CSS Swim Calculator
Threshold pace from a 400/200 testRuns locally · nothing uploaded
Pool unit
CSS pace /100m
1:57.5
Critical speed
0.851 m/s
Training paces
| Intensity | /100m | /50m | /25m | Sample set |
|---|---|---|---|---|
| Endurance CSS +8s | 2:05.5 | 1:02.8 | 0:31.4 | 400–800 repeats, 20–30 s rest |
| Tempo CSS +4s | 2:01.5 | 1:00.8 | 0:30.4 | 4 × 300, 20 s rest |
| CSS / threshold CSS | 1:57.5 | 0:58.8 | 0:29.4 | 10 × 100, 10–15 s rest |
| Hard CSS -3s | 1:54.5 | 0:57.3 | 0:28.6 | 8 × 50, 30 s rest |
Also works as:critical swim speed calculatorcss paceswim threshold paceswim training zones
How it works
Formula
CSS = (400 − 200) ÷ (T₄₀₀ − T₂₀₀)Meters (or yards) per second.
CSS pace per 100 = (T₄₀₀ − T₂₀₀) ÷ 2Worked example
Given: 400 m in 7:20 (440 s), 200 m in 3:25 (205 s)
- Difference = 235 s.
- CSS pace = 117.5 s (1:57.5 per 100 m).
- CSS = 200 ÷ 235 ≈ 0.851 m/s.
Result: Threshold 100s at about 1:57–1:59.
Sources
- Wakayoshi K, et al. Eur J Appl Physiol. 1992;64(2):153–157.
How to use
- 1After a warm-up, swim 400 all out; rest fully, then swim 200 all out.
- 2Choose the pool unit and enter both times.
- 3Read your CSS pace and the per-100/50/25 splits for each zone.
FAQ
What is CSS?
Critical Swim Speed: roughly the fastest pace you can hold for about 30 minutes, close to your swimming threshold.
How often should I retest?
Every 4–6 weeks.
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