Swim CSS (Critical Swim Speed) Calculator
CSS = (400m time − 200m time) / 200, in seconds per metre. Per Salo et al. — the swim equivalent of FTP.
Result
CSS (per 100 m)
1:45
1.050 sec/m · valid only when 400 time > 200 time.
- 200m time3:00
- 400m time6:30
- t400 − t200210 s
- CSS (sec/m)1.050
- CSS / 100 m1:45
- Recovery zone (~CSS+15)2:00/100m
- Aerobic (~CSS+8)1:53/100m
- Threshold (CSS)1:45/100m
- VO2 (~CSS−5)1:40/100m
Step-by-step
- t200 = 180 s; t400 = 390 s.
- CSS = (t400 − t200) / 200 = 210 / 200 = 1.050 sec/m.
- Per 100 m: CSS × 100 = 105.0 s = 1:45.
How to use this calculator
- Swim an all-out 400m, fully rested.
- Rest 30+ minutes.
- Swim an all-out 200m.
- Plug both times in.
About this calculator
CSS (Critical Swim Speed) is the swim analog of running's critical pace or cycling's FTP — the highest steady-state pace before lactate accumulates faster than it clears. Salo et al. defined it as (t400 − t200) / 200 sec/m: do an all-out 400m, rest fully, do an all-out 200m, plug in. A swimmer hitting 6:30 / 3:00 has CSS = 1:45/100m. Train with CSS-based intervals: aerobic at +8s, threshold at CSS, VO2 at −5s.
Frequently asked
200m is more anaerobic than 400m — the difference between them isolates the aerobic component, giving you a stable threshold pace.
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