Critical Swim Speed (CSS) Calculator
Estimate your Critical Swim Speed threshold pace from a 400 m and 200 m time trial — the pace that anchors distance swim training.
How it works
CSS pace per 100 m = (400 m time − 200 m time) ÷ 2 · Speed = 100 ÷ CSS pace
Critical Swim Speed estimates the fastest pace you can hold without fatiguing rapidly — a swimmer’s equivalent of a running threshold. It comes from two flat-out time trials: because the 400 m covers 200 m more than the 200 m does, dividing the time difference by that extra 200 m gives the pace of the sustainable portion, expressed per 100 m. Training sets are then built around this pace, holding it, or a few seconds above and below it, over repeats.
Worked example
A swimmer records 6:00 (360 s) for 400 m and 2:50 (170 s) for 200 m. CSS pace per 100 m = (360 − 170) ÷ 2 = 95 s, i.e. 1:35 per 100 m, a speed of about 1.053 m/s.
Frequently asked questions
How do I run the CSS test?
After a thorough warm-up, swim a maximal 400 m time trial, rest fully, then swim a maximal 200 m. Record both in seconds and enter them here. The gap between the two efforts is what defines your threshold pace.
How is CSS pace different from ordinary swim pace?
A plain swim-pace calculator just divides one swim’s time by its distance. CSS uses two different distances to isolate your sustainable threshold speed, filtering out the fast start that a single short swim would include.
How should I use CSS pace in training?
CSS is the anchor for threshold sets — for example repeats of 100 m or 200 m held right at CSS pace, or "CSS +2s / −2s" to work slightly easier or harder. Retest every 6–8 weeks to keep the pace current as you improve.
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This calculator is for educational purposes. Double-check important results before acting on them.