Critical Swim Speed (CSS) Calculator
Enter your all-out 400 m and 200 m times in seconds to get (400 - 200) / (t400 - t200) in m/s, plus the matching pace per 100 m.
CSS (m/s)
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How the CSS test works
Critical Swim Speed (CSS) is the highest pace sustainable predominantly under aerobic metabolism ā the swimming analogue to running threshold pace. Standard protocol: swim 400 m and 200 m all-out (with full recovery between), then apply CSS = (400 ā 200) / (t400 ā t200) in m/s. The result is a pace sustainable for roughly 1 hour. CSS-paced sets (e.g., 10Ć100 with short rest) build aerobic capacity and lactate threshold. Tools like SwimSmooth, Garmin Swim 2, and FORM goggles pace CSS directly.
Example: 400 m in 6:00 (360 s) and 200 m in 2:45 (165 s) ā (400 ā 200) / (360 ā 165) = 1.026 m/s ā 1:37 / 100 m.
Applications
Triathlon (Ironman, Olympic, 70.3), masters swimming, structured pool training, interval prescription, and progress tracking over the season.
FAQ
How often should I re-test? Every 4ā8 weeks of consistent training, or after a structured training block.
Difference from race pace? CSS sits between 1500 m and 5 km race pace ā sustainable about 1 hour, not maximal.
Why 400 + 200? The two distances bracket the aerobic-anaerobic transition; the math extracts steady-state speed from the difference.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.