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Coil Coverage and Sweep Time Calculator

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Quick answer

An 11 inch coil swept in a 4 foot arc at 20 sweeps per minute with 25 percent overlap covers roughly 5,500 square feet per hour, so one acre takes about 8 hours of swinging. A 15 inch coil cuts that to about 6 hours. Enter your own numbers below.

This is the number that decides whether a permission is realistic. People agree to detect a ten acre field and then discover that covering it once, without digging anything, is a month of weekends. Coverage depends on three things you control: coil width, how wide and how fast you swing, and how much you overlap each pass.

Ground covered per hour

5,500 sq ft

Coils and support for covering ground

Top matches update when you calculate.

A larger coil is the only change that meaningfully increases coverage. A harness is what lets you swing one for more than an hour.

How the calculation works

Each sweep advances you forward by the coil width minus the overlap, and sweeps across the arc width you walk. Multiply those together for the area of one sweep, multiply by sweeps per minute and by sixty, and you have square feet per hour. An acre is 43,560 square feet.

The assumption worth questioning is the sweep rate. Twenty sweeps per minute is a three second cycle, which is a comfortable, unhurried pace. Pushing to thirty covers half as much ground again on paper, but the coil spends less time over each target and you lose depth on exactly the faint, deep, old signals you came for. Treat extra coverage bought with speed as theoretical.

What this means for choosing a coil

On open ground, coverage is the argument for a large coil and it is a strong one. On a trashy park it is the wrong metric entirely, because a 15 inch coil hearing four targets at once reports one confused signal, and the coin next to the bottle cap simply disappears. Match the coil to the site rather than to the acreage, and see the coil size selector if you are deciding between two.

Plan the pattern, not just the pace

Coverage per hour only helps if your passes actually line up. Marking lanes with pegs or a line and working them in order is the difference between covering a field once and covering a third of it three times. The search grid planner turns a site's dimensions into a lane count and a walking distance.

Frequently asked questions

How long does it take to detect one acre properly?

At a realistic pace with an 11 inch coil, a 4 foot sweep arc and a 25 percent overlap, covering one acre once takes roughly 4 to 7 hours of swinging, before you dig anything. That is why experienced detectorists work a small marked-off block properly rather than wandering a whole field, and why a bigger coil is the only thing that meaningfully changes the number.

How much should I overlap each sweep?

Between 20 and 30 percent of the coil width is the usual compromise. A Double-D coil detects along a narrow blade down the middle rather than across its whole footprint, so sweeping with no overlap leaves unsearched strips between passes. Overlap below about 20 percent is how people walk over targets repeatedly and conclude a site is hunted out.

Does a bigger coil really cover more ground?

Yes, and it is close to a linear gain: a 15 inch coil covers roughly 35 percent more ground per sweep than an 11 inch coil at the same speed and overlap. What it costs you is target separation in trash and sensitivity to very small targets, so a big coil is right for open fields and beaches and wrong for a trashy park.

How fast should I actually swing?

Slower than feels natural, and most machines need a sweep of roughly 2 to 3 seconds from side to side. Swinging fast reduces the time the coil spends over a target, which costs depth on the faint signals that are usually the old ones. If you increase sweeps per minute in this calculator, treat the extra coverage as theoretical rather than free.

Why does my coverage estimate not match my real day?

Because this figure is swinging time only. Digging is the other half of a hunt, and on a trashy site it is the larger half. Run the dig time budget calculator alongside this one: a site that takes 5 hours to sweep can easily take 15 to work if it returns a signal every couple of minutes.

Researched, not professional advice. This tool produces an estimate from the figures you enter and from published manufacturer specifications. Depth, ground coverage and battery runtime all vary with soil chemistry, moisture, temperature and how a machine is configured. Any metal value shown is a melt estimate only: it is not an offer, an appraisal or a guarantee of what a buyer will pay, and collectable coins are frequently worth far more than their metal. Get the landowner's permission before detecting on ground you do not own, keep off protected land, and fill every hole.