Sink Rate and Rule Number Calculator Using Archimedes' Principle

This calculator enables accurate determination of fly line diameter, sink rate, rule number, and density. Use it for situations when you are unsure of line diameter and wish to measure that diameter accurately based on Archimedes' principle. If you know line diameter, then you will likely prefer to use our basic calculator or lookup tables for sink rate and rule number. After entering your data in the blue cells below, click Calculate.

Diameter Calculator v1 5.xls

For example, a 9 foot segment of fly line that weighs 103.2 grains above water (show me) and 71.2 grains below water in 70 degree fresh water has an average diameter of 38.7 thousandths of an inch.

This is an exact calculation based on Archimedes' principle, which states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. In this case, the buoyant force is 32 grains (103.2 - 71.2) . The displaced fluid is the volume of the fly line, which is determined by its length and diameter. Given length and buoyant force, the calculator solves for the unique diameter such that an amount of water with the same volume would weigh 32 grains. (Show me equations.)

To measure weight below water, use your same grain scale with a setup like the one below. In this case, we positioned the scale above a tank of water and suspended below it an underwater plastic bag with a weight in it (the weight keeps the bag from floating). The suspension is made from the ends of a pair of taped-together chop sticks using monofilament line. Zeroing out the scale with this setup and then placing the fly line in the submerged bag yields the exact under water weight of the fly line. (In this process, take care to shake any air bubbles out of the bag and off your submerged line).

Once set up, this Archimedes'-principle method for measuring fly line diameter takes about the same amount of time as the calipers method, but is more accurate. It's accuracy is limited only by the accuracy of your grain scale. In this case, the diameter calculation is accurate to about 0.25%, which is 10 to 20 times more accurate than the calipers method.