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

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.

Get your FREE swimming workouts for triathletes book here.


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.

Here’s a FREE Swim Workouts for Triathletes Book with swim workouts, tips, technique, drills, kit and terms.

Would you like a free training plan? Claim your free plan or e-book.


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.


Would you like to talk about your training? Get in touch here.

Karen Parnell is a Level 3 British Triathlon and IRONMAN Certified Coach, NASM Personal Trainer and Sports Technology Writer.

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