Fly fishermen and manufacturers have long used sink rate (also called "type") as a standard by which to compare sinking lines. Simply, sink rate is the speed at which a straight, horizontal section of line sinks in still water. For example, a type 3 sink tip sinks at 3 inches per second; a type 6 sink tip sinks at 6 inches per second.
The manufacturers with whom we have spoken (Airflo, Rio, and Scientific Anglers) all measure sink rates the same way. They drop a short (1"2") piece of line in a tank (or tube) of water in a laboratory setting, and use sophisticated laser technology to determine the exact amount of time required for the line to sink a given distance. The advantages of this method are that it's simple, transparent, and reproducible (doing the same test multiple times for the same line segment yields nearly identical results). However, we have found that the sink rates determined by this method overstate the true sink rate of a longer section of line  as would be used for fishing. We are grateful to Bruce Richards of Scientific Anglers (SA), who helped us test this in a sensible way. In his labs, Bruce measured the sink rates of 1"  2" lengths of three of SA's sink tips. He then shipped these same three sink tips to us. We meticulously measured the sink rates of long (10 ft) segments of these lines in a stillwater swimming pool. We found that the long segments sink more slowly than the short ones by 8% to 17%.
That short line segments sink faster than long ones is also predicted by fluid dynamics theory. The principle is the same in the design of aircraft wings, where theory and experiment have shown that very long, thin wings provide more lift than shorter ones. For short line segments, water flows rapidly around the ends of the line, reducing the vacuum on the high side of the line that contributes to its drag (...more on why short segments sink faster).
For long (e.g. 10 ft) line segments, this "flowing around the ends" effect is negligible. In actual fishing situations, it is totally negligible because both ends of the line are connected to another line. So, while the manufacturers' test is simple and reproducible, their measurement method itself leads to overestimation of sink rates. Jim Havstad independently reached this same conclusion  that short line segments sink faster than long ones  in his seminal study of fly line sink rates.
In addition, we have noticed that in some (but not all) cases, manufacturers "type" designation deviates further from actual (long segment) sink rates than the difference in measurement methods would suggest. For example, a 109 grain, 15' tip we tested sinks at 5.6 inches per second, even though its label says "Type 8."
To support more accurate determination of sink rates, we explain the science of sink rate. Multiple independent experiments have validated the accuracy of it's predictions within a few percent  over many years, over many line types, and across several different experimenters. The results are simple sink rate lookup tables and also a sink rate and rule number calculator, which we publish here for the first time. For any given line diameter and grain weight, you can simply look up its sink rate in a table or calculate it with an online calculator. Such calculations are exact, in the sense that they are determined entirely by known laws of fluid dynamics and known physical constants. (Working through the sink rate equations is not for the faint of heart, but we provide them for those who wish to understand the theory or to program their own sinkrate calculator.)

