Critical Swim Speed

How to establish your swim threshold

For details of the CSS Test workout, click the bar below.
CSS Test


  • 10 minutes of mixed strokes.


  • 6 to 8 x 50m
  • Build by each intervals to finish the last 50 strong


  • 400m Time Trial – Ensure you do the correct distance and pace yourself well. Having someone to time you will help.
  • 5 mins rest.
  • 2 x 25 –  #1 easy #2 moderate just to get yourself ready for the next TT interval.
  • 200m Time Trial – Ensure you do the correct distance and pace yourself well.
Once you have your test results, enter them in the CSS Calculator using the link below.
CSS Calculator

One you have your times for both the 200m and 400m time trials you can enter them in to the CSS Calculator to establish your CSS / Threshold Pace. The calculator is available HERE.


CSS stands for Critical Swim Speed. The CSS Test aims to find out what that Critical Swim Speed is, which should assist you in two key areas.
1) Where you are ‘at’ performance-wise or a benchmark of your current swim ability, and
2) What is a suitable pace for a threshold swim.

The first point is somewhat subjective. As we know, on any given day your performance can vary, especially at different points of the season where your overall training load may be higher or lower. But, that in turn also provides a pointer. Whether you benchmark faster or slower it will help you set a marker for your threshold swims at that time in your training cycle.

he second point is important for pacing. As with all disciplines in endurance sport, applying the right intensity at the right time is an important skill. Knowing your threshold pace gives you a figure on which to base your intervals. If your interval asks for a threshold effort, CSS effort, or an effort around 85% then this will be your threshold pace.



So, what is a threshold swim? This is a rough guide to your threshold pace, and this translates to a pace that in a race you might be able to sustain for 1500m. This threshold pace, or more importantly threshold ‘effort’, is key because it forms the basis of an endurance swimmers workouts. You’ll want to improve your threshold pace if you want to improve your triathlon swims, the longer the race distance, the more imperative this becomes.



We could ask you to simply swim 1500m as fast as you can and time yourself. If done correctly that would give you a great indication of your CSS pace. If you think that sounds like a fairly horrible task then you’d be right, it also requires you to pace the entire 1500m perfectly and count your laps perfectly too. So, instead, we extrapolate the times of two easier time trials. A 400m ‘all out’ timed swim, followed by a 5 minutes rest interval. After this, another ‘all out’ time trial but this time 200m.

By taking the 200m pace and correlating it against the 400m pace we can establish a pattern of how much your pace slows down as you go longer.

Key Part: The scientists have plotted the results of many 400m and 200m tests over many athletes. They have formulated an equation to estimate your threshold speed / CSS Pace using just a 400m and 200m test. Easier mentally, easier physically, easier to pace.

It should be noted that the formula is based on your fastest possible 400m and fastest possible 200m. The reason for having a long break between is so that you completely recover from the 400m prior to doing the 200m. You could actually do them on different days if you wanted, but a good rest interval of 5 minutes or so between the 400m and 200m should be enough.

The Caveat: The test does fall apart a little if your pacing is not spot on. Like we mentioned before, the equation is based on the 200m and 400m times, and the difference between the two. You obviously slow down pace between the 200m and 400m TT,  but how much do you slow down? That’s what the CSS time is extrapolated on.

If you nail an amazing 200m time, but pace your 400m poorly. That slow down in pace is going to be greater, which gives you a worse CSS result. It is perfectly feasible that an athlete beats their previous 200m time by a lot, beats their 400m time by a little and suddenly finds their CSS test result worse than it was before …. because the difference between the two times was greater than before. 

So, the test result needs to be used as a guide, not a fact.



If you have any questions on the CSS Test, let us know via our 140.6 and 70.3 Training Page HERE.


Doing a 140.6 or 70.3?

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We created a group specifically for full and half distance athletes interested in learning more about racing and training. Along with the usual group discussions we'll also be posting tips and advice on key aspects of long course triathlon, alongside some swim, bike and run workouts. A great learning opportunity with pro coach advice from PB+J. So ... dive in, join the group, and we look forward to having you on board.

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