Fly Fishing Devon: Instruction & Guiding on Dartmoor Rivers

  • Balancing rod, line and tippet

  • Fly rods are designed to cast a particular weight of fly line

  • But sometimes it helps to 'overline' a rod

  • Fly size determines tippet diameter

How to select an appropriate weight (i.e. AFTM number) of fly line

Fly rods are designed to cast a particular weight of fly line. The weight of a fly line is described by an AFTM number. This number is printed on the rod just above the handle. The AFTM number is based on the weight of the first 30 feet of the fly line. These numbers range from #0 (the lightest line) to #13 the heaviest line


  • lighter lines (AFTM#2-5)are used to cast flies tied on small hooks (sizes 14 to 23 - a tiny hook)
  • heavier lines (AFTM#6-7 or 8) are used to cast larger or heavier flies
  • On Westcountry rivers we use AFTM#4 or #5 weight fly lines to cast flies tied on size 15 to 19 hooks
  • On local reservoirs /still waters we use AFTM#6-7 weight lines to cast flies tied on larger hooks

Should I 'overline' my fly rod?

Overlining means using a flyline that is heavier (has a greater AFTM# rating) than that recommended by the manufacturer. Do not use a flyline greater than one size above the manufacturer's recommended line rating. It may damage the rod.

Fly rods are designed to cast a particular weight of fly line. The rod loads or bends most effectively when used with this weight of  line. With experience you will 'feel' a 'sweet spot' when you are casting with an appropriate length of fly line outside the rod tip.

The weight of a fly line is described by an AFTM number. The AFTM number is based on the weight (in grains ) of the first 30 feetof the fly line.

Consequently, if you know that you will be consistently making short casts with less than 30 feetof fly line outside the rod then it can help to overline the rod. The shorter length of a heavier fly line compensates for the loss of weight in the lighter fly line.

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Now for the "geeky" bit.

The table below shows that there is not a precise relationship between weight and a particular AFTM number. For example, a line can be described as AFTM#5 if the the first 30 feet weighs between 134 and 146 grains (There are approximately 475 grains to the ounce).

Incidentally, this is probably the source for the constant debate between anglers on what brand of line works best on a particular model of fly rod.

When I am buying a fly line I check the lengths of the front taper and line belly. The relative lengths of these parts of the line can affect their casting characteristics especially when making short casts.

Here is the profile of a Snowbee XS Weight Forward Floating line.

AFTMA Standards
Line # Wt (grains) Range (grains)
1 60 54-66
2 80 74-86
3 100 94-106
4 120 114-126
5 140 134-146
6 160 152-168
7 185 177-193
8 210 202-218
9 240 230-250
10 280 270-290
11 330 318-342
12 380 368-392
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What diameter / breaking strain of tippet should I buy?

This depends on the size, weight and bulkiness of the fly tied on the end of your leader - the tippet section.


  • Small flies are tied onto thin tippets
  • Large, heavy or bulky flies need thicker tippets

If you tie a big or heavy fly onto a thin tippet, it may break off during casting.

A thin tippet may not 'turn over' a bushy fly leading to poor presentation - the fly lands in the middle of a heap of tippet

It is difficult to pass a thick tippet through the eye of a small fly, and a thick tippet may interfere with the behaviour of a small fly.

This is about to get very "geeky" so I might as well  give you the bottom line before your eyes begin to glaze over!

  • For everyday river work I would use nylon monofilament with a breaking strain of 3 to 4lb
  • For day-to-day stillwater fishing I would use nylon monofilament with a breaking strain of 6 to 7lb

Now for another "geeky" bit - you really don't need to know this stuff unless you intend to subject yourself to cross-examination as part of a game angling instructors examination !

Tippet diameter is often described by using the X rating system. The X scale runs from 008X abbreviated to 8X (a very small diameter tippet) to 000X abbreviated to 0X a much thicker tippet. Matching tippet diameter to fly size is important for proper presentation of the fly.

How to calculate the size of tippet to use with a fly - the "Rule of 3"

  • Divide the hook size by 3 to determine the appropriate tippet diameter on the X scale
  • For example, use 6X tippet with a size 18 fly (18/3=6)

(Unfortunately there is no standard way of representing hook size. Sizes vary between hook manufacturers. )

Sometimes the X value is not printed on a spool of tippet material. In that case the diameter of the material will be printed on the spool.

Here is a way to convert X to diameter in inches - the "Rule of 11":

  • Subtract the X (000X) value from .011 inches.
  • For example, 006X subtracted from .011 gives a diameter of .005 inches
  • For example, 005X subtracted from .011 gives a diameter of .006 inches
  • Remember that 006X is normally abbreviated to 6X, and 005X is abbreviated to 5X
  • A 3X (003X) leader has a diameter of .008 inches (i.e. 011-003=008)

Here is how to convert tippet diameter in inches into an X value:

  • Subtract tippet diameter in inches from 0.011
  • Thus tippet with diameter of .004 inches is 7X (i.e. 0.011-0.004= 0.007 or 7X)

How to calculate breaking strain of X rated tippet - the "Rule of 9":

  • Subtract the X number from 9 to get the strength of nylon monofilament tippet.
  • For example, a 5X leader has a breaking strain of 4 pounds (9-5=4)
  • 2X tippet has a breaking strain of 7 pounds (9-2=7)

Bear in mind that putting a knot into nylon monofilament reduces the breaking strain of the line. Therefore do not expect 3lb test line to break at exactly 3lb.

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This table summarizes the relationship between:

  • hook size
  • tippet size (X)
  • tippet diameter (inch)
  • breaking strain (pounds)
  • line weight (AFTM#)

Unfortunately there is no standard way of comparing "hook size" between manufacturers.

The author Datus Proper commented "A true size 14 weighing .18 grains and testing three pounds would make more difference at the trout's end than any amount of miracle fibers in fly rods."

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If you use the clinch knot to tie your fly to the tippet (line at end of the tapered leader), five turns will hold with materials down to 4X.

But with smaller diameter tippets, five turns does not give a knot with sufficient bulk. Use the following X+2 rule with small diameter tippets.

Tippet X rating Clinch knot turns
1X 5 turns
2X 5 turns
3X 5 turns
4X 5 turns
5X X+2=7 turns
6X X+2=8 turns
7X X+2=9 turns