What is Critical Swim Speed?

Critical Swim Speed (CSS) is the fastest pace you can hold for a long swim without fatiguing, roughly your threshold pace. It is found by timing a 400m and a 200m effort and dividing the time difference by the distance difference to get seconds per 100m.

How is CSS calculated from two time trials?

CSS pace per 100m equals (400m time minus 200m time) divided by 2. For example, a 6:40 over 400m and a 3:10 over 200m gives (400 minus 190) seconds over 200m, or about 1:45 per 100m.

What do the zone offsets mean?

Each zone is CSS plus or minus a few seconds per 100m. Aerobic endurance swims sit several seconds slower than CSS, threshold sets sit right at CSS, and VO2max and sprint work run faster than CSS, demanding shorter rest.

Why train at CSS instead of a fixed pace?

CSS scales every interval to your individual threshold, so the same session loads two swimmers of different ability equally. As your CSS improves, all the derived paces tighten automatically when you retest.

How often should I retest CSS?

Retest every four to six weeks. Swimming gains can come quickly with technique work, so stale CSS values will leave your threshold and faster sets too easy to drive adaptation.

Swimming Interval Pace Calculator

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Critical Swim Speed turns a single threshold number into a full set of training paces. This calculator takes your CSS per 100m and offsets it to build aerobic, threshold, VO2max, and sprint targets you can swim to in the pool.

How it works

CSS itself is your threshold pace per 100 metres. Each zone is CSS shifted by a fixed number of seconds:

Aerobic / endurance  CSS + 6 s per 100m  (easier, longer reps)
Threshold            CSS                  (held at CSS)
VO2max               CSS − 3 s per 100m   (faster, more rest)
Sprint               CSS − 8 s per 100m   (short, maximal)

Slower paces build aerobic capacity with short rest, while the faster bands demand longer recoveries between reps. Because everything is anchored to your own CSS, the relative effort is the same for any swimmer.

Example and tips

With a CSS of 1:45 per 100m, aerobic reps come out at 1:51, threshold sets at 1:45, VO2max at 1:42, and sprints at 1:37. Build the week around threshold and aerobic volume, adding a small dose of VO2max and sprint work. Keep rest short on the slower bands to keep them aerobic, and lengthen it on the fast bands so you can hold the target. Retest your CSS regularly so the paces keep pace with your fitness.