Sink Rate and Depth Calculator for Fly Lines
A sinking fly line's type number is its nominal sink rate in inches per second, so a Type 3 sinks about 3 to 4 inches per second and a Type 6 about 6 to 7, both measured in still water. In still water, depth in inches is roughly sink rate multiplied by the sink count in seconds: a Type 3 reaches about 3 feet in 10 seconds. Current lifts the line, so in a moderate river current expect roughly half that depth.
Every fish you cannot catch on a dry fly is a depth problem. Fly line makers make it look simple by printing a type number that equals the sink rate in inches per second, and the arithmetic really is that easy in a lake. On a river it is not, because current lifts line, and the difference between the calculated depth and the real one is why so many anglers fish a sinking line six inches under the surface without knowing it.
How deep is my fly actually fishing?
Sink rates are the manufacturer's still-water figures. The current correction is a working estimate, not a physical model, and real depth varies with line length out, line diameter, retrieve speed and wind drift. Use it to choose a starting count, then let the water tell you.
What does the type number on a sinking line mean?
The type number is the nominal sink rate in inches per second. A Type 3 sinks roughly 3 to 4 inches per second, a Type 6 roughly 6 to 7. It is one of the few conventions in fly fishing that says exactly what it means, and it dates from a time when lines were classified by how far they fell in a tank in one second.
Intermediate lines sit outside the numbering and sink at roughly 1.25 to 1.75 inches per second. That sounds slow and is enormously useful: an intermediate keeps a fly a foot or two under the surface on a windy lake where a floating line would be dragged into a bow across the surface.
| Line type | Sink rate (ips) | Depth at 10s | Depth at 30s | Where it belongs |
|---|---|---|---|---|
| Floating | 0 | Surface | Surface | Dries, indicator nymphing, everything shallow |
| Intermediate | 1.25 to 1.75 | 1ft 3in | 3ft 9in | Windy stillwater, shallow bays, coastal flats |
| Type 2 | 2 to 3 | 2ft 1in | 6ft 3in | Shallow lake shelves, slow rivers |
| Type 3 | 3 to 4 | 2ft 11in | 8ft 9in | The all-round sink tip, most river streamer work |
| Type 4 | 4 to 5 | 3ft 9in | 11ft 3in | Deeper river runs, moderate stillwater |
| Type 5 | 5 to 6 | 4ft 7in | 13ft 9in | Deep lake work, big river holding water |
| Type 6 | 6 to 7 | 5ft 5in | 16ft 3in | Fast heavy water, deep steelhead runs |
| Type 7 | 7 to 8 | 6ft 3in | 18ft 9in | Reservoir depths, heavy sink tips |
| Type 8 | 8 to 9 | 7ft 1in | 21ft 3in | Deep trolling depths, specialist stillwater |
Depths above are still-water figures at the midpoint of each rate band. They are the ceiling on what you will achieve, never the typical result in a river.
Why does current stop a line from sinking?
Because water pushes on the whole length of the line, not just the fly. A fly line is a long cylinder with a lot of surface area, and moving water exerts drag along all of it, lifting the whole system toward the surface. The drag force rises roughly with the square of velocity, which is why a modest increase in current speed produces a large loss of depth.
That is the single most useful thing to understand about sinking lines in rivers. In slow water you might see three quarters of the calculated depth. In moderate current, closer to half. In genuinely fast water, less than a third. Buying a faster sinking line does not fix it, because the extra sink rate is fighting the same drag.
How do I use the countdown method?
Cast, let the line land, and count in seconds before starting the retrieve. That count is your depth control and the only variable you can adjust from cast to cast without changing gear.
- Start with a count of 10 and retrieve.
- Add 5 to the count on each successive cast until you touch bottom or snag.
- Subtract 5 from the count that snagged. That is your fishing count.
- Fish that count consistently, and note it. Depth is repeatable, guesswork is not.
The discipline is in counting the same way every time. Most anglers count faster when they are excited, which is exactly when the depth stops being repeatable.
Sink tip or full sinking line?
A sink tip has a sinking front section, usually 10 to 15 feet, joined to a floating running line. It gets the fly down while leaving a floating section you can mend, which is why sink tips dominate river fishing. A Type 3 sink tip is the single most useful sinking line a river angler can own.
A full sinking line sinks along its whole length, which keeps the fly at a constant depth through a long retrieve. That is what stillwater fishing needs and what river fishing does not, because a fully sunk line cannot be mended and drags the fly out of the seam.
What leader should I use with a sinking line?
Short. Three to five feet, and heavier than you would use with a floating line. A long leader lets a lightly weighted fly ride well above the sunk line, so the fly fishes shallower than the line does, which defeats the entire point of the line.
The opposite holds for floating lines and weighted flies, where a longer leader lets the fly sink freely rather than being tethered up by a buoyant line. Size the tippet through the leader and tippet calculator, and check the sink rate chart for the printable version of the table above.
Where to go next
If you are fishing streamers, the floating against sinking line comparison covers which line gets the fly down. For rivers, indicator depth is usually the more important number, and that is covered in how to rig a nymph setup. If you are still choosing a line class, start with the line weight calculator.
Frequently asked questions
How fast does a Type 3 sinking line sink?
Roughly 3 to 4 inches per second in still water, which is exactly what the type number tells you. The convention is that the type number equals the nominal sink rate in inches per second, so a Type 6 sinks around 6 to 7 inches per second and an intermediate line sinks about 1.5. Those are still-water figures measured with no fly attached, and moving water reduces them substantially.
How do I count a sinking line down to the right depth?
Cast, let the line land, then count in seconds before starting the retrieve. Depth in inches is roughly the sink rate multiplied by the count, so a Type 3 at 3.5 inches per second reaches about 3 feet in 10 seconds of still water. Add counts of 5 until you touch bottom or find fish, then subtract 5 and fish that count consistently. The count is the variable you control.
Why does my sinking line not reach the depth the maths says?
Because current lifts it. Published sink rates are still-water figures, and in a river the drag of the water on the whole length of line and leader holds it up. In a moderate current you may see barely half the calculated depth, and in fast water less than a third. Getting deep in current is done with weight close to the fly and a short leader, not with a faster line.
Should I use a sinking line or a weighted fly?
In rivers, weight near the fly wins. A split shot or a heavy bead head sinks the business end while the line stays where you can mend it, which is why nymphing rigs work at all. On stillwater, a sinking line wins, because you want the whole line at a consistent depth so the retrieve stays in the zone rather than climbing back to the surface.
What is the difference between a full sinking line and a sink tip?
A full sinking line sinks along its entire length, which keeps a retrieve at a constant depth and is the standard stillwater tool. A sink tip has a sinking front section, usually 10 to 15 feet, spliced to a floating running line. That combination gets the fly down while leaving a mendable floating section on the surface, which is why sink tips dominate in rivers.
Does the leader affect how fast the fly sinks?
Considerably. A long leader lets the fly ride above the line rather than tracking with it, so the fly fishes shallower than the line does. With a sinking line, shorten the leader to about 3 to 5 feet so the fly stays in the same zone as the line tip. With a floating line and a weighted fly, the opposite is true and length helps the fly sink freely.
How we choose: we compare published manufacturer specifications, line and leader standards, and verified owner reviews across retailers. We do not test gear in person, and every diameter, grain weight and breaking strength quoted here is the manufacturer's published figure unless we say otherwise, so check the spool label before you trust a number. Nothing here is professional instruction. Wading moving water is the real hazard in this sport: wear a snug wading belt, know the water temperature, and turn back rather than push through a crossing you are unsure of.
