Why this tool asks you for the curve instead of knowing it
Running depth is a property of a specific lure. Lip angle, lip area, body buoyancy, the position of the tow point and the profile all decide how much downward force the water generates at a given speed, and two lures of the same size and weight can run at depths that differ by a factor of three. No general formula recovers that. What a formula can do is take a point you already know for your lure and move it to the conditions you are fishing, which is what happens here.
The reference point can come from a manufacturer dive chart, from published trolling data for that lure, or best of all from your own line counter and a day when you found bottom in known depth. That last source is worth more than either of the first two, because it already includes your line, your rod position and the way your boat tracks.
The three scalings, and what each exponent means
Depth grows with line out, but not proportionally. The first fifty feet of line buys a lot of depth. The next fifty buys less, because as more line goes out the angle it makes with the surface flattens and each additional foot contributes less vertical drop. That behaviour is captured by an exponent below one, and published trolling data generally sits somewhere around 0.6 to 0.85 depending on the lure and the line. The default here is 0.75 and it is an input because it should be.
Depth falls as speed rises, for a lure that has already reached its diving limit. Faster water lifts the line and the lure planes up. An exponent of zero would say speed makes no difference and one would say depth is inversely proportional to speed. Most lures behave somewhere between, and the default of 0.5 is a middle rather than a measurement.
Line diameter matters because water drag on the line is what holds the whole system up. Thinner line is dragged less and lets the lure sink further at the same line out, which is why the same lure on braid runs deeper than on heavy monofilament. The tool applies a square root of the diameter ratio, which is a modest allowance and should be treated as a nudge rather than a calculation.
Added weight is the one figure you have to supply yourself
| Variable | Where the number comes from | How confident to be |
|---|---|---|
| Reference depth | Dive chart or your own records | Good, if it is for your exact lure and line |
| Line-out exponent | Published trolling data or your own two points | Fair, and easy to calibrate |
| Speed exponent | Mostly judgement | Weak, especially outside the lure's designed range |
| Depth gain per ounce | Only from your own line counter records | Meaningless unless you measured it |
An inline weight adds depth in a way that depends on how far ahead of the lure it sits, how much line is out, and the lure itself, so no default is defensible. The field is set to 2.5 feet per ounce only so the calculator produces something, and if you have not measured it on your own gear the honest thing is to set the weight to zero and use the tool for the line and speed scaling alone.
Calibrating this against reality
Two points beat any exponent. Let out a known length of line over a known bottom depth and note where the lure starts ticking. Do it again at a different line out. Those two depths and the two line lengths let you solve for the exponent that fits your own gear, and from then on the scaling is yours rather than a default. The arithmetic is a ratio of logarithms, but you do not need to do it formally: adjust the exponent field until the tool reproduces both of your measured points, and it will be close across the range between them.
Everything else on this page assumes the boat is tracking straight at a steady speed. It is not, on a turn, and the inside lure rises while the outside lure dives, which is why strikes cluster on turns. A line counter and a lure that has told you where it is beat a formula every time. When the rig has to hold still rather than move, the sinker weight calculator handles the current side, and the fishing line strength calculator covers whether the connection can take what a deep lure loads it with. Local rules on gear and method are covered in the fishing rules guide, since they differ by water and change.
Questions people ask
Why does the calculator not know how deep my lure runs?
Because that is not a calculable quantity from any input a person can reasonably supply. Diving depth comes out of the lip geometry, the buoyancy distribution, the tow point position and the hydrodynamics of the specific body, and lures with identical published sizes and weights can run at wildly different depths. Manufacturers measure their own lures in controlled tests, and there is published trolling data that does the same independently. Either of those, or your own line counter, gives you the one point this tool needs. Once you have that point, the scaling to other line lengths and speeds is ordinary arithmetic.
Does more line always mean more depth?
It means more depth with sharply diminishing returns, and eventually almost none. As line goes out the angle it makes with the surface flattens, so each additional foot adds progressively less vertical drop. Beyond a few times the reference length the curve is nearly flat and you are mostly adding line drag rather than depth. That is the reason the tool warns when you extrapolate more than three times past the reference: the exponent model keeps growing when the real lure has stopped.
Faster or slower to run deeper?
Slower, once the lure is already working. A diving lure reaches its designed depth in a fairly narrow speed band, and pushing past that band lifts it, because the increased drag on the line and the increased lift on the body both act upward. Going too slow is not a free win either, since below some speed the lure stops swimming properly and the depth becomes irrelevant. The speed table in the result shows the direction and rough magnitude, but the useful band is the one where the lure has the action you want.
How do I work out the depth gain per ounce for my own gear?
Run the same lure at the same line out and speed twice, once bare and once with a known weight, over water shallow enough that you can find the bottom by feel or watch the depth on a sounder. The difference between the two depths, divided by the ounces, is your gain figure for that line out. It will not transfer to a very different line length, because the weight and the lure are being carried by the same line angle, so measure it near the line-out you actually fish. Until you have done that, leave the added weight at zero rather than trusting a default.
Does this apply to downriggers or diving planers?
Not really, and that is a good thing. A downrigger puts the bait at a depth you set directly with a heavy ball on a separate wire, and the only correction needed is for blowback, the angle the wire takes under drag, which is a different calculation. A diving planer has its own published depth curve and behaves like a very aggressive lure. The page here is for a lure or bait pulled directly on the fishing line, where the depth is an emergent result of line, speed and the lure rather than something you set.