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Critical Swim Speed Calculator

Find your critical swim speed (CSS).

Result

1.02 m/s
1:39 /100m

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.

Worked example

A swimmer completes 400 m in 6:12 and 200 m in 2:55. In seconds that is t400 = 372 and t200 = 175. The difference is 372 − 175 = 197 s. CSS = 200 ÷ 197 = 1.0152, shown as 1.02 m/s. Pace per 100 m = 100 ÷ 1.0152 = 98.5 s, which rounds to 99 s, displayed as 1:39 /100m. The swimmer's threshold training pace is therefore about 1 minute 39 seconds for every 100 m.

Try this example →

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.

Sources

About this calculator

Critical swim speed (CSS) is estimated as CSS = (400m − 200m) ÷ (t400 − t200): the extra distance covered in the extra time between a 400 m and 200 m time trial. It approximates the fastest pace a swimmer can sustain aerobically without accumulating lactate faster than they can clear it — the swimming equivalent of a running lactate threshold.

Swim coaches use CSS to set interval training paces, since threshold sets are typically swum at or just above CSS to build aerobic capacity without the fatigue of all-out sprinting, and competitive swimmers retest it every few weeks to track fitness gains and recalibrate their per-100m target pace.

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Comments (1)

  • Editorial team

    Per 100 m the pace is simply half the gap between your 400 and 200 times, which makes the result very sensitive to timing. A two second error in either swim moves the pace by a full second per 100 m. Swim both tests in the same pool with full recovery between them. How many seconds apart were your two swims?

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