Fish Weight Calculator

What the fish weight calculator does

When you catch a fish and have no scale to hand, you can still get a good idea of its weight from a couple of measurements. This tool does that. You give it the fish's length and girth, choose the species, and it estimates the weight, with a choice of units.

It is especially handy for catch and release, where a quick tape measurement lets you record the fish and get it back in the water fast, without the stress and handling of hanging it from a scale.

How to use it

1. Measure and enter the length, from the tip of the mouth to the end of the tail.
2. Measure and enter the girth, the distance around the fish at its thickest point.
3. Choose the species group closest to your fish, and your preferred weight unit.

Press Calculate for the estimated weight, or Reset to clear the fields.

How the weight is estimated

The estimate comes from a long-standing angler's formula that multiplies the length by the square of the girth and divides by a number that depends on the species:

Weight = (length × girth²) ÷ species divisor

The reason girth is squared is that weight depends on the fish's volume, not just its length. A fish is roughly a long tube, and the girth captures how thick that tube is, so squaring it stands in for the cross-sectional area. Length alone would tell you almost nothing about whether you had a skinny snake of a fish or a deep-bodied slab, which is why this formula uses both.

Why girth matters so much

Because girth is squared in the formula, it has an outsized effect on the answer, much more than length does. A fish that is twenty percent wider around than another of the same length does not weigh twenty percent more, it weighs roughly forty-four percent more, because that twenty percent gets squared. The practical lesson is that an accurate girth measurement matters enormously. Wrap a soft, flexible tape around the fattest part of the fish, usually just behind the pectoral fins, and pull it snug but not so tight that you squeeze the body. A sloppy girth reading, squeezed or guessed, will throw the weight off far more than a small error in length ever would.

Why species matters

The divisor in the formula is really a shape factor, and it differs by species because fish are built differently. A long, lean, streamlined fish like a trout or a pike carries less mass for a given length and girth, so it uses a larger divisor, which produces a lighter estimate. A stockier, deeper-bodied fish packs in more, so it uses a smaller divisor and comes out heavier. Choosing the species group that matches your fish is what tunes the formula to the right body type, which is why the tool groups species with similar builds together rather than using one number for everything.

An example with real numbers

Say you land a trout that is 20 inches long with a girth of 12 inches, using the trout group, which divides by 900.

• Weight = (20 × 12²) ÷ 900
• = (20 × 144) ÷ 900 = 2,880 ÷ 900 = about 3.2 pounds

So your trout weighs roughly 3.2 pounds. Notice how the girth, squared to 144, drives the result, which is why measuring it carefully is worth the effort.

It is an estimate

This is an estimate, and a good one, typically landing within about ten percent of the true weight when you measure carefully and pick the right species. But a real fish is not a perfect tube, and its weight shifts with its condition. A fish that has been feeding heavily, or is heavy with eggs in the spawning season, weighs more than the formula expects, while a lean, post-spawn fish weighs less. For an official record, a tournament weigh-in, or a fish that looks unusually fat or thin, there is no substitute for a proper scale. For everyday catch and release, though, this is a quick and fish-friendly way to log a catch without harming it.

Questions people ask

How do you estimate a fish's weight from measurements?

Multiply the length by the square of the girth, then divide by a species-specific number. Girth is squared because it captures the fish's thickness, which drives its volume and weight.

Why is measuring girth so important?

Because girth is squared in the formula, so it affects the result far more than length. A fish twenty percent wider weighs about forty-four percent more, so a careful girth reading is critical.

Why does the calculator ask for species?

Because the divisor is a shape factor, and fish are built differently. Lean fish like trout and pike weigh less for their size, while stocky, deep-bodied fish weigh more, so the species sets the right factor.

How accurate is the estimate?

Usually within about ten percent with careful measurement, but real weight varies with the fish's condition and the season. For records or weigh-ins, use a scale.

References

1. Wisconsin Department of Natural Resources, Estimating Fish Weight (length and girth weight formulas). https://dnr.wisconsin.gov/topic/Fishing/questions/estfishweight