We’ve all seen it. You are scrolling through your social media feed and see a photo of someone holding up a big bass. They write, “I don’t have a scale. What do you think she weighs?”

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Don’t have a scale handy? If you keep a soft tape measure in your tackle box, you don’t need one to accurately estimate a fish’s weight. Photo by Capt. Richard Simms

As an angler, a fishing guide, and photographer, such posts drive me a little crazy. Sure, I understand everyone wants to show off their big fish. I’m no different. But as a photographer I know that estimating a fish’s weight based upon a photograph is an exercise in futility.

Is the photo taken with a 35mm, wide-angle lens, a standard 55mm lens, or a 125mm telephoto? Is the photographer 5 feet away or 20 feet away? Is the guy, or girl, holding the fish 6 feet tall or 5-foot, 6-inches? Are they holding the fish close to their body or, as many fishermen are prone to do, are they “long-arming” it?

All those factors greatly influence the apparent size of fish in photographs. And there is no way to know the answers. I also understand the frustration of always trying to have an accurate scale on hand. Besides them being expensive, keeping up with them is sometimes a challenge. Digital scales are prone to failure and their batteries seem to quit when you need them the most.

But I’ve got the answer to your fish weight worries, and it only costs a measly $3.59 (on Amazon). More likely, you already have the solution in your own home. It’s called a soft tape measure. It weighs ounces and most important, it can always live in your tackle box. Such a tape measure will provide you with a fish’s reliable weight, plus or minus 5 percent, without fail.

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A soft tape measure is all you need to accurately estimate a fish’s weight.

HOW TO “WEIGH” A FISH WITH A TAPE MEASURE

There are simple formulas that many biologists use for calculating a fish’s weight without a scale. Many states even post those formulas, tables or online calculators on their websites. The only things you need are accurate length and girth measurements.

Formulas do vary depending upon the fish species. Obviously, the formula for a largemouth bass isn’t going to be accurate for a muskie or northern pike.

The Wisconsin Dept. of Natural Resources has these formulas posted on its website:

● Bass weight = (length x length x girth) / 1,200

● Pike/Muskie weight = (length x length x length) / 3,500

● Bluegill/Crappie weight = (length x length x length) / 1,200

● Trout weight = (length x girth x girth) / 800

● Catfish weight = (length x girth x girth) / 800

● Walleye weight = (length x length x length) / 2,700

For example, you catch a 21.5-inch walleye. Using the formula above: (21.5 x 21.5 x 21.5) / 2,700 = 3.68 pounds.

I’ve tested the formula numerous times against accurate digital scales. Provided I’ve taken careful measurements, it usually spot on.

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The author estimated the weight of this blue catfish using the formula above, then checked it against a digital scale to find that it was spot on. Photo by Cat. Richard Simms

As an example, this blue catfish was 52 inches long with a girth of 34.5 inches. According to the formula - 52 x 34.5 x 34.5 = 61,893 Dividing that by 800 = 77.37 pounds

We also weighed that fish (supported in a landing net), and the scale read just over 79 pounds. I also crosschecked those measurements on a Texas A&M University online calculator, and the weight came in at 77 pounds, 8 ounces.

Even if you have a good scale, it is often better for the health of the fish to use a calculation based on length/girth measurements, rather than stressing the fish by actually weighing it.

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WHY WEIGHING FISH CAN BE THE WRONG MOVE

I’m the owner of Scenic City Fishing Charters. Myself and three of my guide partners routinely fish for trophy catfish. When one of our clients is fortunate enough to catch a trophy cat, they understandably want to know what it weighs. But hanging a huge catfish, sometimes weighing 50 pounds or more, vertically from a scale can seriously damage its internal organs.

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Hanging a large fish vertically from a scale can damage internal organs and cause harm to a fish you intend to release. Photo by Richard Simms

Fish live their lives horizontal in a nearly weightless environment. Hanging them vertically on a scale compresses their heavy internal organs in a way the fish have never experienced.

“With a larger fish, holding the fish horizontal with good support under it, you eliminate the risk of damage in that fish,” says Mike Jolley, a fisheries biologist with the Tennessee Wildlife Resources Agency. “There is no doubt about it because that fish lives in an environment where it is never having to support the weight of those internal organs. They are just not designed for that.”

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WHY MEASURING FOR GIRTH IS SO IMPORTANT

Many anglers routinely measure the length of their fish, sometimes because the law requires it to ensure it is a legal fish. However, few people measure the girth of their fish and the fact is, a fish’s weight is determined more by girth than it is length.

We all know that a man who is 5-feet, 10-inches tall can weigh 180 pounds. If, however, another man, of the exact same height, has an extremely large girth, he can weigh 250 pounds. The same variance can apply to fish.

While many states provide a simple table that provides estimated weight based solely on length, they calculate those based upon “average” girth. An extremely fat or skinny fish will weigh dramatically more or less than the tables indicate.

And it is impossible to accurately determine a fish’s girth unless you have a flexible tape measure. But combine the length and girth, using the formulas above for bass, catfish or trout, and your weight estimation is likely to be accurate with maybe a 5-percent margin of error either way.

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In order for the formula to be accurate, it is important to measure both the length and the girth of the fish. Image by Wisconsin DNR

When you can do so without harming the fish, weigh it on a quality scale. But if you don’t have one – or if you fear potentially harming the fish by weighing it – pull out that tape measure for length and girth. Then you won’t have to rely upon the guesses of Internet warriors, and you can document/verify your claims for your trophy catch, with or without a scale.