Swimming
Critical Swim Speed (CSS) Calculator
Estimate your sustainable swim threshold pace from 400 m and 200 m time-trials.
Free · instant · no sign-upDeterministic
Result
Critical Swim Speed1:35/ 100 m
Your sustainable threshold pace.
CSS speed1.053m/s
CSS = (400 − 200) ÷ (T₄₀₀ − T₂₀₀). Do both time-trials rested for a meaningful result.
The formula
Critical Swim Speed
CSS = (400 − 200) ÷ (T₄₀₀ − T₂₀₀)
Source: Wakayoshi et al. (1992); popularised by Swim Smooth
What it means
- CSS approximates the pace you can hold before fatigue accelerates — the anchor for threshold sets.
Limitations
- Both trials must be rested and maximal.
- An estimate, not a physiological lactate threshold.
Educational only
This is a deterministic tool: the same inputs always give the same result. It is for learning and planning, not a substitute for coaching or measurement.
Keep exploring
Follow the threads from this tool into the wider knowledge base.