Swim CSS Calculator
Work out your Critical Swim Speed from a 400m and 200m time trial, then get your endurance, threshold and speed training paces per 100m.
Swim both maximally in the same session, with 10–15 minutes of easy swimming between them.
How this is worked out
Critical Swim Speed is the speed implied by the gap between your 400 m and 200 m times: 200 metres divided by the difference between the two, in metres per second. Because both efforts are maximal, the difference isolates the sustainable part of your speed from the sprint part — which makes CSS a practical proxy for lactate threshold without any lab equipment.
Training paces are fixed offsets in seconds per 100 m either side of CSS. Swimming uses offsets rather than percentages because pool pacing is read off a clock in seconds, not off a percentage table.
Retest every 6–8 weeks, and always in the same pool length — a 25 m pool flatters you against a 50 m one, because turns are faster than swimming.
Information only — not medical advice. These are estimates from population-level formulas, not a substitute for professional assessment. See our full disclaimer.
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