Fly Depth Formula While wading (and waiting for that steelhead take!), have you ever wondered how deep your fly is at hangdown?  Here's a simple way to calculate it based on sink tip length, water velocity, and rule number. Fly Depth (ft) = 3 x Sink Tip Length (ft) /  (Water Velocity (mph) x  Rule Number) For example, suppose you are using 12-feet of rule 6 sink tip in 2 mph water. Then at hangdown, your fly is 3 ft deep, calculated as 3 x 12 / (2 x 6). For 3 mph water, this formula is virtually exact.  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 simulator-based calculations.