Sonar coverage
How much does a 16 degree sonar cone cover in 80 feet of water?
Quick answer
A 16 degree sonar cone in 80 feet of water covers a circle about 22.5 feet across, which is roughly 397 square feet of bottom. That is the typical beam for 455 kHz: down and side imaging, narrow along track.
This is the number that surprises people. Sonar coverage is a cone opening downward from the transducer, so the shallower the water the less bottom it sees. In 80 feet a 16 degree cone is looking at a circle 22.5 feet across, and everything outside that circle is invisible.
It is pure trigonometry, and the cone angle belongs to your own transducer at the frequency you are running rather than to the unit or the brand. Check the specification for your transducer before trusting any general figure, including this one.
On this page
What 16 degrees covers at other depths
Coverage scales linearly with depth: in 80 feet a 16 degree cone covers 22.5 feet across, and in twice that depth it covers twice the diameter and four times the area.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Depth | Coverage diameter | Coverage area |
|---|---|---|
| 5 ft | 1.4 ft | 2 sq ft |
| 8 ft | 2.2 ft | 4 sq ft |
| 10 ft | 2.8 ft | 6 sq ft |
| 12 ft | 3.4 ft | 9 sq ft |
| 15 ft | 4.2 ft | 14 sq ft |
| 20 ft | 5.6 ft | 25 sq ft |
| 25 ft | 7.0 ft | 39 sq ft |
| 30 ft | 8.4 ft | 56 sq ft |
| 40 ft | 11.2 ft | 99 sq ft |
| 50 ft | 14.1 ft | 155 sq ft |
| 60 ft | 16.9 ft | 223 sq ft |
| 80 ft | 22.5 ft | 397 sq ft |
| 100 ft | 28.1 ft | 621 sq ft |
Every common cone angle at 80 feet
At 80 feet the difference between a narrow imaging beam and a wide traditional one is the difference between 14.0 feet and 92.4 feet of bottom.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Frequency | Typical cone | Coverage at this depth | What it is for |
|---|---|---|---|
| 50 kHz | 45 degrees | 66.3 ft | Deep water traditional sonar, wide and low resolution |
| 83 kHz | 60 degrees | 92.4 ft | Wide traditional beam, more water covered and less detail |
| 200 kHz | 20 degrees | 28.2 ft | The common traditional beam, the default on most units |
| 455 kHz | 16 degrees | 22.5 ft | Down and side imaging, narrow along track |
| 800 kHz | 10 degrees | 14.0 ft | High resolution imaging, shallow range |
These are typical cone angles for each frequency. The angle is a property of your own transducer and the maker publishes it, so use theirs rather than these where you have it.
Wide beam or narrow beam
The trade is coverage against resolution, and it is real in both directions. A wide beam at 80 feet covers more water, which finds fish faster, but every return inside that 92.4 foot circle arrives at the transducer at roughly the same time and is drawn at roughly the same place on the screen. A narrow beam sees less and tells you far more about what it sees.
That is why almost every unit runs both: a wide traditional beam for finding, and a narrow imaging beam for identifying. In shallow water like this the wide beam is also picking up returns from the bottom at the edges of the cone, which is where the arch of a false fish mark frequently comes from.
Why shallow water is where imaging earns its money
At 80 feet a traditional cone covers 22.5 feet of bottom, which on a boat moving at four miles an hour is almost nothing. This is exactly the situation where side imaging changes the picture: it looks sideways rather than down, covering a hundred feet or more either side of the boat regardless of how shallow it is.
The general rule that falls out of the trigonometry: the shallower you fish, the less traditional sonar is doing for you, and the more the imaging and live sonar options are worth the money.
Cone coverage
Published figurediameter = 2 x depth x tan( cone angle / 2 ) depth = 80 ft, cone = 16 degrees diameter = 2 x 80 x tan(8) = 22.5 ft area = pi x ( diameter / 2 ) ^ 2 = 397 sq ft
Pure trigonometry. The only figure that is not is the cone angle itself, which belongs to your own transducer.
Transducers and units for this kind of water
Chosen against the numbers on this page rather than from a general list, so the recommendation cannot contradict the arithmetic above. Every figure quoted is the manufacturer own published specification.
Garmin GT54UHD-TM transom mount transducer
$396.71The 800 kHz ClearVu and SideVu transducer that the seven inch ECHOMAP units carry, and a real upgrade over a GT20.
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Garmin GT34UHD-TM transducer
$249.99Ultra high definition scanning without the side imaging element, for a boat where down imaging is what gets used.
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Garmin GT23M-TM transducer
$239.99Mid band CHIRP with ClearVu, and the sensible replacement when a stock transducer gets knocked off a trailer.
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Garmin Airmar P19 plastic thru-hull transducer
$161.59A plastic thru-hull, which is the honest answer on a deep vee where a transom mount ventilates the moment the boat gets on plane.
Check before buying: A thru-hull means a hole below the waterline. Plastic housings go in fiberglass hulls only, never aluminium.
Best for: Fiberglass hulls that run fast enough to aerate a transom transducer
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Airmar P19 thru-hull depth transducer
$95.04 Published specsThe Airmar branded version of the same housing at 200 kHz and 350 W, and the cheapest genuine thru-hull here.
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Garmin Airmar B150M thru-hull with 20 degree tilt
$477.13A tilted element bronze thru-hull, which keeps the beam vertical on a hull with real deadrise instead of pointing it sideways.
Check price on AmazonHow we chose
We did not test this equipment in person and we never claim to. Picks are researched from manufacturer specification sheets, published engineering and regulatory figures, and the recurring themes in verified owner reviews. Every pick is matched to a real thrust rating, a real published amp draw, a real capacity or a real conductor size rather than to a price bracket, and it is placed in the tier where its published capability actually belongs. We rejected the unbranded flood that dominates several of these search results, because a lithium pack whose management board is the only thing between a cell fault and a fire, a breaker protecting a battery cable, a life jacket and a bracket holding a two thousand dollar motor to the bow at highway speed are all parts whose failure has a consequence nobody can walk away from. Where a figure is convention rather than a published standard we label it that way, and where a manufacturer does not publish a figure we leave it out instead of estimating it.
Frequently asked questions
How wide is a 16 degree sonar cone in 80 feet of water?
About 22.5 feet across, covering roughly 397 square feet of bottom. The calculation is twice the depth multiplied by the tangent of half the cone angle, which is pure trigonometry. Coverage scales directly with depth, so the same transducer in twice the water sees twice the diameter and four times the area.
Where do I find my own cone angle?
In the transducer specification rather than the display specification, and it is different at each frequency the transducer runs. A dual frequency transducer commonly has a wide beam around 60 degrees at 83 kHz and a narrow one around 20 degrees at 200 kHz. The manufacturer publishes both, and that figure governs rather than any general table including this one.
Is a wider cone better?
Only for finding, not for identifying. A wide cone covers more water so you drive over fewer fish without seeing them, but everything inside the cone is drawn at roughly the same place on the screen, which is where false arches and confusing returns come from. A narrow cone shows you far more about far less water, which is why most units run both at once.
Why does my fish finder show nothing in shallow water?
Partly because there is so little to see. At 80 feet a 16 degree cone covers only 22.5 feet of bottom, so a boat on the move passes through most of the water without the cone ever touching it. Very shallow water also puts the bottom return and the surface clutter close together on screen. This is the situation side imaging and down imaging exist for.
Does frequency change the coverage?
Indirectly, because cone angle and frequency are linked in practice. Lower frequencies generally use wider cones and penetrate deeper with less detail, and higher frequencies use narrow cones with far more resolution and less range. The coverage arithmetic only cares about the angle, so a 200 kHz beam at 20 degrees and a hypothetical 83 kHz beam at 20 degrees would cover exactly the same circle.
Does boat speed affect what the cone sees?
It affects what you get out of it rather than the geometry. The cone is the same size at any speed, but a fast moving boat gives each patch of bottom fewer pings, so returns get drawn thin and structure smears. That is why people slow down over a spot they want to read properly, and why side imaging has a recommended speed range in its own manual.
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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.