Fishfinder Cone Coverage Calculator
How much bottom your sonar actually sees — beam diameter and coverage area by depth and cone angle, with what the display is really showing.
Geometry: diameter = 2 × depth × tan(half the cone angle) — the familiar rule that a 20° beam covers about a third of the depth. The catch anglers miss: a fish arch at the screen edge can be anywhere on that circle, and in 30 feet of water a 20° cone sees a spot just 10 feet wide — you are looking through a straw, which is why boat position matters more than screen-watching.
The cone is narrower than it sounds
A beam is quoted as an angle, and an angle is hard to picture. Converted to what it actually covers, the numbers are smaller than most people guess:
| Cone | Diameter as a multiple of depth | In 30 ft of water |
|---|---|---|
| 9° | 0.157 × depth | 4.7 ft across |
| 20° | 0.353 × depth | 10.6 ft across |
| 40° | 0.728 × depth | 21.8 ft across |
| 60° | 1.155 × depth | 34.6 ft across |
The common rule of thumb is that the footprint is about a third of the depth. That describes the 20° default and nothing else: the angle for which it is exactly true is 18.92°, which is not one of the options, so the default runs 5.8% wider. A 60° cone is 3.5× what the rule predicts.
Area goes as the square of a tangent, which is very fast
What you care about is the area being searched, and area goes as the square of the diameter, which goes as the square of a tangent. Relative to the narrowest beam on offer:
| Cone | Width factor | Area factor | vs 9° |
|---|---|---|---|
| 9° | 0.157 | 0.019 | 1.0× |
| 20° | 0.353 | 0.098 | 5.0× |
| 40° | 0.728 | 0.416 | 21.4× |
| 60° | 1.155 | 1.047 | 53.8× |
From 9° to 60° is 6.7× the angle, 7.3× the width, and 54× the area. There is no intuition that survives that.
The tangent does part of the work and the squaring does the rest. Going from 20° to 40° is exactly double the angle; if the diameter went linearly with the angle that would be 4× the area. It is 4.26×, because tan is convex — the widening accelerates as the beam opens, and the same doubling at 30° buys more again than it does at 10°.
Depth buys the same coverage, and you do not choose it
The formula is linear in depth and non-linear only in the angle, so the two are interchangeable at a fixed rate. A 60° cone in 30 ft of water covers 942 square feet. To match that with the 20° default you need 98 ft of water.
That is 3.27× the depth, which is exactly the ratio of the two width factors — the substitution is not approximate. It is also what makes the angle argument less interesting than it looks: anyone fishing deep water already has a large footprint from the depth alone, and anyone in the shallows cannot buy their way out of a small one.
And a big footprint is not only good
At 80 feet, the 20° cone covers 28 ft across and 625 square feet; the 60° cone covers 92 ft and 6,702. A fish anywhere inside that circle draws in the same place on the screen, because the display has one horizontal axis and it is time, not position.
So the wide cone is 10.7× the search area and 3.3× the uncertainty about where the thing actually is — and the first number is the square of the second, so it is one trade rather than two. It is also why the default cone stops being a small number with depth: it first covers a whole 20-foot boat length at 57 feet of water.
How to use
- Enter your transducer's cone angle and the depth.
- Read the beam diameter and area covered.
- Compare a narrow and a wide cone at your usual depth.
- Remember the display is a history, not a live picture.
Frequently asked questions
How wide is my sonar beam?
It depends on the cone angle and the depth, and the relationship is simple trigonometry — the beam widens proportionally as it goes deeper. A 20-degree cone covers roughly a third of the depth in diameter, so in 30 feet of water it sees about a 10-foot circle.
Should I use a wide or narrow cone?
Wide cones cover more area and suit shallow water and searching; narrow cones concentrate the energy, give better detail and penetrate deeper. Many transducers offer both frequencies for exactly this reason — search wide, then examine narrow.
Is the fish shown directly below me?
Not necessarily, and this is the most misunderstood part of sonar. A fish anywhere in the cone appears on the display, and in deep water that cone is wide. A mark showing at 40 feet could be well off to the side, which is why arches say more about the boat's movement past a fish than about its position.
What does the display actually show?
A history scrolling right to left, not a live view. The right edge is what the transducer is seeing now, and everything to the left is the past. A stationary boat over a stationary fish draws a straight line, not an arch — which surprises people who expect arches to mean fish.
Why do I see arches?
Because as you pass over a fish, its distance from the transducer decreases and then increases — closest when directly beneath. That changing distance draws the arch. A partial arch usually means the fish passed through the edge of the cone rather than the centre.
Does the cone angle affect what depth I can read?
Yes. A narrow beam concentrates the same power into a smaller area, so more energy returns from depth and the reading holds in deeper water. Wide beams lose definition and bottom lock sooner, which is why deep-water transducers tend to be narrow.
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