What is the difference between speed through water and GPS speed?

Speed through water (STW) is boat motion relative to the surrounding water and is what most directly affects lure action. GPS normally reports speed over ground (SOG). Current can make the two different.

How does this calculator adjust for current?

For current aligned with the course, it uses SOG = STW + current when the current flows with the boat and SOG = STW − current when the boat heads into it. Slack current leaves SOG equal to STW.

What does a negative GPS result mean?

It is a signed along-track result: the opposing current is faster than the requested speed through water, so the boat would move backward over ground while still moving forward relative to the water. It is not an instruction to enter a negative value on a GPS.

Can this calculate a lure’s running depth?

No. Depth depends on the specific lure, line diameter and length, rig weight, speed, and empirical dive data. Use the manufacturer’s dive chart or measured data for the exact setup.

Does it handle cross-current?

No. Cross-current correction is a vector problem requiring desired track and current direction. This simple calculator is limited to current directly with or against the course.

What trolling speed should I use?

Use the lure or rig manufacturer’s recommendation when available and watch the actual action. Published agency figures on this page are context-specific starting references, not universal optimum speeds.

Trolling Speed Calculator

Enter the speed you want through the water and the along-track current to estimate the GPS speed over ground you need to hold.

Calculate GPS trolling speed

Start with the speed your lure or rig should experience through the water.

Optional sourced preset

Presets are context-specific references, not guaranteed optimums.

Target speed through water

Minimum Target Maximum

Current speed

Current relative to your course

Choose only for current directly along the course; cross-current is not modeled.

Unit

mph knots

Speed unit converter

Miles per hour (mph) Knots Kilometers per hour (km/h)

Enter your through-water target and current, then calculate.

Published speed references

These figures apply to the stated fishing context; they are not universal species optimums.

Context	Published guidance	Source
Recreational Chinook salmon	Under 2 mph	WDFW
Recreational coho salmon	2–4 mph	WDFW
Commercial salmon troll gear	1–4 knots	NOAA Fisheries
Flatline or downrigger	Slower than 5 mph; no wake	NPS

Summary

Enter the speed you want through the water and the along-track current to estimate the GPS speed over ground you need to hold.

How it works

The requested minimum, target, and maximum values are treated as speeds through water.
Slack current uses SOG = STW. Following current uses SOG = STW + current. Opposing current uses SOG = STW − current.
A negative signed result is preserved and explained rather than incorrectly clamped to zero.
Exact definitions of 1 knot = 1.852 km/h and 1 mile = 1.609344 km are used for conversion.

Use cases

Compensate a GPS trolling speed for an along-track river or tidal current.
Translate a lure manufacturer speed range into a speed-over-ground target.
Compare the GPS target when trolling with versus against current.
Convert trolling speeds between mph, knots, and km/h.
Use source-labelled salmon speed guidance as a starting reference.

Frequently Asked Questions

Last updated: 2026-09-26 · Reviewed by Nham Vu