Swimming calculator

Critical swim speed calculator

Two time trials, one threshold pace. Enter a maximal 400 m and 200 m and get your CSS in seconds per 100 — plus the training paces to write your sets around. Free, instant, no sign-up.

What is critical swim speed?

CSS is the quickest pace you can hold without falling behind on clearing lactate — swimming's version of threshold, and the pace almost every distance set should be written around. It gives you from the pool deck what lab testing charges for, and unlike a heart-rate zone it is a number you can read off the pace clock in the middle of a set.

How this calculator works

Wakayoshi and colleagues (1992) carried the critical-power model of Monod and Scherrer across to swimming and showed that the slope of the distance-time line through two maximal efforts tracks lactate threshold closely. The field-test version, popularised through the Australian Institute of Sport, uses 400 m and 200 m: CSS = (400 − 200) ÷ (T400 − T200). A 6:00 400 and a 2:50 200 leaves 200 m covered in 190 s — 1.05 m/s, or 1:35 per 100 m.

Running the two trials properly

Both swims have to be genuinely maximal or the whole thing collapses. A paced 200 makes your CSS look faster than it is, and every set written from it will be too hard. Warm up properly, swim the 400 first, take ten to fifteen minutes of easy swimming between them, and push off rather than dive-start both so the timing is consistent. Same pool, same day. Retest every six to eight weeks — CSS moves quickly once threshold work is in the programme.

The anaerobic reserve

The second number this test produces is the intercept of the same line: the metres available above CSS before the tank is empty, usually 15 to 25 m for a pool swimmer. A large reserve next to a modest CSS is a sprinter who needs aerobic work; a reserve near zero is either a pure distance swimmer or a sign the 200 was not maximal. It is also what the 1500 m prediction is built from — time = (distance − reserve) ÷ CSS.