Fish Weight Estimator

Fish Weight Estimator
Photo: Barry & Cathy Beck 

Fishermen may keep their spots a secret, but I haven't met a fisherman yet that was shy about talking about a big fish. To tell a fish story properlyand accuratelyit helps to know the size of your trophy. Some fishermen do this just to brag, others just for their personal records. But deep down inside, we all want to quantify the size of "The Big One."


One option to determine the size is to merely measure the length. A problem with only knowing the length is that you are taking a one dimensional measurement from a three-dimensional object. Only the weight of the fish captures its true three-dimensional size. 

Trout for instance come in all different shapes. Some are long and skinny, sometimes even called "snaky,"especially if the body is smaller (in diameter) than the head. Other trout are football-shaped with rotund bellies that show a season of feeding well.

Measuring only the length adequately describes the size of many species if they are on the small side. For instance, all 12-inch trout are about the same size. What's more it's "just" a 12-inch trout, so it's not that important to be any more accurate than to describe the length.

However, the larger the fish becomesthe closer it is to being a "trophy" the more eager we are to accurately describe the true size of it. That's why in New Zealand, trout are described only in terms of pounds, and the guides regularly weigh the fish in the net. After a time, they become experts at accurately "eyeballing" a fish, and estimating its weight before it ever hits the scale. 


William W. Wood's equation was developed by assuming a fish's body shape is similar to two cones placed together. The equation effectively measures the volume and weight of those two cones. Photo | Jorge Valverde

It's also true with large bonefisha bonefish that has a length of 28 inches to the fork can range from 8 to 13 pounds. That's a large range that can separate a good catch from a stellar one, and that's exactly why guides talk mostly of the weight of the fish in pounds.

The weight of a fish is our primary method of judging the true size of a fish, but weighing a fish is not without its pitfalls. The fish can flap around and damage itself, or with some species you could get poked with a spine or bitten if the fish is not under control at all times.

There are "lipper" devices to subdue the fish by locking onto the jaw, and some devices like the BogaGrip include a scale to weigh the fish. However, when these devices are used to grab the jaw and lift a large fish, it can severely damage the fish's spine, mouth, and its associated feeding processes. In other words, it can potentially harm or even kill the fish.


A safer way to use a lipper device is to place the fish in a net, then use the device to weigh the net and the fish. Then merely subtract the weight of the net to get the fish's weight.

Just be careful what net you use. Some nylon nets cut into the fish's fins and remove scales. A rule to follow for nets is the softer the net, the better. Research has shown that soft rubber nets don't damage the fish as much as nylon nets. Also, be sure the net is wet to prevent removing slime from the fish.

Of course, a net only works on fish up to a certain size. With big trout and bass a net works just fine, but for truly large striped bass, Chinook salmon, or redfish it becomes impractical. 

The Ault-Luo equation is more accurate for tarpon because it was created from tarpon data, and it assumes the fish's shape is an ellipsoid. Photo | Ross Purnell

With larger fish like tuna, tarpon, and billfish, nets are impossible. Just lifting a fish like this out of the water is dangerous to both you and the fish.

If you intend to successfully release the fishand you really want a weightthen measuring the length and girth is the safest and most practical method.

William W. Wood was a pioneer in tarpon fishing, recording his first catch in 1885. He developed an equation for predicting a fish's weight from the length and girth measurements. Wood's formula is also simple enough that it can be calculated on a piece of scrap paper. The equation is: weight in pounds = girth2 x length / 800, with both girth and length measured in inches.

Wood's equation was first developed with tarpon in mind, but it was quickly adopted by catch-and-release fishermen everywhere, because it's an easy way to provide a ballpark estimate of your fish's weight. For example, a steelhead that is 40 inches long with a 24-inch girth weighs approximately 28.8 pounds, according to the Wood equation.

Wood's equation was developed by assuming a fish's body shape is similar to two cones placed together. The closer the fish's body resembles two cones, the more accurate the equation is at predicting the weight.

A problem with Wood's equation is that the bodies of some fish deviate from this shape. If you apply Wood's equation to the popular sportfish tarpon, the result is a weight that can be underestimated by as much as 15 percent.

In other words, a tarpon that actually weighs 180 pounds could be estimated by Wood's equation to be only 153 pounds. That's a big difference!

To fix this problem, scientists at the University of Miami developed a new equation specifically for estimating tarpon weight called the Ault-Luo equation.

Instead of using Wood's assumption of a body shape similar to two cones, Drs. Jerald Ault and Jiangang Luo assumed a tarpon's body is similar to an ellipsoid. The downside of the Ault-Luo equation is you probably won't be calculating it on a piece of scrap paperyou'll need a calculator for this one! [The complete equation is printed on page 65. The Editor.]

For most fish species you don't need the complex Ault-Luo equation. Use the old, reliable Wood's equation, and you'll be darn close to knowing how much that trophy trout or bonefish weighs, and you can successfully release it to keep those trophy genes in the ecosystem.

Michael Larkin received his doctorate degree in fisheries from the University of Miami's Rosenstiel School of Marine and Atmospheric Science. He has worked on a wide range of marine biology projects, with most of his experience coming from bonefish and tarpon research. He currently works as a fisheries biologist in St. Petersburg, Florida. 

Release 'Em Alive

When angling innovator Lee Wulff first popularized it, catch-and-release fishing was a revolutionary way to conserve fish stocks amid a growing American population and dwindling habitat. Today, in many places, it is still a sound way to share the resource with other anglers.

Keep in mind, however, that in some places, a wild fish population (trout, striped bass, panfish) can withstand some harvest. There's nothing wrong with eating a few fish caught in places where a wild fish population can sustain some harvest, or in some put-and-take stocked trout fisheries when the trout are not likely to survive the season due to environmental conditions.

If you are going to release a fish, it's important to do it properly. Releasing a fish that dies due to improper handling is a waste (in some waters over 20 percent mortality), and if you catch-and-release many fish this way during the course of a day, you can kill more fish than a poacher who keeps more than his legal limit.

The best way to release a fish is to do so without touching it. Release the fish by sliding your hand down the leader, grab the fly with hemostats or a Ketchum Release tool (waterworks-lamson.com, $19.99), and twist the fly out. The fish swims away untouched. A barbless hook makes this release technique both easy and effective.

It's easy to release smaller fish untouched. With large specimens, however, you may want to handle the fish to get a quick photo, or you may have to net the fish or otherwise handle the fish to retrieve the fly without breaking the line.

Here are some things you should keep in mind when you release a fish by handling it:

Most fish are covered in a protective slime. If you must handle the fish, keep this protective coating intact, don't touch the fish with dry hands, don't throw it in the grass or snow on the bank, and don't net it unless you have to.

It's simple to get a length and girth measurement from a trout, or any other species, without lifting it from the water. The length measurement is from the tip of the nose to the fork of the tail. The girth is at the widest spot in front of the dorsal fin. Photo | Barry & Cathy Beck

All fish have delicate gills, and their internal organs are not protected by a sturdy rib cage like ours. Never squeeze a fish while attempting to hold it or you will injure the fish. Also, the fish will probably pop out of your hands like a bar of soap. The harder you squeeze, the more likely this is to occur. Instead, gently cradle the trout or other fish with your hands, allowing it to rest/lean on your fingers and hands to distribute its weight. Always hold the fish in the water as much as possible, so the water helps support the fish naturally and all the weight is not on your hands.

If you catch a trophy or even just a memorable fish, you may want a photograph. As above, cradle the fish with your hands and lift it momentarily from the water.

You can also compose excellent photos with the fish in the water, especially if the water is clear. Waterproof digital cameras allow you take photos of a trout in its natural environment without ever removing it from the water.

If you do lift the fish from the water, make sure your fishing companion is ready with the camera turned on and lens cap off before you lift the fish from the water. (Photo tip: remove your sunglasses before you have your picture taken so people can see who is holding the fish.)

If you are fishing alone, don't put the trout up on the bank to photograph it. This makes for a poor picture, and when the trout flops on the snow, rocks, sticks, or sand, it can injure itself. If you are alone, cradle the fish half in/half out of the water with one hand, and shoot a few close-up pictures with your digital camera with the other hand.

With big trout and light tippet, a net is probably better for the fish (as compared to leaving the fly stuck in the fish's mouth) but use a net with a soft rubber mesh basket. Net bags made from cotton or nylon (worse) are abrasive, which is fine if you are netting a fish to keep, but not if you plan on releasing it.

Small fish rarely need to be revived because they are not played to the point of exhaustion. When you catch a large fish on a light tippet, however, you must often tire the fish completely before you can handle it. In these cases, you should carefully revive the fish before you release it. If you release it too soon, it may tumble to the bottom and die.

With wet hands, turn the fish on its side or hold it upside down so it doesn't struggle as much. Hold it facing into moderate-speed clean current so the water naturally rushes through the fish's mouth and over its gills. There's no need to "pump" the fish back and forth unnaturally.

In a stillwater situation, cradle the fish in clean water and allow it to pump water through its gills naturally. Try to keep it out of muddy water.

If you handle the fish, don't stick your fingers into the gills. This is the equivalent of having someone sticking their fingers in your bronchial tubes.

In the summer, when the water is warm, properly reviving the fish becomes critical. When you revive a trout, don't do it in warm backwaters or sloughs off the main river. Revive it near riffles where the water is as cool and oxygenated as possible.

As the fish regains its strength, its gills will work faster and you will feel the fish struggle to get away. When the fish feels strong, allow it to slip behind a rock or along the bank where it has some shelter from the current. Do not release a weakened trout or other fish into fast water.

If the water temperature is near or above 70 degrees, catch-and-release fishing becomes unethical and impractical, as the mortality rate will be high.

Ross Purnell

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