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Swimming Training Books

CSS Calculator

Calculate Critical Swim Speed with our handy calculator

Alex Thurston teaching a swim set
CSS Calculator

Calculate your CSS

Critical Swim Speed (CSS) is an effective tool to measure swimming speed improvement over time and predict time to complete larger distances.

Critical Swim Speed is an estimate of your pace over a long distance.

It's calculated by comparing your 200m time trial to your 400m time trial.

Want a printable workout to help you find your CSS?

What is CSS?
What is Critical Swim Speed? 

What is CSS?

When we train for swimming, it's rare we swim further than 200 meters without stopping. Even at the highest level of swimming training.

Because of this, it can be hard to estimate how fast we can swim comfortably over a long distance like 1500m in a triathlon.

We use the Critical Swim Speed calculator to give a best estimate for how long we will be able to swim these long distances without having to actually complete them.

Once you know your critical swim speed, you can incorporate swimming sets which challenge this time and you can record your improvement over time.

After you have done your warm up and any swimming drills, try completing 10 x 100m at a pace 15 seconds faster than your CSS, then take a 15 second rest at the wall.

You can also incorporate your CSS into our Scale Swimming Waterproof books.

CSS training

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Aerial View of a Pool

Critical Swim Speed Workout

CSS Workout

Test your CSS

Print out the adjacent swimming set and bring it to the pool to test your CSS!

Simply fill in all of the open boxes as you swim, then fill in your values into our calculator at the top of this page.

This workout can be easily extended - simply integrate into another workout.

Take time to regularly check your CSS to monitor your progress, we recommend checking every 3 months.

Many factors can alter your CSS from day to day, these include: hydration, hunger level, fatigue, etc.


Try to keep these factors as consistent as possible between CSS tests to gain an accurate picture of your swimming speed and fitness

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