What this calculator does
A fish can be weighed without scales using length, girth and a species constant. The formula multiplies length by girth squared and divides by a constant that reflects how the species is built: around 800 for bass and general species, 900 for trout and salmon, and 2,700 for long thin fish like pike and muskie.
Girth is squared, so it carries most of the result. If girth is not measured the calculator estimates it as 58% of length, which is a reasonable average but introduces real error for a fish in unusual condition. A fat pre-spawn fish and a lean one of the same length differ substantially in weight, and only a girth measurement captures that.
The formula
Weight in pounds is length in inches multiplied by girth in inches squared, divided by the species constant. Where girth is left at zero, it is estimated as 0.58 times the length. The species constant accounts for body shape: a higher constant means a proportionally slimmer fish, which is why pike uses a much larger figure than bass.
| Term | Meaning |
|---|---|
| Girth | Circumference at the thickest part of the body, which is squared in the formula and so dominates the result. |
| Species constant | A divisor reflecting body shape: 800 for general and bass, 900 for trout and salmon, 2,700 for pike and muskie. |
| Estimated girth | The 58% of length fallback used when girth is not measured. |
The inputs explained
| Field | What to enter |
|---|---|
| Length (nose to tail fork) (in) | Total length in inches, from the snout to the tip of the tail. |
| Girth (0 to estimate from length) (in) | Girth in inches at the thickest point. Leave it at zero to have it estimated from length, at some cost in accuracy. |
| Species group | The species group, which sets the constant. Body shape differs enough between these groups that using the wrong one gives a badly wrong answer. |
When to use it
Recording a catch and release
Weighing a fish on a scale means handling it longer and often lifting it by the jaw, both of which cause harm. Two tape measurements let it go back faster.
Estimating without scales
A tape measure travels more easily than a scale, and for most purposes the estimate is close enough to what a scale would show.
Comparing fish of the same length
Two fish of equal length can differ considerably in weight depending on condition, and girth is what separates them.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does length change the estimated weight?
A range of fish lengths with girth estimated at 58% of length.
| Length | Estimated weight | In kilograms | Girth used |
|---|---|---|---|
| 15 in | 1.42 lb | 0.64 kg | 8.7 in (estimated) |
| 20 in | 3.36 lb | 1.53 kg | 11.6 in (estimated) |
| 25 in | 6.57 lb | 2.98 kg | 14.5 in (estimated) |
| 30 in | 11.35 lb | 5.15 kg | 17.4 in (estimated) |
Questions
How accurate is the length and girth formula?
Usually within about 10% when girth is actually measured, which is close enough for most purposes. Accuracy falls considerably when girth is estimated from length, because it then assumes average condition and cannot distinguish a well-fed fish from a lean one.
Where do I measure girth?
Around the thickest part of the body, usually just ahead of the dorsal fin. Wrap the tape snugly without compressing. Since this figure is squared in the formula, an error here costs roughly twice what the same error in length would.
Why does pike use such a different constant?
Because the constant encodes body shape. Pike and muskie are long and slender relative to their girth, so the same length and girth combination represents a different volume of fish than it would for a deep-bodied bass. Using the bass constant on a pike overestimates it substantially.
Can I use this for a record claim?
No. Official record claims require a certified scale and usually witnesses. This is an estimate for personal records and for returning a fish to the water quickly, which is the better outcome for the fish in any case.
For stocking an aquarium rather than estimating a catch, see the fish tank stocking calculator. For horses, see the horse weight calculator.