Swim CSS (Critical Swim Speed) Calculator
CSS = (400m time − 200m time) / 200, in seconds per metre. Per Salo et al. — the swim equivalent of FTP.
Result
General calculation reads
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How to use this calculator
- Swim an all-out 400m, fully rested.
- Rest 30+ minutes.
- Swim an all-out 200m.
- Plug both times in.
About this calculator
CSS (Critical Swim Speed) is the swim analog of running's critical pace or cycling's FTP — the highest steady-state pace before lactate accumulates faster than it clears. Salo et al. defined it as (t400 − t200) / 200 sec/m: do an all-out 400m, rest fully, do an all-out 200m, plug in. A swimmer hitting 6:30 / 3:00 has CSS = 1:45/100m. Train with CSS-based intervals: aerobic at +8s, threshold at CSS, VO2 at −5s.
Frequently asked
Why two distances?+
What if I'm off pace?+
Why not just FTP-style 30-min test?+
Is CSS race pace?+
How often to retest?+
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