Knowing how deep your fly is running is the key to streamer and stillwater success. This calculator turns a sinking line’s inches-per-second (IPS) rate and your countdown time into a depth estimate, then adjusts for how fast you retrieve.
Understanding fly line sink types
Sinking fly lines are rated by how fast they descend through still water, measured in inches per second (IPS). The sink-type number is an approximation of this rate, though there is some variation between manufacturers:
| Sink type | Approximate IPS | Typical use |
|---|---|---|
| Type 2 | ~2 IPS | Shallow stillwater, swing presentations |
| Type 3 | ~3 IPS | Intermediate depths, rivers |
| Type 4 | ~4 IPS | Most stillwater streamer fishing |
| Type 5 | ~5 IPS | Deeper still water, faster currents |
| Type 6 | ~6 IPS | Saltwater, fast-sinking stillwater |
| Type 7 | ~7–8 IPS | Deep lake trolling, heavy currents |
Full-sink lines (where the entire line sinks) reach a specific depth after a countdown period. Sink-tip lines (where only the front section sinks) behave differently because the floating body limits how deep the tip can go — this calculator models full-sink lines.
How it works
Static sink depth grows linearly with the countdown, and the retrieve lifts the fly back toward the surface:
staticDepth (ft) = (IPS × countdownSeconds) / 12
liftFactor = retrieveSpeed scaled from 0 (dead-drift) to ~0.5 (fast strip)
fishingDepth = staticDepth × (1 − liftFactor)The longer you count down, the deeper the line; the harder you strip, the more the fly planes back upward. A dead-slow retrieve fishes the full countdown depth; a fast strip can reduce fishing depth by 40–50%.
Worked example and the countdown method
A Type 4 line (4 IPS) counted down 30 seconds reaches a static depth of 4 × 30 / 12 = 10 feet. With a moderate strip retrieve (lift factor ≈ 0.25), the fly actually fishes at around 7.5 feet.
The countdown method turns this into a repeatable searching technique: cast, let the line sink for a set count, then retrieve. If you get no response, add five seconds to the next count and try again. When you get a strike, note the count and repeat it exactly — you have found the depth where fish are holding. This calculator tells you what that count translates to in feet, and helps you choose whether a different line type would reach your target zone faster.
Factors that change the real fishing depth
Current. A moving river keeps the line from sinking vertically. Across-current presentation results in shallower actual depths than the countdown predicts.
Leader buoyancy. A long floating leader can hold the fly up significantly relative to the sinking fly line. Keep the leader short when fishing deep, or use a monofilament leader that sinks.
Fly weight and drag. A heavily weighted fly digs deeper than the line calculates; a large, high-drag fly or deer-hair streamer fishes shallower because water pressure resists its sink.
Line belly sag. At long distances, the line develops a belly before it sinks fully, which reduces the depth the tip reaches. The estimate is most accurate for medium-distance presentations.