Critical Swim Speed Calculator

How it is calculated

You enter two time trials as plain seconds: t400 for 400 m and t200 for 200 m. The speed is the extra 200 m divided by the extra time: CSS = 200 ÷ (t400 − t200) in metres per second. The pace is 100 ÷ CSS seconds per 100 m, rounded to the nearest whole second and shown as minutes:seconds. The 400 m time must be longer than the 200 m time and the 200 m time must be above zero, otherwise nothing is shown. Because both times enter as a difference, a small timing error of a second or two shifts the pace noticeably. The tool cannot read times typed as 6:12; convert minutes to seconds first.

Common mistakes

• Typing times as 6:12 or 6.12. The fields take seconds, so write 372; a value with a colon returns nothing, and 6.12 would be read as just over 6 seconds.
• Swapping the two trials. The 400 m time must be the larger number, or the tool returns no result.
• Using trials done on different days or after different warm-ups. The method assumes both are maximal efforts in similar condition; a weak 200 m inflates the difference and lowers CSS.
• Forgetting that the pace is per 100 m. Pace is rounded to a whole second per 100 m, so it is not the 200 m or 400 m split.
• Treating the result as exact. The pace on the display is rounded to the second, and a one-second error in either trial changes the speed, so retest rather than trust small differences.