Trolling Lure Depth Calculator: Lead-Core Colors, Speed and Depth
Estimate how deep a lead-core trolling rig runs from the colors out and your speed, or how many colors you need to reach a target depth, with speed in mph, km/h or knots, a depth-by-colors table and a setting you can calibrate to your sonar.
How it works
Depth ≈ colors × 30 ft × sin φ · φ = line angle, from the towed-line balance cos φ ÷ sin² φ = C × (speed ÷ 2 mph)², with C set so that sin φ = (depth per color at 2 mph) ÷ 30 · Colors for a target = target depth ÷ depth per color, rounded up to the next half color
Lead-core line changes color every 10 yards (30 ft), so trollers can count how much is out and repeat a depth that caught fish. The usual starting rule for 18 lb lead core with a small spoon or stickbait is about 5 feet of depth per color at about 2 mph. Speed matters because the water pushes harder on the line as you go faster: drag grows with the square of speed while the line’s weight stays the same, so the line planes up to a shallower angle. The calculator uses the standard balance for a heavy towed line (weight × cos φ = drag × sin² φ) to work out the line angle at your speed, calibrated to the depth per color you enter at 2 mph. For normal trolling speeds this is close to “depth is inversely proportional to speed” (2.5 mph runs about 20% shallower than 2 mph), but it never lets the line hang deeper than it is long at very slow speeds. Use “Colors for a target depth” to work backwards from the depth where your sonar marks fish. The leader adds line length but little depth, so it is only counted in the total line out. Treat the answer as a starting point: lure drag, diving plugs, flashers, current and the speed over the water (not GPS speed over ground) all shift the real depth. Calibrate by touching bottom at a known depth and adjusting the depth per color under More options.
Worked example
Five colors (150 ft) of lead core at 2 mph with the 5 ft-per-color rule: at the reference speed sin φ = 5 ÷ 30 = 0.1667, so the line runs at 9.6° and each color drops 5 ft; 5 × 5 = 25 ft deep. Speeding up to 2.5 mph raises the drag factor to 1.5625 × and the depth per color falls to 4.0 ft, so the same 5 colors run about 20 ft deep. To reach 25 ft at 2.5 mph you would need 25 ÷ 4.01 = 6.2, rounded up to 6.5 colors (about 26 ft).
Sources: Wikipedia: Fishing line — lead-core and copper trolling lines · Wikipedia: Trolling (fishing) — speeds and depth control
Frequently asked questions
How many feet is one color of lead core?
Each color is 10 yards (30 feet, about 9.1 m). A full spool of lead core usually has 18 or 27 colors, often with a half or partial core used on smaller reels.
How deep does lead core run per color?
A common rule is about 5 feet per color at about 2 mph for 18 lb lead core with a light lure, so 5 colors run near 25 feet. Heavier cores and slower speeds run deeper; faster speeds and lures with more drag run shallower.
Why does speed change the depth?
Faster trolling creates more drag on the line and lure, which planes the whole rig upward. Drag rises with the square of speed, so a small speed change makes a noticeable depth change. Slowing down lets the lead sink, so the same colors run deeper.
Should I use GPS speed or speed through the water?
The depth depends on speed through the water at the lure. GPS shows speed over the ground, which differs when there is current or wind drift; a probe at depth or a paddle-wheel speedometer gives the more useful number.
Does the leader change the depth?
Only a little. A mono or fluorocarbon leader is light and runs at a shallow angle behind the lead core. A long leader (50–100 ft) mainly puts distance between the lead core and the lure so fish are not spooked.
Is 5 feet per color exact?
No, it is a field rule, and anglers report anywhere from about 5 to 7 feet per color depending on the core, lure and speed (slower trolling, around 1.5 mph, runs nearer 6–7 ft). Line brand, lure type, current and how the boat is moving all matter, so check your depth with sonar or by lightly touching bottom at a known depth and adjust the depth per color in More options.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.