Critical Swim Speed (CSS) Calculator
CSS estimates your sustainable pace using two time trials. Swim a 400m and a 200m (ideally on the same day, both at maximum effort) and enter your times below.
What is Critical Swim Speed?
CSS is the pace you could theoretically sustain indefinitely without your speed dropping, a genuinely useful, standardized number for pacing distance sets rather than guessing. It's calculated from the difference between your 400m and 200m times, since that difference isolates your aerobic pace from the anaerobic burst of a shorter sprint.
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