How to estimate a fish’s weight from its length

Most of us do not carry a scale, and plenty of regulations only care about length anyway. But when a good fish comes over the side, the first question from everybody in the boat is how much it weighs. The good news is that length gets you most of the way there. A fish of a given species has a predictable shape, so its weight follows its length pretty closely.

Why length predicts weight

A fish grows in three directions at once: longer, deeper and thicker. Double the length and you roughly double the other two as well, so the weight goes up about two times two times two, or eight. That is the whole idea behind every length to weight chart. Biologists write it as a power formula:

Weight = Length ^ b / c

With length in inches and weight in pounds, b sits close to 3 for every fish, usually a touch over because fish fill out as they get older. c carries the body shape. A deep, chunky largemouth has a small c. A long, skinny pike has a big one. Plug in a 20 inch largemouth and you get about 5.1 lb. The same 20 inches on a walleye is only about 2.9 lb, and a 30 inch pike comes out near 7.6 lb.

You do not have to do the math by hand. The fish weight calculator has the formula for 32 species, and every species page has a full chart.

Add a girth and the number gets better

Length alone assumes a fish in average shape. Real fish are not average. A female loaded with eggs before the spawn, a bass that just ate a gizzard shad, or a thin fish coming out of a hard winter can all sit well off the chart. The fix is a girth measurement, taken with a soft tape around the fattest part of the body, usually just ahead of the dorsal fin.

With a girth you can use the old standby:

Weight = Length x Girth x Girth / 800

Take that same 20 inch largemouth. With a 15 inch girth it works out to 5.6 lb. With a 13 inch girth it is 4.2 lb. Same length, almost a pound and a half apart. That gap is why tournament anglers who cull by eye still get surprised at the scale.

How to measure length

Most bad weight estimates start with a bad length. A few habits fix it:

• Use a flat bump board or a tape laid flat. Do not drape the tape over the fish and follow the curve of its back. That adds length the fish does not have.
• Close the mouth and push the nose to the bump. An open mouth on a big-jawed fish like a bass adds length that is not really there.
• Pinch the tail. Squeeze the tail lobes together and read the length at the tip. That is total length, which is what these formulas expect and how most freshwater regulations measure.
• Read it once the fish settles. A fish that is flopping on the board will give you a different number every second.

How to measure girth

A cloth tailor's tape is perfect. Wrap it snugly around the thickest part of the body without squeezing. If you only have a rigid tape, a piece of fishing line works: wrap it, pinch the spot where it meets, and measure the line against the tape. Do it quickly and keep the fish wet. More on that in catch, photo, release.

How close is close enough?

For a fish in typical shape, the species formula gets you close. A fish in unusually good or poor shape can be off by 10 to 20 percent, and that is the gap a girth measurement closes. What no formula will do is replace a certified scale. If you are chasing a record or weighing in at a tournament, only the scale counts.

One more tip: pounds and tenths are not pounds and ounces. A tenth of a pound is 1.6 ounces, so the 5.1 lb bass above is about 5 lb 2 oz. The calculator shows both so nobody at the ramp gets confused.

Or skip the tape

If you would rather keep the fish in the water and get on with it, fishline does all of this from a photo. It identifies the species, finds something in the frame of known size for scale, works out the length, and runs the same species formulas you see here to get the weight. If you did take a length or girth, you can type it in and fishline uses your numbers instead. Try it free.