Pro Tips: How to Quickly Measure a Fish

To measure your arm, put your elbow against a wall, and then put the end of a tape measure against the wall.
Photo by Rachel Barlow

During my time as a guide, I learned quickly that most anglers have no idea how to gauge the size of a fish by sight. Two-thirds of my clients would look at a 12-incher and say, “Man, that’s gotta be fifteen, sixteen inches, right?” while the other third would land a 22-inch rainbow and ask, “Do you think it’s eighteen inches?” In these days of Internet trollery, accidentally overestimating a fish’s length can lead to an avalanche of abuse, which is no fun. So it’s worth learning how to measure a fish, but the key is to do it quickly.

This is important because the amount of time you keep a fish out of the water is extremely important. The “Keep ‘Em Wet” ethos is strong these days, and studies by the Colorado Division of Wildlife showed that leaving the fish in the water while you unhook it can double its chances of survival. So what you need is a way to get a nearly instantaneous measurement on the fish you catch.

When I was a guide, I futzed around with several methods, until I finally settled on what I thought was the quickest and easiest: I measured my own forearm, from the point of my elbow to the tip of my middle finger. (Some of you old-timers or Latin buffs may recognize this measurement as a cubit.) The total distance is just under 19 inches. From the elbow to the bump in my wrist is 10 inches, to the base of my pinky is 15 inches, and to the end of my pinky 17 inches. I easily memorized these numbers and could make a very educated guess for an inch or two to either side of these points.

A tape measure on a zinger means less time wasted fumbling through pockets.
Photo via

Whenever I landed a fish for a client, I could hold the fish just under the surface, lay my forearm alongside it, and get an immediate rough measurement. Sure, it wasn’t exact—and, as a guide, I may have rounded up on occasion—but this method lets you know immediately whether a fish is closer to 15 inches than it is to 18. I also know that the width of my spread hand, from end of pinky to end of thumb, is 8.5 inches. Anything over 19 inches, of course, is a little more complicated.

In this case, I’d mark the 19-inch spot, slide my hand to the fish’s nose, and then estimate in reverse. Simply add the two numbers and you’re good to go. (Example: tail forward to mark = 19 inches, nose back to mark = tip of middle finger to base of pinky, or 4 inches. 19+4 = 23-inch fish.) Anything over, say 25-inches and, face it, you’re breaking out the damned tape.

If you do need to use the tape measure, make sure you keep the fish in the water.
Photo by Sandy Hays

What I particularly like about this method is that you don’t have to worry about holding anything but the fish. However, many anglers like to use their rod as a measuring stick. Jay “Fishy” Fullum, a well-known fly tier and angler, recommends using thread wraps on your rod blank. He makes narrow wraps at 12, 14, 16, 18, and 20 inches. You can use easy-to-read hi vis colors, or, if you think that ruins the aesthetics of your rod, you can use colors to match the guide wraps. When you want to measure a fish, simply lay the rod alongside the fish in the water and estimate based on your wraps.

Both of these methods require a lot less fumbling through pockets or arranging stuff on your vest, saving valuable seconds separating the time you land the fish and the time you let is swim away. And if your buddies can’t live without an absolutely, scientifically verifiable measurement, just get new buddies. It’s better for the fish that way.

8 thoughts on “Pro Tips: How to Quickly Measure a Fish”

  1. Back when I was in my 20s and still kept quite a few trout, I learned to measure a six inch keeper by holding it in my hand with my thumb outstretched. The edge of my palm to tip of my thumb is 6 1/4″ so if a fish was that big or longer I knew it was legal & the extra 1/4 kept me certain of it. These days I’m not keeping fish but know the lengths of parts of the butt section on my favorite fly rod that are 10, 12, 15, 18 & 20 inches. I do carry a small tape as well but haven’t used it in years.

  2. I have a net that has a number line in it. 0-12 from middle to top, both right and left. Total would be 24″. Keep the fish submerged in the net and remove the hook. The lift the net out of the water enough to read the number. Add from middle of the fish to the head to middle of fish to the tail. For example 7″ middle to head + 6″ middle to tail = 13″. Revive the fish submerged in the net then release. The net cost me around $42 and it extends and is very light. I carry it between my safety belt and waders near the swell of my back. Love it!

  3. This is why I invented the Aqua-Ruler. It was a marketing fail; apparently very little interest in exact measurements while keeping fish submerged and breathing comfortably. There are also a few different fish measuring apps that make us of AR (Augmented Reality) technology. No idea how accurate they are.

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