Swim pace and CSS calculator

What pace per 100 m tells you

Swimmers measure speed in minutes and seconds per 100 metres because pools are swum in laps of that length. A pace of 1:40 per 100 m means each 100 m lap takes 1 minute 40 seconds. US pools are often 25 yards rather than 25 metres, so pace per 100 yards is the common unit there, which is why the first tab calculates both.

The formula is pace (s/100 m) = time (s) / distance (m) × 100. For 100 yards the result is multiplied by the conversion factor 1 yard = 0.9144 m.

Projecting onto other distances

The projection table shows how long 400 m, 750 m, 1.5 km, 1.9 km and 3.8 km would take at the same pace. These are the standard triathlon swim distances: 750 m for sprint, 1.5 km for Olympic distance, 1.9 km for half distance and 3.8 km for full distance. The calculation assumes a constant pace without accounting for current, wetsuit or open water conditions; most athletes swim slower in open water than in a pool.

To plan a full triathlon finish time, use the triathlon time calculator, which adds up swim, bike, run and transitions.

Critical Swim Speed (CSS): origin and calculation

Critical Swim Speed is a threshold beyond which fatigue accumulates sharply during swimming. It was scientifically described by Kenji Wakayoshi and colleagues at the University of Tokyo in the early 1990s and validated as a simple field test. The coaching brand Swim Smooth popularised the concept for recreational and triathlon swimmers.

CSS is calculated from two time trials, a 400 m and a 200 m test, each swum at maximum, even effort: CSS (m/s) = 200 / (t400 − t200), where t400 and t200 are the test times in seconds. The difference between the two times isolates the part beyond the pure starting speed, giving a robust threshold speed.

Example: 400 m in 6:00 (360 s) and 200 m in 2:50 (170 s) give CSS = 200 / (360 − 170) = 200 / 190 = 1.0526 m/s, a pace of 1:35 per 100 m.

Training zones and limits of the method

CSS sits close to the aerobic threshold, which makes it a useful reference for training paces. There is no universally standardised percentage-zone model like VDOT for running, but coaching practice has settled on rules of thumb in seconds relative to CSS, for example from Effortless Swimming: recovery at CSS + 8 to 10 s/100 m, aerobic sets at CSS + 4 to 7 s/100 m, threshold sets at CSS pace, and speed work at CSS − 2 to 5 s/100 m.

The method depends on an honest, even pacing strategy in both test swims. If the 400 m test is started too fast and fades towards the end, the resulting CSS is artificially low. It is recommended to swim the 400 m test first and recover fully between the two tests.

Frequently asked questions

How do I swim the CSS test correctly?

Warm up properly, then swim the 400 m test from a push start (not a dive) at an even, maximal effort. Recover fully afterwards, for example with an easy swim, before the 200 m test. Enter both times into the calculator.

What does a CSS of 1:35 per 100 m mean?

With 400 m in 6:00 and 200 m in 2:50, CSS works out to 200 / (360 − 170) = 1.0526 m/s, a pace of 1:35 per 100 m. This is the speed considered a threshold pace over longer distances.

How long would a 1500 m swim take at this pace?

At a pace of 1:35 per 100 m, a 1500 m swim takes 15 × 1:35 = 23:45, assuming the pace is held over the whole distance. The projection table in the first tab shows this automatically for 400 m to 3.8 km.

Why is my open water pace slower than my pool pace?

Open water has no wall to push off from, plus current, waves and navigation. Without a wetsuit most athletes swim 5 to 10 percent slower in open water than in a pool; a wetsuit can partly offset that.