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Critical Swim Speed (CSS): the 400 m and 200 m Test

If you have trained in a pool for a while, you will have heard of CSS. It is one number, worked out from two swims, that tells you what pace to use for many of your sets. Here is what it is, how to measure it and how to use it.

What CSS is

CSS stands for critical swim speed. It estimates the speed you can hold for a fairly long time before fatigue climbs steeply. It comes from the critical speed idea in exercise physiology, adapted to swimming with a practical test: two distances and one calculation.

A caution: CSS is an approximation, not a lab measurement. It usually corresponds to a hard but sustainable pace over a few tens of minutes, and it differs from person to person. Use it as a training reference, not as a verdict.

How to run the 400 m and 200 m test

You need a pool, a stopwatch or pool watch and, ideally, someone to time you. Always test in the same pool length, 25 m or 50 m, so results can be compared over time.

  1. Warm-up. About 400-600 m easy with a few short accelerations, then a couple of minutes to recover.
  2. 400 m. Swim 400 m freestyle at the fastest pace you can hold evenly. Do not go out too fast: even pacing gives a more reliable time. Record the time (T400).
  3. Rest. Take a full recovery, swimming gently or standing at the wall. The exact length varies between coaches: 5 to 10 minutes or more is commonly suggested. The point is to start the second swim rested.
  4. 200 m. Swim 200 m the same way, as fast as you can while staying even. Record the time (T200).
  5. Cool-down. A few easy lengths.

The test is hard. If you cannot yet swim 400 m continuously, or you have health concerns, it is not for you right now.

The formula

With times in seconds:

CSS pace per 100 m = (T400 - T200) / 2

The reasoning is simple: the difference between the two times is how long the extra 200 m took. So your speed is 200 metres divided by that difference, and the pace per 100 m is half the difference.

Worked example

Say you swim the 400 m in 7 minutes 20 seconds (440 seconds) and the 200 m in 3 minutes 30 seconds (210 seconds).

  • Difference: 440 - 210 = 230 seconds
  • Divided by 2: 230 / 2 = 115 seconds
  • Your CSS is 115 seconds per 100 m, or 1:55 per 100 m

If you would rather not do the sums, use the CSS calculator.

Using CSS in training

In general terms, CSS is a reference for choosing repeat paces:

  • Repeats at CSS pace. Sets of 100 m, 200 m or longer swum at CSS pace with short rests (for example 10-20 seconds). This is hard aerobic work, close to race feel.
  • Slower than CSS. For base volume, long sets and technique, swimmers usually stay a few seconds per 100 m slower than CSS pace.
  • Faster than CSS. For short repeats (25, 50 m) with longer rests, you go faster than CSS pace.

How many seconds slower or faster, and with what rests, depends on the coach and the goal: there is no universal table. If you have a coach, ask them. Either way, if you cannot finish a set at CSS pace, either the starting pace is too ambitious or the test was not accurate.

How often to retest

Many coaches retest every 4-8 weeks or so, or at the end of a training block, to update paces. If your fitness changes quickly (you are new to training, or coming back after a break) you may check sooner. Always retest under the same conditions: same pool, same time of day, same freshness.

Common mistakes in the test

  • Going out too fast in the 400 m. If your first 100 m is much quicker than your last, the time does not reflect an even effort and the CSS comes out wrong.
  • Too little rest. If you start the 200 m tired, the time suffers and the calculated pace comes out slower than your real one.
  • Changing conditions between tests. A 25 m pool one time and a 50 m pool the next, or testing at the end of a hard day: comparisons lose their meaning.
  • Poor timing. An error of two or three seconds in one time moves the CSS pace by one or two seconds per 100 m, because the difference is halved. Use the same timing method every time.
  • Treating the pace as an obligation. Tired or unwell days happen: on those days it is normal for CSS pace to feel harder.

Limits to know about

CSS rests on only two data points. Uneven pacing, timing errors or holding back in the 200 m will skew the result. And a number on a page does not replace how the swim feels: if a pace feels too hard or too easy, talk to whoever coaches you.

A plan that adapts

SwimSensei builds a plan from your level, the days you train and your pool length (25, 33 or 50 m), and adapts it every week. See how it works on the SwimSensei for swimmers page and join early access.

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