Trolling Depth Calculator
Estimate running depth for lead core, downriggers and diving crankbaits from the standard rules of thumb, with what actually shifts the answer.
These are the standard rules of thumb — 5 feet per color of lead core at ~2.25 mph, downrigger blowback growing with speed, crankbaits reaching rated depth around 100 feet of line out. Line diameter, current and lure drag all shift real depth; when a bait starts ticking bottom, the lake has given you the exact answer.
The three methods are not the same kind of model
They look parallel and are not. Two are linear in speed with different slopes, and the third does not take speed as an input at all. Over the same change from 1.5 to 3.5 mph:
| Rig | At 1.5 mph | At 3.5 mph | Change |
|---|---|---|---|
| Lead core, 10 colours | 57.5 ft | 37.5 ft | -35% |
| Downrigger, 60 ft cable | 57.8 ft | 48.8 ft | -16% |
| Crankbait, 12 ft rated | 12.0 ft | 12.0 ft | none |
A real crankbait absolutely does run at a different depth at 1.5 mph than at 3.5, so the zero in that last row is a limitation of the model rather than a fact about crankbaits. Worth knowing before you trust the third method as hard as the first two.
And they are calibrated at different speeds
Each formula has a speed where its multiplier is exactly 1.0 — where the rule of thumb was measured, and either side of which is extrapolation. Lead core reads 1.000 at 2.25 mph; the downrigger reads 1.000 at 1 mph.
| Speed | Lead core factor | Downrigger factor |
|---|---|---|
| 0.5 mph | 1.300 | 1.000 |
| 1 mph | 1.250 | 1.000 |
| 1.5 mph | 1.150 | 0.963 |
| 2.25 mph | 1.000 | 0.906 |
| 2.5 mph | 0.950 | 0.887 |
| 3 mph | 0.850 | 0.850 |
| 3.5 mph | 0.750 | 0.813 |
| 4 mph | 0.650 | 0.775 |
| 5 mph | 0.600 | 0.700 |
| 7 mph | 0.600 | 0.550 |
| 9 mph | 0.600 | 0.550 |
Which means there is no trolling speed at which both sit on their anchor. At a routine 2.5 mph the lead core is 5% off its calibration point and the downrigger is already modelling 11% of blowback.
The downrigger also has a quiet floor. Below 1 mph its factor is pinned at 1.000 and slowing further changes nothing — the right shape, since a cable at rest hangs straight down and cannot do better than straight down, but it does mean the speed input is inert across the bottom of its range.
The one limit you can actually hit
All three formulas clamp somewhere. Two of the clamps are unreachable in practice:
| Method | Stops responding | Reachable? |
|---|---|---|
| Lead core | above 4.25 mph and below 0.75 mph | no |
| Downrigger | above 7 mph | no |
| Crankbait | past 156.25 ft of line out | yes |
Nobody trolls at 7 mph. Everybody lets out more than 156.25 feet of line. Past that point the crankbait model is flat — another fifty yards changes the predicted depth by exactly nothing.
| Line out | Depth multiplier |
|---|---|
| 25 ft | ×0.500 |
| 50 ft | ×0.707 |
| 75 ft | ×0.866 |
| 100 ft | ×1.000 |
| 125 ft | ×1.118 |
| 150 ft | ×1.225 |
| 156 ft | ×1.249 |
| 175 ft | ×1.250 — capped |
| 200 ft | ×1.250 — capped |
| 300 ft | ×1.250 — capped |
The square root ahead of the cap is front-loaded. Any doubling of line out multiplies depth by exactly √2 — 25 to 50 feet and 50 to 100 both give 1.414× — while 100 to 150 gives only 1.225×. So line out is a strong control early, a weak one from 100 feet to the cap, and no control at all after it.
How to use
- Choose your method — lead core, downrigger or diving lure.
- Enter line out, speed and weight.
- Read the estimated running depth.
- Confirm with sonar or a temperature probe where you can.
Frequently asked questions
How accurate are these estimates?
Roughly indicative rather than precise. Every rule of thumb assumes particular line diameter, lure drag and speed, and real conditions differ. They get you into the right zone, and sonar or a depth probe is what confirms it.
What is blowback on a downrigger?
The rearward angle the cable takes under drag, which means the ball runs shallower than the counter shows. It grows with speed and with cable diameter, and at higher speeds the error can be substantial — which is why the counter reading is not the fishing depth.
How deep does lead core run?
The common rule is around five feet per colour at moderate speed, but it varies considerably with speed, with the lure's own drag and with the leader. Slower trolling runs deeper for the same line out, which is the largest single variable.
Does line diameter matter?
Substantially. Thinner line has less water resistance, so it cuts down more steeply and the lure runs deeper for the same length out. Switching from mono to braid of the same rating can change running depth noticeably without anything else altering.
Why does speed change depth so much?
Because drag rises sharply with speed, pushing the line and lure toward the surface. A half-knot change can move a lure several feet, which is why holding a consistent speed matters more than most people expect — and why trolling speed is worth measuring rather than estimating.
How do I know I am in the right zone?
Sonar showing fish at a depth, and a temperature probe finding the thermocline, are the two most useful checks. Many species hold at a preferred temperature band rather than a fixed depth, so the thermocline often tells you more about where to fish than the depth reading does.
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