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Critical swim speed (CSS) calculator

Short answer

Critical swim speed is the slope of distance against time across two all-out swims: CSS = (400 − 200) ÷ (T400 − T200). A 6:20 400 m and a 3:00 200 m give 1.00 m/s, or 1:40 per 100 m. In the original study, swimming at that speed held blood lactate steady, which is why swimmers use it to set threshold pace.

Last reviewed 27 October 20262 published sourcesNot medical advice

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Critical swim speed calculator

Swim an all-out 400 m, rest fully, then an all-out 200 m (or the other way round), in the same stroke.

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Enter your 400 m and 200 m times.

Runs entirely in your browser. Nothing you type is saved or sent anywhere.

CSS pace from 400 m and 200 m times
400 m200 mCSS per 100 mSpeed
5:202:301:251.18 m/s
6:203:001:401.00 m/s
7:253:301:580.85 m/s
8:254:002:130.75 m/s
CSS = 200 m ÷ (400 m time − 200 m time).

Where CSS comes from

Critical speed is the slope of the straight line through the distances you swim against the times they take, all out (Wakayoshi et al., 1992). With two swims, 400 m and 200 m, that slope is 200 m divided by the difference in their times.

In eight trained swimmers, the speed from those two swims matched the maximal lactate steady state: at 100% of it, lactate held steady over 1,600 m, while at 102% it kept climbing (Wakayoshi et al., 1993).

Running instead? The pace calculator turns a time into pace, and the race time predictor estimates other distances.

Every swim, next to your recovery

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Common questions

How do I do a CSS test?

Warm up well, swim 400 m as fast as you can hold, rest until you feel recovered, then swim 200 m all out, in the same stroke. Some swimmers do the 200 m first. Enter both times here.

What is a good CSS pace?

It depends on age, training and stroke, so compare yours with itself over a season. A faster CSS after a block of training means your threshold pace has improved.

How do I use CSS in training?

Because CSS sits at about the fastest pace your lactate stays steady, swimmers use it as their threshold pace: repeats at or near CSS on short rest. Retest every few weeks so the pace follows your fitness.

Sources

Figures on this page were checked against these publications on 27 October 2026.

  1. Wakayoshi K, Yoshida T, Udo M, Kasai T, Moritani T, Mutoh Y, Miyashita M. A simple method for determining critical speed as swimming fatigue threshold in competitive swimming. International Journal of Sports Medicine. 1992;13(5):367–371. doi.org/10.1055/s-2007-1021282
  2. Wakayoshi K, Yoshida T, Udo M, Harada T, Moritani T, Mutoh Y, Miyashita M. Does critical swimming velocity represent exercise intensity at maximal lactate steady state? European Journal of Applied Physiology and Occupational Physiology. 1993;66(1):90–95. doi.org/10.1007/BF00863406

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