Trolling puts your lure where the fish swim, but only if you know how deep it really runs. A lure towed behind a moving boat does not hang straight down. Water drag pushes it back and up, and a downrigger ball swings back on its cable like a pendulum. This trolling lure depth calculator models those two effects: the blowback of a downrigger cable and the running depth of a flat line with no downrigger. It turns boat speed, line out, and ball weight into numbers you can set and repeat.
Start with the downrigger, because it is the most precise way to control depth. A heavy ball on a cable carries your lure down, and the cable should hang near vertical when the boat is slow. As speed rises, water drag on the cable and ball grows with the square of speed, so doubling speed roughly quadruples drag. The cable then leans back at a blowback angle, and the ball rides shallower than the cable length. The tool estimates that angle from speed and ball weight, then gives ball depth as cable length times the cosine of the angle.
Here is a worked example. You let out 30 metres of cable with a 5 kilogram ball at 2 knots. The tool estimates a blowback angle near 12 degrees, a ball depth near 29 metres, and a horizontal setback near 6 metres behind the boat. Raise speed to 4 knots with the same ball and the angle grows past 20 degrees, so the ball rises to about 28 metres and trails further back. Swap in a 10 kilogram ball at 4 knots and the angle shrinks again, putting the ball deeper. Weight is your main weapon against blowback.
The second mode covers flat-line trolling with no downrigger. Here the lure itself does the diving, and the maker usually rates it like this: dives to 4 metres on 30 metres of line. Letting out more line takes it deeper, but not in a straight line, because drag grows as more line enters the water. The tool scales the rating with a power of 0.7, the same practical rule used by trolling guides. Speed matters too: faster water flow lifts the lure, so the tool applies a speed factor centred on 2 knots, capped so it never claims too much.
Try a flat-line example. Your lure is rated at 4 metres on 30 metres of line, you let out 60 metres, and you troll at 2 knots. The tool gives about 6.5 metres, not 8 metres, because of the drag curve. Speed up to 4 knots and the same setup gives about 4.9 metres, since the faster flow lifts the lure. That is why two boats trolling the same lure on the same line length can fish very different depths: speed is a depth control, whether you planned it or not.
Several things change real depth beyond this model. Thin braid has less drag than thick mono, so it runs deeper on the same length out. Current can push the lure up, sideways, or down depending on direction. A lure with a big diving lip runs deeper than a slim lure at the same speed, and a fouled lure with weed on the hooks may not dive at all. The tool also assumes the cable and line behave simply, while real drag depends on cable thickness and ball shape. Treat every answer as a starting setting, not a measurement.
Good habits make the numbers useful. Mark your line or count rod pulls so you can repeat a length that produced fish. Change one thing at a time: speed, line length, ball weight, or lure. Watch the rod tip for a steady lure action, and check for weed often, because a fouled lure fishes shallow and dead. When you find fish on the sounder, note speed, line out, ball weight, and lure together, so you can return to the same presentation on the next pass.
In short, this calculator separates the two trolling jobs. With a downrigger it gives blowback angle, ball depth, and setback from speed, cable length, and ball weight. Without one it gives flat-line lure depth from the dive rating, line out, and speed. Use those numbers to set your first pass near the fish, confirm with your sounder, and adjust in small steps. Depth in trolling is not luck; it is a setting you control.