Fly Sinking Rate Calculator
This calculator gives a simplified estimate of sinking speed and depth over time from fly weight, fly length, tippet length, casting angle, freshwater temperature and river discharge. It can be used to compare setups for rivers such as the Aaroy River.
This is a theoretical comparison tool, not a measurement of the fly's actual path. Line tension, fly materials, hook shape, turbulence, mending and salinity are not modelled.
How to read the estimate
The result is an estimate produced by one fixed model. It is most useful for comparing inputs while changing one value at a time. It should not be read as the exact depth of a fly in a real river.
What the model includes
- Estimated effective sinking speed: a terminal-speed estimate adjusted for fly weight, the selected angle, tippet length and river discharge.
- Estimated depth over time: speed multiplied by time, sampled every 5 seconds up to 60 seconds.
- Casting angle: the model uses the vertical component of the selected angle, between 45° and 90°.
- Freshwater temperature: used only to approximate the water density applied to the buoyancy and drag calculations.
Important limits
- Simplified shape: the fly is treated as a cylinder with a fixed radius of 2.5 mm.
- Fixed drag: the calculation uses a drag coefficient of 0.8 for every fly.
- River discharge: the selected value is normalised across the calculator's 0–190 m³/s range. It is a comparison factor, not a conversion to local current speed.
- Display cap: 10 metres is a display limit, not the depth of the pool or proof that the fly reaches the bottom.
Using the comparison at the river
Use the result to compare relative changes within the same model. For example, keep the fly and tippet unchanged while comparing two casting angles or river discharges. The model can show the direction and scale of its own estimate, but it cannot confirm the fly's real depth.
- Change one input at a time so the comparison remains meaningful.
- Treat the displayed speed and depth as estimates, not measured values.
- Expect real flies with different materials and shapes to behave differently.
- Read a result of 0.00 m/s as a limitation of the simplified buoyancy model, not proof that the physical fly cannot sink.
For practical context rather than calculated certainty, see low water late season fishing and the salmon fly Kursk. Season-by-season landings from the same river are in the catch charts.