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Swimming CSS Calculator

Calculate your critical swim speed (CSS) from a 400 m and 200 m time and derive your training pace per 100 m.

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Inputs

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Live
CSS from 400/200 TestCritical swim speed from two test times

400 m Time

Test time over 400 m.

min
s

200 m Time

Test time over 200 m.

min
s
Load example

For illustration only – your own calculation is what counts.

Explanation

How Swimming CSS Calculator works

The swimming CSS calculator determines critical swim speed from a 400 m and 200 m test. It describes your approximate threshold pace and serves as a reference for training intensity.

How CSS is Calculated

CSS SpeedCSS = (400 − 200) ÷ (t₄₀₀ − t₂₀₀)

The distance difference of 200 m is divided by the time difference of the two tests (in seconds). The result is a speed in m/s.

Pace per 100 mPace = 100 ÷ CSS

From speed, the familiar pace specification in minutes and seconds per 100 m results.

Practice

Practical examples

For illustration only – your own calculation is what counts.

CSS from 400 m in 6:00 and 200 m in 2:50

A female swimmer completes a 400 m test in 6:00 and a 200 m test in 2:50.

Result: Critical swim speed is around 1.05 m/s, so a CSS pace of about 1:35 min per 100 m.

Pace for 1500 m in 25:00

A swimmer covers 1500 m in 25:00 and wants their average pace per 100 m.

Result: This corresponds to an average pace of 1:40 min per 100 m (speed 1.00 m/s).

Notes

Common mistakes

  1. Swimming test distances unevenly

    Too fast a start on the 400 m distorts the time difference and thus CSS.

    Swim both tests evenly and fully maximal.

  2. Swimming both tests consecutively without recovery

    Without sufficient rest, the second time is too slow due to pre-fatigue.

    Plan sufficient recovery or distribute tests over two days.

FAQ

Frequently asked questions

What is critical swim speed (CSS)?

CSS is the highest speed you can theoretically maintain for an extended time without progressive fatigue. It corresponds approximately to your threshold and serves as an anchor pace for training control.

How is CSS calculated?

From two time tests: CSS speed = (400 − 200) / (Time over 400 m − Time over 200 m). The distance difference (200 m) is divided by the time difference. The result is a speed in meters per second.

Why exactly 400 m and 200 m?

The two distances are long enough to predominantly represent aerobic endurance and short enough to swim them fully when rested. The time difference describes how much pace drops with distance.

How do I perform the test correctly?

Swim both distances as evenly and maximally as possible, ideally with sufficient recovery in between or on different days. Uneven pacing or too little rest distorts the result.

How do I use CSS in training?

CSS pace is suitable for threshold intervals. Somewhat slower (CSS + a few seconds per 100 m), you train base endurance, significantly faster anaerobic capacity. The calculator shows you CSS pace as a starting point.

How accurate is the value?

CSS is a good estimate but not a laboratory threshold. Daily form, technique, pool length, and pacing influence the result. Repeat the test regularly to track progress.

Limits

Limitations

  • CSS is an estimate of threshold pace, not a laboratory-measured threshold.
  • Pool length, technique, daily form, and pacing influence test times.

Sources

Sources and references

  • Wakayoshi, K. et al. (1992): Determination and validity of critical velocity as an index of swimming performanceEuropean Journal of Applied Physiology 64:153–157

    Establishes critical swim speed as a performance index from two distance tests.

    Retrieved: 09/23/2026 · Verified on: 09/23/2026
  • Ginn, E. (1993): Critical Speed and Training Intensities for SwimmingAustralian Sports Commission

    Popularized CSS from 400 m and 200 m test for swimming training control.

    Retrieved: 09/23/2026 · Verified on: 09/23/2026