How to use your Critical Swim Speed to optimise your swim training

By Dave Colley | 5 Minute Read

Critical Swim Speed (CSS) is your threshold pace when swimming and the guys from the PH office have been using the Swim Smooth Guru app to help guide their swim training.

They recently did a Critical Swim Speed Test -  the swimming equivalent to an FTP or threshold pace test - and Swim Smooth founder, Paul Newsome, has stepped in to help explain how a CSS Test can help tailor training and monitor progress...



  1. What is Critical Swim Speed?
  2. How to do a Critical Swim Speed Test
  3. How to calculate your Critical Swim Speed pace
  4. How to use your Critical Swim Speed pace in training
  5. Using the Swim Smooth Guru app for Critical Swim Speed training
  6. Further reading 


What is Critical Swim Speed?

Ok, so, if you don't know already, your Critical Swim Speed (CSS) is your threshold pace when swimming. It's roughly the pace you can swim a flat-out 1,500m and is a single number which you can base all of your training intensities around.

For instance, in our squads in Perth, swimmers in lane 1 have a threshold pace around 1:50-2:00 /100m, lane 2 1:40-1:50 /100m, lane 3 1:25-1:40 /100m and lane 4 1:15-1:25 /100m. You'll find a similar split between the lanes in every Swim Smooth squad around the world.


CSS training: A huge range of speeds but one accurate way of training for everyone.


If you have trained with a power meter on the bike you probably know your FTP (Functional Threshold Power) and when it comes to running you might be aware of your threshold pace, the pace you can sustain for a race around 10km to 10 miles in length.

CSS, FTP and threshold running pace are the equivalent thing in all three sports. Know them and you can train different energy systems accurately and track your progress over time.

So how do you find your CSS pace? Swim a 1500m time trial? Well you could, but it would be mentally challenging to do and there's another way of doing it that gives more insight into your fitness...

How to do a Critical Swim Speed Test

After a thorough warmup and after a few key drills, swim a 400m time trial. Pace it well and remember this is a flat out effort, you shouldn't finish and think "maybe I could have gone harder"!

If at all possible, have a friend, coach or lifeguard time your first 100m split of the 400m. As we'll come to later, this will give you real insight into your pacing abilities. Record your overall time and also your 100m split.

Spend a few minutes recovering with some easy laps of freestyle until you feel you are ready for part 2; a 200m time-trial. Again, well paced and a good hard effort. Record your time.

Note: if you're in a yard pool, swim 400yd and 200yd time-trials instead.

Swim an easy cool down to recover from your efforts.

How to calculate your Critical Swim Speed Pace

Armed with your times, enter your numbers into this basic CSS calculator.

For example, the lads shared their results with me:

James: 4:46 400m / 2:19 200m = CSS 1:13.5/100m

Andy:  5:10 400m / 2:29 200m = CSS 1:20/100m

Jonny: 5:36 400m / 2:44 200m = CSS 1:26/100m

Now you can find out where you rank vs the guys who make your electrolyte drinks ;-)

Or, for a much more thorough analysis (including pacing analysis and performance predictions over different distances), use the Swim Smooth Guru (Pro subscription required, see below for details of an exclusive discount for Precision Hydration athletes!).

The Guru gives you Swim Smooth's full analysis of your CSS test.

The Guru will use your 100m split from the 400m to assess your pacing abilities - if these are poor then it's critical you address them to make decent headway with your swimming.

How to use your Criticial Swim Speed pace in training

So, how do you control your pace to swim at CSS?

The secret is to use a Finis Tempo Trainer Pro set to beep at regular intervals such that you pace things out to be passing every 25m (or 25 yards) when it beeps.

For instance, if your CSS pace is 2:00 /100m then set your beeper to 30 seconds. Get ahead of the beep every 25m and you're going to fast. Behind the beep and you're travelling too slowly.

The key to CSS training is sustained speed with short recoveries. This is a little different to traditional master's swimming, where the swims are faster but with much longer recoveries (sprint training).

Here's an example CSS set:

3x (100m + 200m + 300m) - all swum at CSS pace with 10 seconds rest after every swim

That's 1,800m in total, suitable for swimmers with a CSS pace around 1:30 /100m.

If your CSS pace is around 1:20 /100m, design sets around 2000m in length.

If your CSS pace is around 1:45 /100m we suggest a set around 1600m.

If your CSS pace is around 2:00 /100m try 1400m in total.

The key to these sets is sustained speed with short recoveries between swims. This will effectively develop your "diesel engine".

How to use the Swim Smooth Guru app for Critical Swim Speed training

With a Pro Guru subscription you can take CSS training to the next level.

First up, there's hundreds of fun and effective training sessions in the Guru for all ability levels. Each uses your CSS pace to set your intensity accurately.

Whenever you use a Tempo Trainer, the Guru pops up and tells you what to program in (no maths required!) to accurately control your training intensity:

Tell the Guru how your session went and it'll automatically adjust your CSS pace up or down for the next time. This means it's not necessary to perform the CSS test regularly as the Guru will actually track your CSS pace as you go along. We call this process "CSS tweaking", something that's unique to Swim Smooth.


CSS isn't a concept that Swim Smooth invented, but we like to think that we've popularised it for swimmers and developed a whole system of focused, effective training around it to help you achieve your potential in the water.

In essence it:
  • Provides focused training to improve your distance swimming
  • Is quicker to recover from than many other training sets
  • Allows you train scientifically using a Tempo Trainer Pro
  • Inherently develops your pacing skills


If you do want to give the CSS features of Guru Pro a try, we've hooked PH customers up with an exclusive discount on the Guru coaching platform. Just use the code 69VC-K4EU to get 30% off a 3 months Guru Pro subscription or 10% off an annual subscription

To activate your Swim Smooth Guru license just visit, hit ‘Sign Up’, create an account and enter the code at the next step. If you're already a Guru subscriber, or have any problems activating your license, contact and mention our promo code.

No excuses now, get to work...

Swim Smooth!


Further reading

Paul Newsome is the founder of Swim Smooth, the popular online swim coaching program and provides detailed and personalised coaching at 

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