For a conventional downward-looking sonar with an approximately circular conical beam, the nominal coverage diameter increases with distance below the transducer. Calculate it at the jig's depth, not automatically at the bottom. A broad footprint on the lake bed can coexist with a much smaller cross-section around a suspended jig.
The useful estimate is diameter = 2 × vertical distance below the transducer × tan(full beam angle ÷ 2). This is ideal-cone geometry, not a promise that everything inside will be detected or that nothing outside can produce a return.
Jump to calculation inputs, worked examples, detection limits, tackle selection or FAQs.
Use the right distance and the full beam angle
Measure down from the transducer, not from the rod tip
The distance in the formula is the vertical separation between the transducer and the layer you are considering. It is not line released, handle turns or the length between your hand and the jig. A depth display can use a configured reference, and an off-axis target's sonar range is not automatically its vertical depth. Treat an uncertain input as an estimate rather than reporting a precise footprint.
If the jig is 5 m below the transducer while the bottom is 10 m below it, use 5 m for coverage around the jig. Keep units consistent: metres produce a diameter in metres; feet produce a diameter in feet.
Halve the angle inside the calculation
Use the full included beam angle from the exact transducer and operating mode's specification, then divide that angle by two before taking the tangent. In a calculator, use degree mode when entering degrees. Do not substitute the frequency number in kHz for an angle.
Manufacturer specifications also need context. The Humminbird ICE 55 specification lists 9° and 19° coverage at −3 dB. Those are model-specific figures, not universal narrow and wide settings for every ice sonar. Keep the stated reference level with the angle; figures reported at different reference levels are not necessarily like-for-like comparisons.
Worked examples: the same beam at two jig depths
The following values use ideal, vertical circular cones and are rounded to two decimal places. They describe nominal diameter, not a measured field detection area.
At 5 m below the transducer
- 9° full beam: 2 × 5 × tan(4.5°) ≈ 0.79 m across.
- 19° full beam: 2 × 5 × tan(9.5°) ≈ 1.67 m across.
At 10 m below the transducer
- 9° full beam: 2 × 10 × tan(4.5°) ≈ 1.57 m across.
- 19° full beam: 2 × 10 × tan(9.5°) ≈ 3.35 m across.
Doubling the distance doubles the diameter for the same angle. It does not merely add a fixed allowance. In the 9° example, the nominal diameter at 5 m is only about half its value at 10 m.
Diameter is not the distance from the centre
A diameter of 0.79 m means a nominal radius of about 0.39 m from the cone's central axis at that layer. It does not mean 0.79 m of reach on each side. This distinction matters when thinking about a presentation offset from the transducer.
Do not turn the radius into a drilling instruction. Ice integrity, access and hole arrangements require current local safety assessment. The calculation only describes a simplified acoustic geometry.
Why the cone is not a hard detection boundary
The drawn sides make the calculation easy to understand, but a real beam does not behave like a solid-walled tube. The published angle describes a specified beam characteristic. Return strength, target orientation, settings, interference, bottom conditions and processing still affect what appears.
A target inside the nominal cross-section is not guaranteed to be distinguishable. A strong return does not prove that the target sits on the central axis, and disappearance does not measure the distance the jig moved sideways. Ordinary conventional sonar does not turn this diameter estimate into a three-dimensional lure position.
Nor is a wider beam automatically better. It samples a broader region, which can add spatial ambiguity. The Vexilar FLX-28 manual explains how wider coverage can increase bottom-related masking on slopes. For that separate interpretation problem, see our ice-fishing slope guide.
Turn the estimate into a better fishing decision
- Identify the actual beam. Record the unit, transducer, operating mode, full angle and its stated reference. Do not borrow another model's number.
- Choose the working layer. Estimate vertical separation below the transducer. Do not substitute total water depth when targeting suspended fish.
- Calculate diameter, then radius. Keep the rounded result as a scale reference, not a sharp boundary.
- Compare supported beam choices. If your system permits selection, follow its manual and preserve the same jig, working layer and physical transducer arrangement during the comparison.
- Observe before interpreting. A newly visible return after a setting change can reflect changed detection, not a fish arriving. Record the setting change with the observation.
Changing gain and changing beam selection are different actions. A brighter trace is not evidence that the nominal beam angle increased. If a busy display is the main problem, use the separate sonar clutter and gain guide rather than repeatedly changing the geometry calculation.
Choose the rod for the presentation, not the beam width
A larger calculated footprint does not justify a heavier jig or a stronger rod. Match the complete lure-and-bait load and line system to the outfit. For example, the GOOFISH Iceseeker ice fishing spinning rod has two nominal 32-inch, one-piece Fast-action options:
- Green-wrap ML: 2–6 lb line; 1/32–1/4 oz lures, approximately 0.9–7 g.
- Blue-wrap M: 4–8 lb line; 1/8–3/8 oz lures, approximately 3.5–10.5 g.
These ratings describe tackle selection, not sonar visibility. Neither model guarantees detection of a compatible jig. Use a suitable spinning reel and controlled line handling; winding a particular number of turns does not verify vertical depth or keep the lure on the beam's centreline.
Ice sonar beam-width FAQs
Can I use the bottom footprint for a suspended jig?
No. Calculate at the jig's vertical distance below the transducer. A shallower layer has a smaller cross-section in the ideal-cone model.
Does 19° mean 19° on each side?
Not when the specification gives a full included angle. The half-angle is 9.5°. Confirm the manufacturer's convention before calculating.
Can I use this formula for live sonar or an imaging fan?
Do not assume so. This circular-footprint calculation concerns approximately conical conventional beams. Directional, fan-shaped, scanned and other coverage patterns need their own specifications and geometry.
Does an empty cone mean there are no fish?
No. Non-detection does not establish absence, even inside the nominal footprint. The estimate helps you understand the scale of coverage; it does not identify everything in the water.
The useful habit is to keep three ideas separate: nominal beam geometry, actual detection and the presentation you can control. Calculate the first, observe the second and choose tackle for the third.
