Fly Casting Math From the Archive

This is my answer to one of our Group members who is a big game salt water fly fishing expert (FFF CCI ) about to take the MCCI exam next month.

(If you are interested in the original quiz and answers you can find it at: )

My answer addresses his concern over the amount of mathematics needed to pass this test in view of the various formulae and equations which have appeared in our many Group messages over the past few years.

Also addressed, is his concern over the fact that he uses heavy salt water tackle on an almost daily basis, but will be taking the exam with the designated equipment  (7 wt. system or lower.)

In preparation for your MCCI exam, I wouldn’t worry about the math at all.  You may not be asked any of it .

Some very simple arithmetic is all you need such as:

# The “rule of elevens” : (subtracting the X number of the tippet from 11 and bring it to the thousanths place for the diameter of the material)

Example:  Tippet label is  4X.       11 – 4 = 7  So the diameter is .007 inches.

# The “rule of nines” : (Subtracting the X number of the tippet from 9 to get the approximate pound strength of the material.)

Example:  Tippet label is 5X.    9 – 5 = 4     So the approximate pound test is 4 lbs.
(Of course, with more modern materials, this becomes far less accurate …. just rough approximations.)

# The “rule of fours ( or threes)” :  To approximate the size of your tippet to the hook size of your fly,  divide the hook size by either 4 or 3 .

Example:  Fly hook size is 12.     12 / 4     = 3   (or)     12 / 3    = 4    So I’d consider using a 3X or 4X tippet.

Loop speed related to fly (top) leg and rod (bottom) leg speed :  Add the speed of the fly leg to that of the rod leg and divide it in half:-

Loop speed =  Fly (top) leg speed + Rod (bottom) leg speed /  2

Example:   Fly leg =  80 meters/second.      Rod leg = 20 meters/second

80 + 20    =    100   / 2  =  50 meters/second loop speed.
(You probably will not be asked this one, anyway.)


Since you are primarily a salt water fly fisherman (as I am) I’d not expect you to use the formulae above very often.  As a true Master, however, you need to know how to advise your students who ask you about fly tackle for trout fishing in the mountains, etc.

Some rough, “rules of thumb” for salt water leader material are probably already familiar to you:-  (Even so, I doubt you will be asked this.)  These are some approximations which, of course, will vary with different nylons and brands.

40 Lb. test nylon monofilament = 18.2 Kg. = .64mm diameter = .025 in. diameter.
25  ”     ”      ”            ”            =  11.3 Kg. = .54  ”       ”         = ..021″        ”
20  ”     ”       ”           ”            =   9.0  Kg. = .47 mm    ”        =  ..018″        ”
12  ”     ”       ”           ”            =   5.4 Kg   = .33 mm    ”        =  ..013″        ”


Fly lines:

You already know that fly lines are rated by the grain weight of the first thirty feet regardless of design and excepting some shooting heads.  Only tiny bit of, “math” to consider is that for most lines, every time you subtract 5 to 6 feet by carrying that much less line as you cast you are casting with one Wt. less line.

As you carry more line while casting than the first 30 feet, every time you add 5 to 6 feet you are really casting with one additional weight line.

Let’s use a WF6F long belly line as an example………

Example:  Casting with 30′ of a 6 wt. line  =  6 wt. line  (about 160 grains)
Casting with 40′ of the same 6 wt. line  =  8 wt. line. (about 210 grains)
Casting with 50′ of the same 6 wt. line  =  10 wt. line. (about 280 grains)
Casting with 20′ of the same 6 wt. line  =    4 wt. line.(about 120 grains)

The detailed calculus equations which have appeared in some of the past Group messages over the past few years are strictly the product of physics research of casting parameters.  They have no place whatsoever in teaching students fly casting and, therefore, will NOT appear during your exam!


In the short time you have before your Sept. exam, you should do all the tasks on the exam many times over while striving for perfection each time.  Best to do these with the outfit you will be using at test time.

Going, as you are, from the daily use of heavy salt water tackle to the 7 wt. and a tiny yarn fly will be less of a problem if you keep practicing to perform the tasks with only the minimum power needed.  Some of these tasks require polishing your, “tip casting” techniques so rarely used in the salt.


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