Swim CSS calculator
Swim a 400 m and a 200 m time trial with a good rest between them, enter both times, and the calculator gives your critical swim speed as a pace per 100 m, per 100 yards and in metres per second.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Swim a 400 m time trial, rest fully, then a 200 m time trial.
- Enter the minutes and seconds of each swim.
- Read your CSS pace per 100 m. The per-100-yard figure assumes the same speed over yards.
How it works
Speed = (400 − 200) ÷ (T400 − T200) in metres per second. Pace per 100 m = 100 ÷ speed. Pace per 100 yd uses 1 yd = 0.9144 m. Results are rounded to the second.
Worked example
400 m in 6:00 (360 s) and 200 m in 2:50 (170 s): speed = 200 ÷ 190 = 1.0526 m/s. Pace per 100 m = 95 s = 1:35. Pace per 100 yd = 91.44 ÷ 1.0526 = 86.9 s, 1:27. Pace per 50 m = 47.5 s, shown as 48. At CSS, 1500 m takes 1,425 s = 23:45.
Tips
- Use the same pool length for both trials and for later retests.
- Re-test every month or two to keep the pace current.
- Train in the way your coach prescribes; the pace is a reference, not a diagnosis.
Frequently asked questions
What is critical swim speed?
Critical swim speed, or CSS, is a pace swimmers use as a threshold for training. It is estimated from two time trials as the slope of distance against time, so it is the distance gained per extra second of swim time between the two trials. The idea was described in research in the early 1990s.
What is the CSS formula?
CSS in metres per second is (400 − 200) ÷ (T400 − T200), with times in seconds. The pace per 100 m is 100 divided by that speed. For example 6:00 for 400 m and 2:50 for 200 m gives 200 ÷ 190 = 1.053 m/s, a pace of 1:35 per 100 m.
How should I do the time trials?
Swim each as fast as you can hold evenly, after a proper warm-up, with plenty of rest between them or on separate days. Use the same pool length and the same push-off conditions for both. Errors in either time change the result a lot because it comes from their difference.
Is the 1500 m time a prediction?
It is the time you would take at exactly CSS speed over 1500 m, which is only a rough reference. Real open-water or pool races depend on pacing, turns, conditions and more.