Sonar cone angle and coverage width is the piece of sonar physics most fish finder users don't understand — and it directly affects how you interpret what you're seeing on screen. Here's the math, translated into practical coverage at real fishing depths.
How Cone Angle Works
A sonar transducer doesn't beam in a single direction — it emits a cone of sound that expands with depth. The cone angle (typically expressed as degrees) describes the total width of the coverage zone. A 20-degree cone angle means 10 degrees to each side of vertical, for a total coverage zone of 20 degrees.
The width of coverage at any given depth follows basic trigonometry: Coverage Width = 2 × (depth × tan(half angle)). For a 20-degree cone at 20 ft of depth: 2 × (20 × tan(10°)) = 2 × (20 × 0.176) = 7 ft wide.
Practical Coverage Chart
| Depth | 20° Cone (Standard) | 40° Cone (Wide) | 9° Cone (Narrow/Ice) |
|---|---|---|---|
| 10 ft | 3.5 ft wide | 7.3 ft wide | 1.6 ft wide |
| 20 ft | 7 ft wide | 14.6 ft wide | 3.2 ft wide |
| 40 ft | 14 ft wide | 29 ft wide | 6.3 ft wide |
| 100 ft | 35 ft wide | 73 ft wide | 15.8 ft wide |
| 200 ft | 70 ft wide | 146 ft wide | 31.5 ft wide |
Practical Implications
At shallow depths (under 20 ft): Even a 20-degree cone covers only 3.5–7 ft — you're essentially seeing directly below the boat. Fish 10 ft to the side of the boat are completely outside the cone. Side imaging is essential for coverage in shallow water; 2D sonar only shows the immediate below-boat column.
At deeper depths (100+ ft): The same 20-degree cone covers 35 ft — fish can be 17 ft to either side of the boat and still show on 2D sonar. This is why 2D sonar works better for finding fish in deep water than shallow.
Ice fishing (9-degree cone): The narrow cone is essential for ice fishing — a wide cone returns signals from the hole wall edges, creating false returns that obscure fish. A narrow 9-degree cone keeps returns centered directly below the hole.