How this instrument works
When you can't put a fish on a scale — a common situation during catch-and-release fishing — length and girth measurements can estimate weight reasonably well, because a fish's body volume (and therefore its weight) scales predictably with both how long and how thick around it is. This calculator uses two widely published angler's formulas: for bass-type fish, weight in pounds equals length times girth squared, divided by 800; for trout-type fish, the same length-times-girth-squared calculation divides by 900 instead, reflecting trout's typically slimmer body shape relative to bass at the same length.
For pike-type fish — species with a notably elongated, torpedo-like body shape where girth varies less predictably relative to length — this calculator instead uses a length-only formula: weight in pounds equals length cubed, divided by 2700. If you haven't measured girth directly, a commonly used rule of thumb estimates it at roughly 58% of the fish's length, though a direct girth measurement (wrapped snugly around the widest part of the body) will always be more accurate.
These formulas are widely published estimation tools across fishing and wildlife-agency educational resources — useful and reasonably accurate (often within about 10% for typical body shapes) but not a substitute for an actual scale reading. Individual fish vary in body condition (a well-fed fish of a given length weighs more than a lean one), so treat the result as a solid catch-and-release estimate rather than a certified weight, especially for a potential record-fish claim.
- Enter Length in inches — measured from the tip of the snout to the fork (or tip) of the tail.
- Enter Girth in inches — wrapped snugly around the fish's widest point; estimate as roughly 58% of length if you can't measure it directly.
- Select Species group — bass-type, trout-type, or pike-type (length-only).
- Read Estimated weight in pounds.
- For pike-type species, note that girth isn't used in the calculation — only length matters for this body-shape category.
Worked example — a 20-inch, 12-inch-girth bass
Enter 20 inches for length, 12 inches for girth, and select the bass-type formula. Weight comes out to 20 x 12² / 800 = 20 x 144 / 800 = 2880/800 = 3.60 lb — a healthy, respectably sized largemouth or smallmouth bass.
A trout of similar proportions — 24 inches long, 13 inches of girth — uses the /900 divisor instead: 24 x 13² / 900 = 24 x 169 / 900 = 4056/900 = 4.51 lb. A 30-inch pike, using the length-only formula, comes out to 30³ / 2700 = 27000/2700 = 10.00 lb exactly, with the girth input ignored entirely since pike-type species use only length.
Questions
How accurate are length-girth weight formulas?
For typical, healthy-condition fish of the species they're designed for, these formulas are commonly cited as accurate to within about 10% — close enough for practical catch-and-release estimates, tournament weigh-in approximations, or general curiosity. They're less reliable for unusually skinny or exceptionally fat individual fish, since the formula assumes a 'typical' body-condition ratio between length and girth that any specific fish may not perfectly match.
Why do bass and trout use different divisors (800 vs. 900)?
The divisor reflects each species group's typical body density and shape relative to its length and girth — bass tend to carry a bit more mass for a given length-and-girth combination than trout do, so a smaller divisor (800 vs. 900) produces a proportionally higher weight estimate for the same measurements. These divisors come from empirical fitting against real weighed fish of each type, published across fishing and wildlife-agency educational resources, rather than a pure geometric derivation.
Why don't pike-type species use girth in the calculation?
Pike, muskellunge, and similarly elongated, torpedo-shaped species have a body form where girth doesn't add much predictive accuracy beyond what length alone already captures — their body proportions are more consistent relative to length than rounder-bodied species like bass. A length-only cubic formula (length cubed divided by a fixed constant) is the standard approach for this body-shape category in angler weight-estimation guides.
What if I don't have a way to measure girth?
A commonly used rule of thumb estimates girth at roughly 58% of a fish's length when a direct measurement isn't practical — useful in a pinch, but a real girth measurement (a flexible tape or string wrapped snugly around the fish's widest point, typically just behind the gill covers or pectoral fins) will always give a more accurate weight estimate than the 58% approximation.
Can I use these formulas for species other than bass, trout, or pike?
The bass-type and trout-type formulas generalize reasonably well to other similarly shaped fish — walleye and panfish are often estimated with a bass-type-style formula, for example — while the pike-type length-only formula suits other elongated species like muskellunge or northern pike relatives. For very differently shaped fish (catfish, for instance, with their own distinct body proportions), a species-specific formula from a wildlife agency or fishing resource will generally be more accurate than forcing one of these three general categories.