Sonar
How wide is your sonar cone at any given depth?
Quick answer
A 20 degree cone, the common 200 kHz beam on most fish finders, covers a circle about 3.5 feet across in 10 feet of water, widening to 7.1 feet at 20 feet and 14.1 feet at 40 feet. The math is simple trigonometry: coverage diameter equals twice the depth times the tangent of half the cone angle.
Sonar cone coverage is the diameter of the circle a transducer's beam actually reads at a given depth. It is published trigonometry rather than a guess: coverage diameter equals two times depth times the tangent of half the cone angle. The part almost every angler gets wrong is how small that circle is in shallow water, and how much it grows as the boat moves into deeper water on the same beam.
The cone angle itself belongs to your own transducer at the frequency you are running it. A manufacturer publishes that angle for each frequency its transducer transmits, and the specification sheet for the actual unit on your boat governs, not a generic table. The five frequencies below are the common values published across the mainstream lineup, which is why a MEGA imaging unit and a plain 200 kHz unit read completely different amounts of bottom at the same depth.
On this page
What decides how wide the cone is?
Two things only: the depth of the water and the cone angle of the transducer at the frequency it is transmitting. Nothing about boat speed, water clarity or bottom hardness changes the geometry. A wider angle sees more water at the same depth and resolves less detail in exchange, which is the entire trade-off between a deep water search beam and a high resolution imaging beam.
Cone angle by frequency
These are the cone angles published across the common frequencies most fish finders and chartplotters use. A specific transducer can differ, and its own documentation is the authority over this table.
The default 200 kHz beam most fish finders ship with runs about 20 degrees, while a 50 kHz deep water beam opens out to 45 degrees.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Frequency (kHz) | Typical cone angle | What it is used for |
|---|---|---|
| 50 | 45° | Deep water traditional sonar, wide and low resolution |
| 83 | 60° | Wide traditional beam, more water covered and less detail |
| 200 | 20° | The common traditional beam, the default on most units |
| 455 | 16° | Down and side imaging, narrow along track |
| 800 | 10° | High resolution imaging, shallow range |
These are typical values for the common frequencies, not a reading off any particular transducer. Check the specification sheet for the transducer you actually run, since a UHD transducer and an entry level unit at the same stated frequency can publish different angles.
How much bottom is under the cone at every common depth?
The full grid below runs the trigonometry across nine depths and all five common frequencies. Coverage diameter equals two times depth times the tangent of half the cone angle, so every cell here can be checked with a calculator and the two published numbers.
At 40 feet a 200 kHz beam covers a circle 14.1 feet across, while the same depth under a wide 50 kHz beam covers more than 33 feet.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Depth (ft) | 50 kHz, 45° | 83 kHz, 60° | 200 kHz, 20° | 455 kHz, 16° | 800 kHz, 10° |
|---|---|---|---|---|---|
| 5 | 4.1 | 5.8 | 1.8 | 1.4 | 0.9 |
| 10 | 8.3 | 11.5 | 3.5 | 2.8 | 1.7 |
| 15 | 12.4 | 17.3 | 5.3 | 4.2 | 2.6 |
| 20 | 16.6 | 23.1 | 7.1 | 5.6 | 3.5 |
| 30 | 24.9 | 34.6 | 10.6 | 8.4 | 5.2 |
| 40 | 33.1 | 46.2 | 14.1 | 11.2 | 7.0 |
| 50 | 41.4 | 57.7 | 17.6 | 14.1 | 8.7 |
| 75 | 62.1 | 86.6 | 26.4 | 21.1 | 13.1 |
| 100 | 82.8 | 115.5 | 35.3 | 28.1 | 17.5 |
Coverage diameter equals two times depth times the tangent of half the cone angle. Use the angle published for your own transducer at the frequency you are actually running rather than assuming one of these five values applies to your unit.
How little bottom a 200 kHz beam actually covers in shallow water
The area under the cone shrinks with the square of the diameter, which is why the shallow end of this table looks almost useless next to the deep end. A boat idling through five feet of water on a plain 200 kHz beam is reading a patch of bottom you could stand inside.
A 200 kHz cone only covers about 2.4 square feet of bottom in 5 feet of water, climbing to 244 square feet by 50 feet.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Depth (ft) | Cone diameter (ft) | Cone area (sq ft) |
|---|---|---|
| 5 | 1.8 | 2.4 |
| 10 | 3.5 | 9.8 |
| 15 | 5.3 | 22.0 |
| 20 | 7.1 | 39.1 |
| 30 | 10.6 | 87.9 |
| 40 | 14.1 | 156.3 |
| 50 | 17.6 | 244.2 |
The same 20 foot depth, five different beams
Holding depth constant and changing only the frequency shows the trade-off cleanly: a wide search beam covers far more water at the cost of resolution, and a narrow imaging beam trades coverage for detail. This is why a boat running a unit with SideScan, DownScan and CHIRP in the same package gets three different pictures of the same 20 feet of water rather than one.
At the same 20 foot depth, switching from a wide 60 degree beam to a narrow 10 degree beam shrinks the footprint from 23.1 feet down to 3.5 feet across.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Frequency (kHz) | Cone angle | Diameter at 20 ft | Typical use |
|---|---|---|---|
| 50 | 45° | 16.6 ft | Deep open water, wide search |
| 83 | 60° | 23.1 ft | Wide area search in moderate depth |
| 200 | 20° | 7.1 ft | Everyday bottom and fish reading |
| 455 | 16° | 5.6 ft | Down and side imaging structure detail |
| 800 | 10° | 3.5 ft | High resolution close range imaging |
Frequently asked questions
What is a sonar cone angle?
The cone angle is the width of the beam a transducer transmits, measured in degrees. It decides how much of the bottom the sonar actually reads at a given depth: a wider angle covers more water with less resolution, and a narrower angle covers less water in sharper detail. The angle is a property of the transducer at the frequency it is running, published by its maker, not a fixed number for every unit.
Why does my fish finder show almost nothing in shallow water?
Coverage diameter shrinks in direct proportion to depth. A 200 kHz beam that covers 35 feet across at 100 feet only covers about 3.5 feet across at 10 feet, and the area under that circle shrinks even faster since area falls with the square of the diameter. Shallow water is exactly when a narrow traditional beam is least useful, which is why down and side imaging exist.
Does a wider cone angle make a better fish finder?
Not automatically. A wider angle reads more water at any depth, which helps in deep open water where structure is sparse, but it also mixes more of the water column and the bottom into one return, reducing target separation. A narrower angle at the same depth reads less water in sharper detail, which is why imaging sonar runs narrower angles than traditional CHIRP.
How do I find the actual cone angle for my transducer?
Check the transducer's own specification sheet or manual rather than a general chart. Manufacturers publish the cone angle at each frequency the transducer transmits, and it is usually listed alongside the frequency itself, for example 200 kHz at 20 degrees. If two frequencies are listed, such as 50 and 200 kHz on a dual frequency transducer, each one has its own separate angle.
Why do down imaging and side imaging use a narrower beam?
Down and side imaging aim to resolve fine detail in structure and bottom composition rather than search a wide area quickly. A narrow beam, typically 16 to 10 degrees against 45 to 60 degrees for traditional deep water sonar, concentrates the sound into a thin slice of water, which is what produces the picture-like detail these modes are known for.
Does water depth change the cone angle itself?
No. The cone angle is fixed by the transducer and the frequency it transmits, and it does not change with depth. What changes with depth is the diameter of the circle that fixed angle produces on the bottom, which grows in direct proportion to depth. A 20 degree cone is always 20 degrees; it simply covers more feet of bottom the deeper the water gets.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.