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CSS Swim Pace Calculator

Critical Swim Speed Test

Calculate your training paces and zones

Calculate your swim training paces and zones

When you look at your swimming program, have you ever wondered what the best pace should be swimming for different sets, for drills, for long intervals and sprints?

As no two athletes are the same, you could benefit most from individualising the paces to your ability or current fitness.

What are your individual training zones?
The CSS swim test can help you calculate your optimal swimming paces so that you get the most out of each training session.

Table of Contents

What do you need to focus on to be a faster swimmer?

For a swimmer or triathlete to make significant progress they need to focus their work in three main areas.
In order of importance, these areas are:
  • body position,
  • aerobic fitness, and
  • strength.

Body Position

Body position determines the amount of drag you create while trying to propel yourself forward. The goal is always to create as little drag as possible. Holding a body position that allows you to “cut” through the water with the least amount of energy spent is what every swimmer should aim for.
Tips and techniques on how to improve your body position are discussed in-depth in our many swimming related blogposts.

Aerobic Fitness

This is the “fitness” component which is most important to long-distance swimming and triathletes. This includes your muscle endurance and the efficiency of the lungs and heart.


The force and the direction of that force that you are able to place on the water to propel you (hopefully) forwards.

This is the combination of how well you “catch”, “pull” and “push” the water as well as the strength of the muscles used in these actions.
This is also discussed in-depth on our many swimming related blogposts.

What is the CSS swim test?

The CSS swim test is a feedback loop we use to gauge improvements in aerobic fitness
The CSS swim test is a simple, effective and easily repeatable field test that is used to determine your optimal training zones for endurance swimming. The test can be performed with a coach, in a squad situation, or unsupervised. 
The data from the test is put into a formula to calculate the fastest speed at which you should be training to keep improving your aerobic capacity.
The Critical Swim Speed (CSS) is defined as the swimming speed that can be maintained continuously without exhaustion. This test can be used by coaches and swimmers as a measure of aerobic capacity and also to calculate training intensities.
The purpose of the test is to record your swim speed as a measure of endurance fitness so that it can be compared to future tests. The results of the test will help set your swim training paces from now until the next batch of testing.
While not appreciated by all coaches, the CSS swim test has been shown to correlate well with the swimming velocity corresponding to the onset of blood lactate accumulation (OBLA) and the maximal lactate steady state.
We like the test for its simplicity and repeatability

How to do the CSS swim test

Equipment required:

  • a 25m or 50m swimming pool
  • a stopwatch.

The testing procedure:

After a standardised warm-up, the swimmer starts each time trial in the water with a push start from the wall (no diving).
Record the overall time for the 400m TT (Time Trial) as well as each 100m split time i.e. your time at 100m, 200m, 300m and 400m.
Record the overall time for the 200m TT (Time Trial) as well as each 100m split time i.e. your time at 100m and 200m.

The recorded data is then submitted in the form below for analysis.

CSS Swim Test Protocol

WU: (Warm-Up)

200m swim long and easy
200m kick with a board
200m swim at a medium pace
100m kick with a board
6 x 50m building to race pace on 60 secs
Rest 1 min

MS: (Main Set)

400m Time Trial
Rest 1 min
200m swim long and easy
Rest 1 min
200m Time Trial

LD: (Loosen Down)

200m swim long and easy

What you will receive via email

Upon completion of the form below, you will receive an analysis of your 2 time trials as well as your training paces below:

Swimming paces based on your CSS swim test results.

Warm Up & Loosen Down Pace:

Use this pace for your warm up and loosen down as well as anything denoted as “easy swimming”.
This is also your pace for doing drills and kicking sets.
Drills should always be performed slowly so that you can give 100% concentration on mastering the actions.

Aerobic Pace:

Use this pace for long intervals of 600m or longer, open water swims, paddles & pull buoy sets.
You should never feel out of breath when performing these sets. These are the swimming sets that build your stamina for CSS and T-Pace training so it is very important to control your aerobic pace very rigidly.

CSS Pace:

CSS swim sets are usually 100m, 200m, 300m, 400m or even 600m repeats.
The pace should not feel super hard at first, but you will feel fatigued towards the end of the set, so try to reign yourself in at the beginning in favour of holding an even pace all the way through the set. Examples of CSS swim workouts can be viewed on our swim blog posts


T-Pace swim sets of 100m, 150m, 200m, 250m repeats
The pace will feel hard, but you will get long rest periods to recover. The T-Pace sets are usually short so “Hang Tough”.
Be aware of keeping your style as strict as possible for as long as you can. If you feel you are starting to “thrash” the water and forcing your stroke, rather slow your pace by 5 seconds per 100m

How to improve your CSS pace

Depending on your swimming ability, there are many areas you can address to improve your CSS swim pace.
They are:

  • body position
  • pacing
  • technique
  • fitness
  • strength

Body position and how to improve it is discussed in many of our swimming blog posts.

Pacing is usually an issue for novice swimmers or triathletes not familiar with this test. From our data, we often note that those with CSS paces of 2 minutes per 100m or slower start far to fast and lose pace after 150m to 200m in the 400m time trial. Also, because they have gone out so hard in the 400m, their 200m split times can be up to 20% slower than their 400m times. The best pacing strategy for the 400m time trial we have learned is to start at a medium pace and to keep increasing the pace after each 10om. 
The best pacing strategy for the 200m is to increase the pace after every 50m. 

Your Technique will generally degenerate with increased fatigue levels. It is therefore important to get the pacing right.

Fitness improvements will influence your times positively, especially in the 400m time trial. 

Strength improvements will influence your 200m time trial.

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