Anything Fly Fishing

Guessing the Size of Your Fish

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Anglers, as a group, tend to be inconsistent at estimating fish sizes. Partly, this is because most anglers are people and people are notoriously bad at this. For instance, people tend to view anything in their grasp as small, things just out of their grasp as larger, and anything beyond their grasp as enormous.

But over the years, I’ve learned that an estimate of size is a key component of any fish tale so a skilled angler will at least need to have a good guess. Having done this for some time, I’ve developed a scientific method for estimating fish size which I will share here.

To begin, catch a fish.

If you don’t at least start with a fish then you’re just lying. So, let’s assume you have a fish and guess it’s eighteen inches long. As long as you’re within a foot of the correct answer, it’s as good a place to start as any.

Now, begin to apply the following factors.

First, were there any witnesses who are now present where you are telling the tale? Witnesses, especially fishing buddies, tend not to be as generous as you might with fish estimates. So, subtract two inches if witnesses were present. Likewise, if you were fishing alone, add two inches. What can it hurt?

Taking a photo will affect your estimate of size.

The second factor is whether you took a photo. Cameras can be used to exaggerate the size of the fish or to shrink it, so unless you are an expert at the former, I suggest subtracting two inches from your guess if you took a picture. If you left your camera in the truck, a wise move in this process, add two inches.

The third factor involves the use of the fish photo. If you posted your fish on any social media sites everyone will have seen it by now and will have formed their own opinions. As noted above, these people tend not to be generous in their own estimates of size and may try to interject their estimates into the telling of your fish tale. To preempt that, go ahead and subtract three inches from your guess if the photo has already been on social media. Likewise, add three if you kept it to yourself.

With any good fish tale, you should keep track of what you said previously and who heard the story. Once you share an estimate, you are at least locked in until the audience changes. So, if your audience includes friends who already heard the story before, use the same estimate that you did last time. If not, add two inches.

If the fish got away, guess any size you want.

I will argue there is one exception to these calculations and that is for the one that got away. When your tale involves the Moby Dick of your angling exploits, feel free to add any length that you want. You have no photos, no witnesses, and no social media detractors. Therefore, any estimate in the vicinity of the current state record could apply.

Your friends will discount the size of your fish.

Finally, there is the factor of time. Your friends will discount the size of fish in old tales so you have to compensate for this. I find that adding one inch for every year since the fish encounter is about the same factor listeners use to diminish your estimate.

You can tell from my formulas that estimating fish size is a serious, scientific business. But by forgoing photos, witnesses, and social media, my eighteen-inch trout becomes twenty-five inches long and grows an inch per year. Even more if it got away.

With a bit of practice, you too can become an expert at estimating fish size. When I’ve presented this formula in the past I’m sometimes asked why I don’t just measure the fish. After explaining that the extra handling is bad for survival rates, I come to the real reason you should always guess.

Anglers who measure their fish simply lack imagination. And who wants to fit into that group?

Jim Mize never carries a ruler fishing. You can purchase Jim’s new book, The Jon Boat Years at or, or buy autographed copies at

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