the result is converted back, so the divisors stay as published.
Species-formula reference: The Wisconsin Department of Natural Resources publishes species-specific fish weight formulas and describes the results as approximate. Its differing methods also show why a formula should be matched to the species and the source it came from.
Preset selection guide
Which fish formula should you choose?
Choose a named species preset when it matches the catch. If the species is not listed, pick the general body shape that most closely matches the fish rather than choosing by length alone. Divisors are model assumptions, so two presets can produce different estimates from identical measurements.
| Preset | Best match | Measurements used | Selection note |
|---|---|---|---|
| Bass / salmon / fat-bodied | Deep, thick-bodied fish | Length and girth | A measured girth matters most when body condition varies. |
| Trout / slender-bodied | Streamlined fish with a narrow profile | Length and girth | Use the named trout option where it fits; otherwise the general slender preset. |
| Sunfish / bluegill | Small, laterally deep panfish | Length and girth | Do not substitute a bass preset just because both fish look deep-bodied. |
| Pike / muskie / gar / walleye | The listed length-only species | Length only | Girth is ignored by these presets, so proportions are assumed. |
If you are unsure: run the two closest plausible presets and treat the difference as model uncertainty. For anything that counts, use a species-specific method published by the relevant fisheries agency or record organisation.
Worked examples
Fish weight examples by species
The first two rows use girth estimated as 58% of length. A measured girth will move the result for an unusually lean or unusually heavy fish.
| Fish | Length | Girth | Formula | Estimated weight |
|---|---|---|---|---|
| Largemouth bass | 27 in | 15.66 in estimated | L × G² ÷ 800 | 8.28 lb |
| Trout | 20 in | 11.60 in estimated | L × G² ÷ 900 | 2.99 lb |
| Northern pike | 30 in | Not required | L³ ÷ 2,700 | 10.00 lb |
| Salmon | 45 in | 25 in measured | L × G² ÷ 800 | 35.16 lb |
Better measurements
How to measure fish length and girth
A consistent method improves the estimate and makes repeat catches comparable. It also keeps handling time short, which matters more than the extra decimal place.
1. Lay the fish straight
Use a wet measuring board or smooth surface, and do not follow the curve of the body with the ruler.
2. Measure total length
From the most forward point of the closed mouth to the farthest point of the tail, in a straight line.
3. Wrap the widest girth
Run a flexible tape around the thickest body section without compressing the fish.
Length definition matters: total, fork, and standard length end at different tail landmarks. Use whichever the formula, regulation, record programme, or comparison table expects — mixing them silently biases every estimate you record.
Measurement reference: Texas Parks & Wildlife publishes an illustrated guide to measuring fish length, covering jaw position, tail handling, and the straight-line endpoints used for total length.
Accuracy and uncertainty
How measurement errors move the estimate
Small differences matter more than they look, because girth is squared in length-and-girth formulas and length is cubed in length-only formulas. Record the raw measurements before rounding anything.
5% more girth
Produces about 10.25% more estimated weight when length is unchanged, because the term is squared.
1% more length
Produces about 3.03% more estimated weight in a length-only model, because the term is cubed.
Girth at 58% of length
Is a fallback proportion, not a measurement of this individual fish's condition. Measure it whenever you can.
Interesting fact
No single formula can cover every fish
A whale shark is a shark, but it is also the world's largest fish — NOAA Fisheries reports that the species can reach about 46 feet in length and nearly 24,000 pounds. Put that beside a bluegill of a few ounces and you have a size range spanning roughly five orders of magnitude. That is precisely why one length-to-weight relationship cannot represent every species: the divisors here encode an assumed body shape, and that assumption breaks down the further you get from the fish it was fitted to. The presets in this calculator are intended for the listed recreational species and comparable body types, not for whale sharks or other protected megafauna.
Source: NOAA Fisheries, whale shark movements in the Gulf of Mexico.
Reproducible records
What to record with a weight estimate
An estimate is far more useful when someone can reconstruct it later. Save the measurements and the method, not just the final number, if you are comparing catches, tracking growth, or keeping a fishing log.
Minimum log fields
- Species, or the body-type preset you selected
- Length, length definition, girth, and units
- Formula, divisor, and whether girth was measured or estimated
- Date, waterbody, and a measurement photo if appropriate
When an estimate is not enough
Use an approved, calibrated scale and the governing measurement procedure whenever weight affects a tournament, official record, legal declaration, commercial transaction, scientific dataset, or regulated harvest report. Keep the scale reading and the formula estimate as separate fields — never replace a measured weight with a calculated one.
Example entry: largemouth bass · total length 27 in · girth 15.5 in measured · L × G² ÷ 800 · estimated weight 8.11 lb · date and location saved with photo.
Record-documentation reference: The International Game Fish Association's International Angling Rules set out the measurement, weighing, and photographic requirements for record claims, and make clear that estimated weights are not accepted for weight records.
Interpret the result carefully
An estimate is not a weight. Two fish of identical length and girth can differ by a pound or more depending on season, sex, and condition.
The preset carries assumptions. Divisors are fitted to typical fish of a body type; an exceptional individual will not match the model.
Check units before you calculate. Inches entered as centimeters is the single most common way to get a wildly wrong number.
The fish comes first. Keep handling brief, minimise air exposure, and follow local catch-and-release guidance over any measuring ambition.
Frequently Asked Questions
How can an angler estimate a catch-and-release fish's weight without a scale?
Measure the fish's total length and its girth at the widest point, where girth is the body's maximum circumference. Record both measurements before returning the catch to the water, then apply the formula for the closest species or body type. If girth is unavailable, this calculator can estimate it as 58% of length for formulas that require both measurements. The resulting weight estimate is an approximation, not a certified weight.
How much does a 20-inch trout weigh, and is that a trophy size?
Using the slender-fish formula and an estimated girth of 11.6 inches, a 20-inch trout weighs approximately 3 pounds. An unusually lean or deep-bodied trout may differ from this estimate, and what qualifies as a trophy varies between fishing programmes and regions.
How much does a 27-inch largemouth bass weigh?
With girth estimated as 58% of length, the calculation gives approximately 8.28 pounds for a 27-inch largemouth bass. Entering a measured girth produces a more fish-specific result. Do not assume the same bass relationship fits salmon, carp, or catfish; choose the closest matching species or body-type preset.
How heavy is a 30-inch pike, and do walleye or muskie use the same method?
The pike length-only formula estimates a 30-inch fish at 10 pounds, because 30 cubed divided by 2,700 equals 10. Actual weight depends on the fish's condition, population, and season. This calculator also applies a length-only method to walleye and muskie, although published species-specific divisors may differ.
Is a fish weight calculator accurate enough for a fishing tournament or record?
It provides a field estimate rather than a precise scale reading. Accuracy depends on correct measurements, a suitable species formula, and whether the individual fish has typical proportions. A measured girth usually captures body condition better than length alone. For a tournament result or official record, follow the event rules and use an approved, calibrated scale.
How does metric conversion work in the fish weight calculator?
Select centimeters and enter the measurements; the calculator converts them to inches internally before displaying the estimated weight in both pounds and kilograms. The field formulas themselves are defined in inches and pounds, which is why the conversion happens behind the scenes rather than in the formula.
Practical accuracy checklist
- Measure the fish straight, never over a curve
- Use the same tail endpoint every time
- Take girth at the widest point, without squeezing
- Confirm inches versus centimeters before calculating
- Record raw numbers before rounding them
- Note whether girth was measured or estimated
Other useful calculators
Stride Length Calculator
Find your step length and turn step counts into distance.
Marathon Pace Calculator
Plan splits and finish times for your target marathon pace.
Bike Gear Calculator
Compare gear ratios, development, and cadence versus speed.
Batting Average Calculator
Work out batting average from hits and official at-bats.
Meeting Cost Calculator
Price the labor and annual cost of a recurring meeting.
Winch Size Calculator
Size a winch from vehicle weight and pulling conditions.
Disclaimer
This fish weight calculator provides approximate, formula-based estimates for informational and recreational use. It does not replace a properly calibrated scale and should not be used to certify a record, tournament result, harvest declaration, or legal weight.
Formulas and measurement definitions vary by species, population, jurisdiction, and organisation. Check local fishing rules, size limits, seasons, licensing requirements, protected-species restrictions, and approved measurement procedures before fishing or retaining a catch.
Handle live fish carefully, minimise air exposure, and follow local catch-and-release guidance. Individual fish may differ substantially from a generalised estimate because of body condition, sex, season, habitat, and measurement technique.