Metal Detector Target Depth Estimator

Most depth calculators online, including an earlier version of this one, multiply your coil size by a series of factors for soil, frequency, sensitivity and target size, and hand back something like "12.4 inches". Those factors are invented. No detector manufacturer publishes a formula that converts kHz and a sensitivity percentage into inches, and there is a good reason for that.

Detection depth for a real target in real ground depends on at least these, none of which a web form knows:

• How the target sits. A coin lying flat presents its full face to the field; the same coin on edge can be almost invisible. Nothing else on this list matters as much.
• What it is made of. A silver crown and an aluminium pull-tab of the same size do not behave alike, because conductivity governs how strongly eddy currents form.
• How long it has been there. A long-buried copper or silver object leaches into the surrounding soil and creates a halo that reads larger than the object itself. A coin dropped last summer has no halo.
• Your ground balance. A machine balanced properly to the ground hears weak targets that the same machine, badly balanced, buries in noise.
• The detector's own electronics. Two machines running the same coil at the same frequency are not equivalent.

So this page gives you the published rule, tells you which direction each variable pushes, and then tells you how to measure the rest. A precise-looking number with nothing behind it is worse than no number.

The air test takes five minutes and tells you what your machine and coil can do with the ground taken out of the equation. Switch on away from metal and interference, ground balance as usual, then have someone pass a target (the actual kind of target you hunt) towards the coil from below while you note the distance at which you first get a repeatable signal. Sweep the target past the coil rather than holding it still; most detectors only respond to movement. Repeat with the target flat and on edge, and note both.

Your air-test figure will be better than anything you get in the ground. That is expected, and the gap between the two is the number that actually tells you about your soil.

The test garden is the real answer. Bury known targets (a copper coin, a silver coin, a nickel, a small thin target, an iron nail for contrast) at measured depths in your own ground, note exactly where they are, and leave them. Sweep it when you get a new coil, after you change a setting, or when a site is reading oddly. After a season the buried targets even begin to acquire the halo effect that freshly buried ones lack, which makes the garden more honest over time.

Mark the layout on paper. Everybody thinks they will remember where the nickel went.

Mineralised ground (red clay, iron-rich soil, wet salt sand) reduces depth, and the direction is not in doubt even though the size of the effect is not published. Magnetic iron oxides and dissolved salts both produce a background signal the detector has to work through, and the usual consequence is that you turn sensitivity down to stop the machine chattering, which costs depth again.

Coil configuration matters here and this part is documented. Garrett describes the Double-D as "highly recommended when hunting over mineralized ground commonly found when prospecting and relic hunting", and says concentric coils are "most susceptible to interference from ground minerals, which results in substantial loss of performance when used over heavily mineralized ground".

So on clean sand or mild soil, coil choice is mostly about coverage and small-target sensitivity. On red dirt or a wet salt beach, a DD coil is the recommended configuration and a concentric one will cost you real depth. The calculator flags that combination.

Coil size is a trade, not an upgrade. The published rule tells you a bigger coil reaches deeper on a coin, up to that 15-inch ceiling, but the same physics that pushes the field deeper also spreads it out.

A larger coil also separates adjacent targets less well, which is why an experienced detectorist working an old trashy homestead often puts on a *small* coil and moves slower. On a clean pasture the reverse is true.

If you navigate to permissions or mark find spots by coordinate, the distance and bearing calculator gives the great-circle distance and initial bearing between two points, and the waypoint projection calculator does the reverse: a point from a start, a bearing and a distance.

Walking a big permission? The hiking time estimator is a reasonable way to budget the day.