About the CSS Swimming Calculator

Critical Swim Speed (CSS) estimates the fastest pace a swimmer can sustain aerobically, computed from two maximal time trials: CSS = (D₂ − D₁) ÷ (T₂ − T₁). It is a stronger predictor of open-water and distance performance than a single speed test.

Training by CSS

Endurance sets swim at CSS + 5–8 s per 100 m, threshold work sits at CSS itself, and speed work is faster. Retest every 4–6 weeks to keep paces honest as fitness improves.