Hangdown Movies

Sunken Line Simulator

Fly Depth Results

Rule Numbers

Fly Depth Formula

Sink Tip Length Formula

Estimating Water Velocity

Fluid Dynamics Principles

Rule Number Lookup Tables


Sink Tip Length Formula

How much sink tip do you need to deliver your fly to a desired hangdown depth?  The calculation is simple based on rule number and water velocity.
Sink Tip Length (ft) = Desired Fly Depth (ft) x Water Velocity (mph) x Rule Number / 3
For example, suppose you are using rule 9 sinking line. You are fishing in 2 mph water and would like to get your fly to a depth of 2.5 ft. According to this formula, you would need to attach 2.5 x 2 x 9 / 3 = 15 ft of this sink tip to your floating head.

Note that this formula is the fly depth formula, but just turned around. So it has the same accuracy: for 3 mph water, it is virtually exact by definition.  Over a wide range other practical fishing conditions (e.g. water velocities from 1 to 5 mph), this formula yields estimates that are accurate to within about 10% of our more-precise simulation-based calculations.