What is Critical Swim Speed and how to use it

What is Critical Swim Speed and how to use it

Q. What is Critical Swim Speed ?

The short answer is we use it to get an understanding of effort levels and pace

Q. But what actually is it and how do I get to know mine?

If you can’t make one of the club testing sessions and still haven’t done a CSS test scroll down to the session below.


It’s a calculation that not only looks at your swim speed but also how much you are slowing down between the 400m and 200m sets and It assumes that this drop of will continue. This means you can workout your 750m/1500m/3800m time and also get a pacing strategy for easy, medium, hard, very hard.


So as triathletes we are in the main long-distance swimmers, 1500m is classed as long distance, so to use old terminology we are the Diesel engine not the drag racer which is why the 100m gold medal winner does not enter the 1500m race.


The whole point of CSS sessions is working at a level for roughly 1600m+ continuously (over reps of 100, 200,or 400) that will become increasingly harder. If you’re already fairly good for 200m you don’t need to do lots of faster reps. You need to do some slower reps, but way more of them, with no extended rest so a strict 10s rest per interval.


If you followed the CSS sessions, you might not make any significant improvement on your 200m time but might bring your 400m time down and that would reduce your CSS and change your pacing chart. Doing those same sessions again would then shift more towards the anaerobic training level, you might not even be able to finish a session, and then you’d see more improvement in your 200 times. CSS will then slow you down a bit, which then allows your endurance base to catch up with your short new interval speed. It will continue to seesaw.


You need to be strict on the limited rest and mix in the 200 and 400m sessions. Below would be an example of your key training session in that week, the 2000m sets does not include a warmup or down also feel free to change the number of sets but not the distance or rest interval.


Wk1 - CSS test

Wk2 - 200 session (10x 200 20sec ri)

Wk3 - 400 session (5x 400 40 sec ri)

Wk4 - 100 session (20x 100 10 sec ri)

Wk5 - 200 session (10x 200 20sec ri)

Wk6 - 400 session (5x 400 40 sec ri)

Wk7 - 100 session (20x 100 10 sec ri)

Extras :

3x 600 60 sec ri

100,200,300,400, 100,200,300 100,200, 100 ri 10,20,30,40,10…

400, 2x300, 3x200, 4x100 ri 40,30…



**ri = rest interval, strictly at 10 seconds rest per 100 swam.


The CSS Test


200 easy

2x 50 choice drill



4 x 50 freestyle (25 fast + 25 easy) RI 10

4 x 100 freestyle RI 10



400m Race Pace RI 30

5 minutes easy swimming

200m Race Pace RI 30

100m Easy RI 45


2x (50m sprint 50m easy) RI 30 Repeat to end of the set



The Critical Swim Speed is defined as the theoretical swimming speed that can be comfortably maintained over a long distance like 1500m in a triathlon.

Purpose: to calculate critical swim speed as a measure of endurance fitness, and to help set training swim intensities.

CSS for a swimmer is about 80 to 85% of maximum 100m swim speed and 90 to 95% of their 400m swim speed.


Calculators here: