What Critical Swim Speed is
Critical Swim Speed (CSS) is the swimmer's threshold pace, the fastest speed you can hold for a prolonged effort. It is the pool's FTP and anchors every swim zone.
CSS is the slope between two maximal time trials: CSS = 200 ÷ (t400 − t200) meters per second. Pace per 100 m is 100 ÷ CSS. The method comes from Wakayoshi et al. (1992) and Ginn (1993).
Why both trials must be maximal
Sandbag the 400 m and its time falls too close to the 200 m, so the formula reports an unrealistically fast CSS. A well-swum 200 m is only moderately faster than 400 m pace. Rest fully between trials.
Pace, not heart rate, is the anchor
Heart rate is unreliable in water. Body position, water pressure and the dive reflex pull it below land values, and a monitor is hard to read mid-stroke. CSS uses seconds per 100 m instead. Every band is an offset: easy slower, threshold at CSS, VO₂max and sprint faster.
How to run the CSS swim test
Two maximal time trials in one session, with full recovery between, from a push start.
- 1
Warm up
400–600 m easy with a few build-ups.
- 2
Swim a maximal 400 m
All-out, evenly paced. Record the time in seconds.
- 3
Rest fully
Easy swimming and rest for several minutes.
- 4
Swim a maximal 200 m
All-out. Record the time.
- 5
Enter both times
The calculator returns your CSS and pace zones.
Worked example
400 m in 6:00 (360 s) and 200 m in 2:50 (170 s):
| t400 − t200 | 360 − 170 = 190 s |
| CSS speed | 200 ÷ 190 ≈ 1.05 m/s |
| CSS pace per 100 m | 100 ÷ 1.05 ≈ 1:35 /100 m |
| Easy band | ≈ 1:43–1:47 /100 m |
| VO₂max band | ≈ 1:29–1:33 /100 m |
Threshold sets are swum at about 1:35 /100 m. Easy and fast bands offset from it.
Frequently asked questions
What is a good CSS pace?
A developing triathlete often has a CSS around 2:00–2:20 per 100 m, a club swimmer 1:30–1:45, and competitive pool swimmers under 1:20. About 1:35 per 100 m is a solid mid-pack triathlon threshold.
How often should I retest my CSS?
Retest CSS every 4–6 weeks during focused swim training, or whenever sets feel consistently easy at threshold pace. CSS improves with fitness and technique, and a stale number sets every band too slow.
Why use a 400 m and 200 m test?
The 400 m and 200 m bracket the threshold effort and give the slope between them: CSS = 200 ÷ (t400 − t200) meters per second. The extra 200 m over the time gap is your sustainable speed. Both trials must be maximal.
Why not just use heart rate for swim zones?
Heart rate is unreliable in water. Body position, water pressure and the dive reflex suppress it by roughly 10–15 bpm versus land, and reading a monitor mid-stroke is impractical. Critical Swim Speed uses seconds per 100 m instead.
Can I use CSS for triathlon swim pacing?
Yes. CSS is your threshold swim pace. Sprint and Olympic triathlon swims sit at or just below CSS, Ironman-distance swims a touch easier. Adjust for wetsuit buoyancy and open-water conditions.
Sources
- Ginn (1993). “Critical speed and training intensities for swimming.” Australian Sports Commission, origin of the 400 m / 200 m CSS field test.
- Wakayoshi et al. (1992). “Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer.” Eur J Appl Physiol 64:153–157.