What is Critical Swim Speed?
Critical Swim Speed (CSS) is your threshold pace in the pool — the fastest pace you can sustain for an extended duration without accumulating excessive fatigue. It is the swimming equivalent of FTP in cycling.
CSS is calculated from two time trials: a 400-meter and a 200-meter all-out effort. The formula is: CSS = (400 minus 200) / (T400 minus T200), where T400 and T200 are the times in seconds. This gives you a pace in meters per second, which converts to a time per 100 meters.
Once you know your CSS, you can set swim training zones just as cyclists use FTP for power zones. Endurance sets are done below CSS, threshold sets at CSS, and VO2max intervals above CSS. Tracking CSS over time is the simplest way to measure swimming fitness improvement.