Swim CSS calculator: your threshold pace per 100 m
CSS (Critical Swim Speed) is calculated from two times: CSS = (400 - 200) / (T400 - T200), which gives your threshold pace per 100 m. Enter your 400 time and your 200 time, and the tool gives you the pace you can hold for a long time without blowing up, per 100 m or per 100 yards depending on your pool.
Fill in the fields to see the result.
How to use it
- After a good warm-up, swim a 400 all out, front crawl, and write down your time.
- Recover fully (about 10 minutes of very easy swimming), then swim a 200 all out and write down your time.
- Choose your pool (25 m, 50 m or 25 yards) and enter both times.
- Read your CSS pace per 100: it's your reference for threshold sets.
The method
The math is simple: the difference in distance (200) divided by the difference in time between the 400 and the 200 gives a speed. Brought back to 100, that's your CSS pace. The idea comes from the critical power concept of Monod and Scherrer (1965), adapted to swimming by Wakayoshi and colleagues in 1992: by plotting distance swum against time over several maximal efforts, the slope of the line gives a speed you can hold for a long time. The two-distance test, 200 and 400, is the short version that has become common among coaches.
Why it works: by subtracting the 200 from the 400, you remove most of the “sprint” part of the effort, the part you can't repeat. What's left is the speed your endurance can sustain. Wakayoshi et al. (1993) showed that it roughly matches the intensity of the maximal lactate steady state, and Dekerle et al. (2002) found that the speed held over 30 minutes isn't far from it.
An example
You swim the 400 in 6:40 (400 s) and the 200 in 3:10 (190 s). The time gap is 210 s for 200 m, so a speed of 200 / 210 = 0.95 m/s. Per 100 m, that's 100 / 0.95 = 105 s: your CSS is 1:45 per 100 m. A set of 10 x 100 at threshold is therefore swum around 1:45 per 100, and a 1,500 held at that pace would take about 26:15.
Limitations
CSS depends a lot on how well the test goes: a 400 started too fast or a 200 swum tired skews the result. Dekerle et al. (2002) found that CSS from the 200 and 400 overestimates by about 3% the speed actually held over 30 minutes: it's not a race pace for an Ironman. It applies to front crawl and to the pool where you did the test: in a 25 m pool the turns help, while in a 50 m pool and in open water (swell, sighting, wetsuit) you'll swim at a different speed. In a 25-yard pool, the pace is per 100 yards (91.44 m), not per 100 m. Redo the test every 6 to 8 weeks, since it changes with your fitness.
Frequently asked questions
What is CSS in swimming?
CSS (Critical Swim Speed) is the swim speed you can hold for a long time without wearing out, close to your threshold. It's expressed as a pace per 100 m (or per 100 yards). It's the reference for pacing your sets.
How do you do the CSS test?
Warm up, swim a 400 all out, recover well, then swim a 200 all out. Write down both times and enter them in the calculator. Do it in front crawl, in the pool where you usually train.
What pace should you swim based on your CSS?
Threshold sets are swum around your CSS, with little rest (for example 10 to 20 seconds between 100s). Easy swimming is slower than CSS, and speed sets are faster.
Is CSS the same in a 25 m pool and a 50 m pool?
No. In a 25 m pool, you do twice as many turns and push-offs, which save time. Do the test in the pool where you swim your sets, and redo it if you change pools.
How often should you redo the CSS test?
About every 6 to 8 weeks, or after a training block. If your threshold sets start feeling easy, it's a sign your CSS has improved.
Sources
- Wakayoshi K., Ikuta K., Yoshida T., Udo M., Moritani T., Mutoh Y., Miyashita M. (1992). Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer. European Journal of Applied Physiology, 64, 153-157.
- Wakayoshi K., Yoshida T., Udo M., Harada T., Moritani T., Mutoh Y., Miyashita M. (1993). Does critical swimming velocity represent exercise intensity at maximal lactate steady state? European Journal of Applied Physiology, 66, 90-95.
- Dekerle J., Sidney M., Hespel J.M., Pelayo P. (2002). Validity and reliability of critical speed, critical stroke rate, and anaerobic capacity in relation to front crawl swimming performances. International Journal of Sports Medicine, 23(2), 93-98.
What if ELYO worked it out for you?
ELYO reads the swim workouts recorded by your watch, tracks your pace per 100 and your training load week after week, and adjusts your plan as your CSS improves.