How to complete a Swimming Time Trial (TT) and Swim Pacing Guidance (CSS)

Karen Parnell November 10, 2021

By Karen Parnell

The words “time trial” can make even the most experienced triathlete or swimmer shudder but they are extremely valuable. They are not a test you pass or fail so it´s best just to relax into it and just do the best you can on the day. There is no right or wrong. I like them because they put a stake in the ground for your current fitness. You can set goals based on your fitness rather than some obscure number you got from a magazine or a friend. Once you've set your goals, it's time to think about workouts that help move you toward your race goals and swim leg target times.

After a few weeks of training, you can do the time trial again to see if your strategy led to the improvement, you were looking for.

I do understand that some athletes dislike the thought of time trials. Some of these athletes get performance anxiety and time trials are a way to help them work through the nerves that come with a test. Other athletes may just be terrible testers but great racers. For these athletes it may be best to use races as benchmark time trials for the non-aerobic tests.

What is CSS and why is it useful?

Critical Swim Speed (CSS) is defined as the theoretical fastest pace that a swimmer can maintain continuously without exhaustion, your aerobic swimming threshold. Your CSS is computed using your time for a 400 m or yard time trial, and your time for a 200 m or yard time trial.

CSS is a pace that’s tough enough to develop your aerobic capacity but not so hard that it’ll take you days to recover. So you can improve your swim fitness and still have enough energy to go running or cycling (or swimming again).

CSS is a race-specific training pace. It may not make you the fastest 50 or 100 swimmer, but it will train you to sustain a moderately high speed for longer distances which is ideal for triathlons.

CSS training teaches you about pace awareness the hard way (which is usually the best way) Go off too fast and you’ll pay the price later. Thankfully you can test your current CSS pace without having to swim a solo 1500 Time Trial. See below for instructions.

Once you know your CSS pace, you can use a Finis Tempo Trainer Pro to help you train. It’s a small beeping device that attaches to your goggles. Dial in your CSS pace and it’ll happily beep every lap so that you can maintain perfect pace.

To test your current CSS pace, you need to swim a 400 and 200 Time Trial within the same session. Ideally, get a friend or coach to time you and record your 50 splits and strokes per minute (SPM). Failing that, simply record the 400 and 200 times yourself. Once you have completed the TT, enter your 400 and 200 times into the ChiliTri swim calculator.


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The Time Trail Session


Warm Up

300 easy freestyle as 200 fins as 50 choice drill, 50 freestyle

4 x 50 freestyle (25 fast + 25 easy) +10 seconds rest then 

4 x 100 freestyle (20 seconds rests)

Do these 100’s at what you perceive to be the AVERAGE pace that you can sustain for a 400 Time Trial.


Main Set

Swim your 400-time trial as a continuous 400 swim and remember there is no right or wrong!

Take 5 to 8 minutes easy swimming/stretching. Feel fully recovered, then:

Swim your 200-time trial as a continuous 200 swim


Warm Down 

100 easy choice of stroke (it´s nice to throw in a few lengths of back stroke to relax off your shoulders and chest.


CSS Sample Workouts

CSS: 'Critical Swim Speed' - this pace (for swimming) is comparable to 'threshold' speed on the bike or run. It's a good marker for a pace to aim for during your repeats. This pace will be worked out after undertaking a 'CSS test' as detailed above.

It is expressed as pace per 100m, so a CSS pace might be 1.40 (1 minute 40 seconds per 100m)

Sometimes pace guidance will be given like this, in terms of ‘how the swim pace feels’ :

Easy: CSS +12s

Steady: CSS + 8s

Moderate: CSS + 4s

Hard: CSS pace

Fast / Very Hard: CSS - 4s

'CSS+4' means 'CSS pace plus 4 seconds per 100m.'

If your CSS pace was 1.40, CSS+4 would be 1.44 per 100m  


Once you know your CSS pace, you can use a Finis Tempo Trainer Pro to help you train. It's a small beeping device (it used to be called a Wetronome) that attaches to your goggles or under your stuffed under your swimming hat because if you are like me you lose the clip! Enter in your CSS pace and it will beep every lap so that you can maintain the pace you require foe that session.


Using CSS in your Swim Training


Including CSS based swimming sessions into your triathlon training can help you improve the speed you can sustain over your race distance and also develop your pace judgement. In addition to CSS training, it's also important to include various different swimming sessions such as speed, threshold and endurance workouts in order to meet the needs of your target races. Technique work including drills and open water training are also important.


If you prefer Heart Rate or RPE here's how it relates to CSS:



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My Finis Tempo Training Pro being used sea swimming to keep a steady arm cadence


Would you like a FREE swimming workouts for triathletes e-book with tips, workouts, drills and more – download it here.

Swimming training plans on TrainingPeaks.

Swimming training plans on FinalSurge.


Karen Parnell is a Level 3 British Triathlon Coach, 8020 Endurance and IRONMAN Certified Coach, WOWSA Level 3 open water swimming coach and NASM Personal Trainer and Sports Technology Writer. 

Need a training plan? I have plans on TrainingPeaks and FinalSurge marketplace:



8020 Polarised Training Plans

I also coach a very small number of athletes one to one for all triathlon distances, open water swimming events and running races, email me for details and availability.



FAQ - Critical Swim Speed (CSS)


What is Critical Swim Speed (CSS)?

Critical Swim Speed (CSS) is a term used in swimming to describe the speed at which a swimmer can sustainably swim without experiencing significant fatigue for a prolonged period.

How is CSS determined?

CSS is typically determined through a swim test that involves swimming multiple distances at varying intensities. The times for these distances are then used to calculate the swimmer's CSS.

What are the units of CSS?

CSS is usually expressed in meters per second (m/s) or minutes per 100 meters (min/100m).

What is the significance of CSS?

CSS is an important parameter for swimmers as it helps in setting training intensities and planning workouts. It provides a benchmark for determining the appropriate pace for different types of training sets.

How is CSS used in training?

CSS is used to define training zones based on different percentages of the CSS pace. These zones help swimmers target specific energy systems and improve their endurance and speed.

Can CSS be used for all swimmers?

CSS is most applicable to intermediate and advanced swimmers who have a good level of cardiovascular fitness. Novice swimmers or those with limited swimming experience may find it challenging to accurately determine their CSS.

How often should CSS be retested?

CSS testing should be done periodically to track progress and make necessary adjustments to training zones. It is recommended to retest every 8-12 weeks or after significant improvements in performance.

Are there any limitations to CSS?

CSS is primarily a measure of aerobic capacity and endurance. It may not accurately reflect a swimmer's performance in sprint events or races that require different energy systems.

Can CSS be improved?

Yes, CSS can be improved through targeted training. By incorporating specific workouts that focus on endurance and speed, swimmers can gradually increase their CSS over time.

Should CSS be used as the sole measure of swimming performance?

While CSS is a valuable metric, it should not be the only measure of swimming performance. Other factors such as technique, strength, and race strategies also play crucial roles in competitive swimming.

When was CSS first devised?

CSS is a concept that dates back to at least 1965 when a distance-time model was proposed by Monod and Scherrer as they evaluated world records in swimming, running, speed skating and cycling.


Please note that while this FAQ provides general information about CSS, it is always recommended to consult with a qualified swimming coach or expert for personalized guidance and assessment.



Tempo Trainer Pro (

Critical Swim Speed (CSS) and Deliberate Swimming | Steel City Endurance Coaching - Triathlon & Endurance Training


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