Swimming
Swim CSS pace per 100.
Enter your 200 m and 400 m time trial results to get critical swim speed as a pace per 100 m and per 100 yd, plus the time to swim common distances at that pace. Runs in your browser.
Time trial results
Critical swim speed
Fill the form and press Calculate.
Worked example
A 200 m in 2:50 (170 s) and a 400 m in 5:50 (350 s). The extra 200 m took 350 − 170 = 180 s, so CSS = 200 / 180 = 1.11 m/s, which is 1:30.0 per 100 m ((350 − 170) / 2 = 90 s) or about 1:22.3 per 100 yd. At that pace 400 m takes 6:00.0.
How it works
Critical swim speed (CSS) is the slope of distance against time across maximal swims. With two distances it is CSS = (400 − 200) / (T400 − T200) in metres per second, so the pace per 100 m is (T400 − T200) / 2 seconds. Wakayoshi and colleagues proposed CSS in 1992 as a swimming equivalent of critical power and related it to the pace at maximal lactate steady state, which coaches use as a threshold pace for sets.
Do both trials on separate days or with full recovery, from a push start in a pool, and keep the conditions the same. The 400 m time must be more than twice the 200 m time. The pace per 100 yd multiplies by 0.9144 and the “time at CSS pace” rows are for planning an evenly paced set, not a prediction of what you can hold for that distance.
Sources and basis (checked 2026-10-05): Wakayoshi et al. (1992), Int J Sports Med 13(5):367-371, critical swimming speed; Topend Sports, Critical Swim Speed test (method and references).
FAQ
How do you calculate CSS for swimming?
Swim a maximal 200 m and a maximal 400 m. CSS pace per 100 m is the 400 m time minus the 200 m time, divided by 2.
What is a good CSS pace?
It depends on the swimmer. Use it as your own benchmark, retest under the same conditions and track whether your pace per 100 improves.
Can I use yards?
Yes. Swim 200 yd and 400 yd, enter those times, and read the per 100 m result as your pace per 100 yd (ignore the converted yd row). The formula is the same in any unit.
Why must the 400 m time be more than twice the 200 m?
Because your 400 m pace should not be faster than your 200 m pace. If it is, a time was likely mistyped or one effort was not maximal.
Is CSS the same as lactate threshold?
CSS is a field estimate related to maximal lactate steady state, not a lab measurement.
Disclaimer
Not medical advice. Maximal time trials are hard efforts. Build up gradually, check with a healthcare professional before starting hard training if you have a health condition, and never swim alone.