Critical Swim Speed (CSS)
The pace you could theoretically hold in a maximal-effort swim: swimming's equivalent of FTP, used to set swim training paces and zones.
Critical swim speed (CSS) is the pace, usually expressed as time per 100m, that you could theoretically sustain in an all-out swimming effort lasting somewhere around 15 to 30 minutes. It's swimming's direct equivalent of FTP: a single reference intensity that the rest of your swim training zones get built around. Where FTP is measured in watts because cycling has power meters, swimming doesn't have an equivalent tool in general use, so CSS is measured in pace instead, derived from how your speed holds up between two different maximal-effort distances.
How it's actually tested
The standard protocol pairs a 400m time trial with a 200m time trial, usually swum in the same session with a short rest between them, both at genuinely maximal, honest effort. The idea, based on the same critical-power concept used elsewhere in exercise science, is that the difference in distance divided by the difference in time between the two swims isolates your sustainable swimming speed from the extra anaerobic contribution that shorter, harder efforts draw on. The 200m swim finishes faster per 100m than CSS, boosted by anaerobic capacity that can't hold up over the full 400m; subtracting that gap out gives a pace estimate that reflects your aerobic swimming speed specifically.
Why it needs retesting
Like FTP, CSS isn't a fixed number. It improves with training, particularly aerobic swim volume and technique work, and drifts the other way during a break from the pool. Training off a stale CSS number means every swim zone calculated from it is off too, the same problem a cyclist runs into training off an outdated FTP. Retesting periodically, especially after a focused swim block, keeps your pace zones honest rather than anchored to a fitness level you've already moved past.
How it's used in training
Once you have a CSS number, swim sets get built as an offset from it rather than picked by feel: an easy aerobic set might sit several seconds per 100m slower than CSS, while a threshold-focused interval set sits right at it or just above. That structure turns "swim faster in training" into specific, testable target paces the same way FTP turns "ride harder" into a wattage range. For a triathlete juggling all three disciplines, CSS matters most where swim pacing in a race actually gets decided: going out too fast off the start, relative to your own sustainable pace rather than the swimmer next to you, is a common way an otherwise solid swim leg gets more expensive than it needed to be. See the 70.3 training plan guide for how swim pacing fits into a longer-course build overall.