How to do a CSS swim test and use your critical swim speed

Critical swim speed (CSS) is the closest thing swimmers have to a runner’s threshold pace: roughly the fastest pace you can hold for a long, continuous swim without falling apart. Knowing it turns vague sets like “swim hard” into specific target times — and you can find it with two time trials and a stopwatch.

Where CSS comes from

CSS is based on the idea of critical velocity, which Wakayoshi and colleagues applied to swimming in 1992. When you plot distance against time for a few all-out swims, the points fall on a nearly straight line. The slope of that line is a speed you can theoretically sustain for a long time. The popular field version uses just two points: a 400 and a 200.

How to run the test

1. Warm up well — around 10–15 minutes of easy swimming, drills and a few short build-ups.
2. Swim 400 all-out from a push start (no dive). Pace it evenly: going out too fast is the most common mistake. Note the time.
3. Recover fully with 5–10 minutes of easy swimming.
4. Swim 200 all-out from a push start. Note the time.

Use the same pool length and stroke (usually freestyle) each time you test, and ask a friend to time you if you can.

Calculate your CSS

CSS pace per 100 = (400 time − 200 time) ÷ 2

With a 6:00 400 and a 2:50 200: (360 − 170) seconds ÷ 2 = 95 seconds, so your CSS is 1:35 per 100. More examples:

Your 400 should take a little more than twice as long as your 200. If it takes less than about 1.8 times as long, one of the swims probably wasn’t all-out. Retest before relying on the result.

Using CSS in training

CSS is most useful as the anchor for threshold sets: repeats at or just around CSS pace with short rests. A typical example is 8–10 × 100 at CSS pace with 10–15 seconds’ rest. Easier aerobic swimming sits several seconds per 100 slower than CSS, and short, fast repeats sit faster.

• If you can’t hold CSS pace to the end of a threshold set, the rests may be too short.
• If it feels easy every time, your CSS has probably improved — time to retest.
• Retest every 6–8 weeks, or after a block of focused training.

A 400/200 test tends to give a CSS a little faster than your true pace for a 30-minute or longer swim, so for open-water races and long swims, set your target slightly slower.

General fitness information, not medical advice. Check with a doctor or qualified coach before major changes to diet, supplements or training.