Printed cone width at depth

Diameter of the circle the printed cone covers at each depth. This is pure geometry and the same for every brand, as long as the cone angle is measured the way the label says it is.

Diameter in feet covered by each cone angle at each depth
Cone	at 10 ft	at 20 ft	at 30 ft	at 50 ft	at 75 ft	at 100 ft	per ft of depth
8°	1.4 ft	2.8 ft	4.2 ft	7.0 ft	10 ft	14 ft	0.14 ft
12°	2.1 ft	4.2 ft	6.3 ft	11 ft	16 ft	21 ft	0.21 ft
16°	2.8 ft	5.6 ft	8.4 ft	14 ft	21 ft	28 ft	0.28 ft
20°	3.5 ft	7.1 ft	11 ft	18 ft	26 ft	35 ft	0.35 ft
30°	5.4 ft	11 ft	16 ft	27 ft	40 ft	54 ft	0.54 ft
40°	7.3 ft	15 ft	22 ft	36 ft	55 ft	73 ft	0.73 ft
60°	12 ft	23 ft	35 ft	58 ft	87 ft	115 ft	1.15 ft

diameter = 2 · depth · tan(cone/2). For cones under about 30°, diameter ≈ depth × cone ÷ 57.

The printed cone is not where your fish shows up

A cone angle is a convention: the angle at which the beam has lost a set amount of power. Sound keeps going past it, weaker. Whether a fish out there gets drawn depends on how loud its echo is and how much gain you are running. Here is the same 20° cone at 30 ft, whose printed footprint is 11 ft across, painting fish of different sizes:

Width in feet across which a fish of each length is painted by a 20 degree cone at 30 feet, by sensitivity
Fish length	50% sens.	65% sens.	75% sens.	85% sens.	100% sens.
3 in	not drawn	not drawn	not drawn	not drawn	7.4 ft
6 in	not drawn	not drawn	not drawn	8.1 ft	13 ft
12 in	not drawn	6.7 ft	11 ft	13 ft	17 ft
20 in	5.1 ft	11 ft	14 ft	16 ft	19 ft
30 in	9.6 ft	14 ft	16 ft	18 ft	21 ft

Two things fall out of that table. First, the cone on the box describes one fish at one setting; for small fish at modest gain your effective cone is much narrower than the label, and for big fish at high gain it is wider. Second, turning sensitivity up does not just make marks brighter: it widens the water you are actually searching. The model behind this is the one on the arch simulator: a beam that fades smoothly past the printed edge, with the echo counted out and back.

Wider cone: more water, more error

A wide cone covers more bottom per pass, and it pays for it twice. Anything near its edge is drawn at its slant distance, so it shows deeper than it is. And near the bottom, a fish at the edge is further away than the bottom straight below, so it isdrawn into the bottom band and disappears. The calculator's "hidden band" is that height for a flat bottom at the depth you set.

Try a 60° cone at 40 ft againsta 12° cone at 40 ft: the wide one searches roughly 30 times the bottom area, and hides fish 5.4 ft off the bottom at its edge instead of 0.2 ft.

What this calculator leaves out

One way of measuring a cone. The printed edge here is where one-way power is down 3 dB. Manufacturers don't all use the same cut-off, so two "20°" labels can mean different beams. The geometry table holds whatever the definition; the painted widths depend on it.
A smooth Gaussian beam, no side lobes. Real transducers leak energy at wider angles. Far outside the printed cone the painted width here is an extrapolation, and the calculator says so when you get there.
Echo strength is fish length squared, nothing else. Real echoes depend mostly on the swim bladder, the species and the angle the fish presents. Same-length fish can differ by several dB.
The sensitivity scale is ours. 75% is 0 dB here and each percent is 0.36 dB. Your unit's percent means something else, so read the sensitivity columns as "more" and "less", not as a setting to copy.
Flat hard bottom, perfect range compensation, 15 pings per second. The same assumptions as the arch simulator, listed there in full.

How wide is your sonar cone at depth?

Printed cone width at depth

The printed cone is not where your fish shows up

Wider cone: more water, more error

What this calculator leaves out

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