How Much of the Bottom Does My Fish Finder See While Ice Fishing?

Ice anglers would be lost without electronics, but many misunderstand exactly how much of the water their particular transducer can show them. Different transducers have different cone angles, which affects how much of the bottom your fish finder actually sees. The difference can be dramatic.

A depth finder on the ice next to an ice fishing hole and a pile of fresh caught bluegills. — A flasher is a useful tool for marking and catching fish, but it's important to understand what it's telling you. | Joe Shead

Why Cone Angle and Depth Matter

A 19-degree ice fishing transducer — Transducers come in different cone angles. This 19-degree transducer provides a wide view of the bottom. | Joe Shead

Modern boat transducers now sometimes send out multiple cones, so for this discussion, we'll keep it simple and talk about traditional single cones used by ice fishing electronics.

Transducers typically come in wide-angle (19 or 20 degree), medium-angle (12 degree) and narrow-angle (9 degree) models. A wide-angle transducer gives you the widest-possible view of the bottom. It's best when fishing in shallow water to maximize the area the fish finder "sees." A narrow-beam transducer reduces interference. It's also ideal when fishing around structure because the transducer returns the shallowest bottom reading. For example, if you're in 20 feet of water but there's a 3-foot-tall rock under you within the cone, it will read as 17 feet. A narrower cone angle reduces the chance of inaccurate readings like this. And of course, a mid-range transducer is a happy medium.

Doing the Math (Without Getting Lost)

A Vexilar fish finder marks fish under the ice — This Vexilar is operating in only 6.5 feet of water. Even with a wide-angle transducer, the unit sees only a limited amount of the bottom. | Joe Shead

To calculate the area our electronics see, we have to employ simple trigonometry. Now, before you start groaning, realize that it's not that complicated and there are plenty of online calculators that make this a snap.

First, we start with what we know. We are essentially trying to calculate the base of a right triangle. So we know straight down from the transducer to the bottom forms a right angle (90 degrees). We know our cone angle at the top of the triangle (let's use a 19-degree transducer for this example). The sum of the angles of a triangle measure 180 degrees, so simple math tells us the third angle is 71 degrees.

The height of the triangle is the depth. For this example, let's say we're in 10 feet of water. We don't really care about the hypotenuse; we really just want to find the measurement of the base of the triangle, which will give us the radius of our cone.

Remember SOH CAH TOA from trigonometry? Probably not. We can figure out the tangent of the triangle (the base of the triangle/lake bottom). Tangent of the angle equals opposite side divided by adjacent side. (Seriously, just punch this all in an online calculator.)

The base of the triangle equals the adjacent side (10 feet) divided by the tangent of the angle (in this case tangent of 71). Long story short, the base is 3.44. So we are seeing a radius of 3.44 feet with a 19-degree transducer in 10 feet of water. But that's just showing half the picture. We are actually seeing a cone, not just a right triangle, so we see 3.44 feet to either side, or a total bottom width of 6.88. And if you do the math to find the area of the cone, we are actually seeing 37.17 square feet of the bottom.

Bottom Coverage Details by Cone Angle

Illustrated representation of how a 12º cone work under the ice at various water depths. — For some people, this many numbers can get overwhelming without a little visual explanation. Hopefully, this representation of how a 12º cone works is helpful. | Kurt Mazurek

That's all way too complicated for on-the-water calculations, so we've compiled the following data for easy reference that shows the bottom coverage you're seeing with various transducer cone angles at various depths.

19º Transducer Cone

Cone Angle: 19º
Water Depth (Ft.): 5
Radius (Ft.): 1.72
Diameter (Ft.): 3.44
Area (Sq. Ft.): 9.29

Cone Angle: 19º
Water Depth (Ft.): 10
Radius (Ft.): 3.44
Diameter (Ft.): 6.88
Area (Sq. Ft.): 37.17

Cone Angle: 19º
Water Depth (Ft.): 20
Radius (Ft.): 6.89
Diameter (Ft.): 13.78
Area (Sq. Ft.): 149.14

Cone Angle: 19º
Water Depth (Ft.): 30
Radius (Ft.): 10.33
Diameter (Ft.): 20.66
Area (Sq. Ft.): 837.77

12º Transducer Cone

Cone Angle: 12º
Water Depth (Ft.): 10
Radius (Ft.): 2.13
Diameter (Ft.): 4.26
Area (Sq. Ft.): 14.25

Cone Angle: 12º
Water Depth (Ft.): 20
Radius (Ft.): 4.25
Diameter (Ft.): 8.5
Area (Sq. Ft.): 56.75

Cone Angle: 12º
Water Depth (Ft.): 30
Radius (Ft.): 6.38
Diameter (Ft.): 12.76
Area (Sq. Ft.): 127.88

9º Transducer Cone

Cone Angle: 9º
Water Depth (Ft.): 10
Radius (Ft.): 1.58
Diameter (Ft.): 3.16
Area (Sq. Ft.): 7.84

Cone Angle: 9º
Water Depth (Ft.): 20
Radius (Ft.): 3.17
Diameter (Ft.): 6.34
Area (Sq. Ft.): 31.57

Cone Angle: 9º
Water Depth (Ft.): 30
Radius (Ft.): 4.75
Diameter (Ft.): 9.5
Area (Sq. Ft.): 70.88

Cone Angle: 9º
Water Depth (Ft.): 50
Radius (Ft.): 7.92
Diameter (Ft.): 15.84
Area (Sq. Ft.): 197.06

What This Means on the Ice

An angler focuses on reading his electronics while ice fishing. — A narrow cone angle works best in deep water or around structure. A wider cone angle gives you the widest view of the bottom and is best in relatively shallow water. | Joe Shead

The numbers may seem daunting, but understanding the concept makes it much easier to interpret what your depth finder is showing you. Knowing how cone angles work helps explain missed fish, false bottom readings, and why fish can seem to suddenly appear or disappear on your flasher. A clearer understanding of your electronics leads to more consistent success on the ice.