Fish Weight Calculator

How this calculator works

A fish is roughly a cylinder, so its weight scales with the square of its girth multiplied by its length: weight (lb) = length × girth² ÷ 800. That divisor of 800 is the standard field constant used by fisheries agencies and tournament anglers for decades.

Two things move the answer away from that constant. First, body shape — a long slender pike carries far less mass at the same length than a deep-bodied crappie, so each species gets its own divisor and its own length-to-girth ratio. Second, condition — a pre-spawn female or a fish that has been feeding hard all fall can be 20–30% heavier than a thin fish of identical length. That is why the calculator asks for both species and condition, and why girth beats length when you can only measure one thing carefully.

If you cannot measure girth (hooked deep, fish thrashing, no tape), leave it blank: the calculator assumes the average girth for that species from published length-to-girth ratios and labels the result as a ballpark.

Sources

• Equipment ratings and capacities follow the published specifications for each rod, reel and line.
• Unit conversions follow NIST (National Institute of Standards and Technology) SI conversion factors.
• Length and girth measurement conventions follow IGFA (International Game Fish Association) record-keeping practice. The divisor of 800 is the field constant used in the standard length x girth squared formula. The girth-to-length ratio for each species is back-solved from the North Dakota Game and Fish Department fish length-weight table; the muskellunge curve follows the Wisconsin DNR average growth curve (33 in = 10 lb, 41 in = 20 lb, 47 in = 30 lb). These are population averages, not laboratory constants — an individual fish can run 20% either way.

Full detail for every calculator is on the methodology page.

From measurement to record

A length and girth is enough to log a fish you release. The species size chart shows where that weight sits for the species, the fillet yield calculator converts it into meat, and the best fishing scale guide covers the hardware if you want certified numbers.

Mistakes to avoid

• Measuring length carefully and guessing the girth. Weight scales with the square of girth, so a sloppy girth measurement moves the answer far more than a sloppy length one. If you can only do one well, do the girth.
• Using the generic 800 divisor for every species. A long slender pike and a deep-bodied crappie carry very different mass at the same length, which is why the calculator asks for the species and not just the tape numbers.
• Treating a pre-spawn fish the same as a spent one. Condition alone can swing the estimate by 20 to 30 percent at identical length and girth, so use the condition option rather than leaving it on the default.
• Rounding the measurement down "to be safe". The formula is already an estimate. Rounding the tape introduces a second, hidden error on top of it, and the result stops matching either the fish or the chart.
• Presenting the result as a certified weight. This is a field estimate for a fish you intend to release. If a record or a tournament number is at stake, weigh it on a scale and follow the certified scale route instead.

Common questions

How accurate is the length-and-girth formula?

Typically within 5–10% of a certified scale when both measurements are taken carefully. Drop the girth and it widens to roughly 20%, because the assumed girth may be wrong by a full inch in either direction.

Where exactly do I measure girth?

At the widest point of the body — usually just behind the pectoral fins, in front of the dorsal. Snug, not compressed. Girth is squared in the formula, so a half-inch tape error swings the weight more than a half-inch length error does.

Why does the same length fish weigh different amounts?

Body condition, season and sex. Two 20-inch largemouth bass can differ by more than a pound depending on whether one is full of eggs in early spring or thin after spawning.

Can I use this for catch-and-release records?

Yes — that is exactly what these formulas are for. Length on a flat board, girth with a cloth tape, numbers in your phone, fish back in the water. For a certified record you still need a certified scale.

Does this work for saltwater fish?

Yes. Pick the closest body profile — striped bass and other fusiform fish sit near 0.54 girth-to-length. Measure fork length for tuna and other pelagics where the fork, not the tail tip, is the standard.

Where do I measure girth on a fish?

Measure the largest circumference of the body, usually just behind the pectoral fins and in front of the dorsal fin. Wrap a flexible tape snug against the body without compressing it. Girth is squared in the formula, so a small tape error changes the weight more than the same error in length.

Weighing and measuring kit

Length and girth give you the number. A scale gives you the exact number — without either, you are guessing.

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