Last week we talked about Critical Swim Speed CSS. If you haven’t read that post I suggest you go back and read it first <here>.

If you want to use CSS more effectively, this post helps to understand how the testing works and what additional information you can extract from it.

I’ve created a calculator you can use here:

CSS_Calculator_Any_Distances_Aligned_v2

Unlike most online calculators, this one allows different test distances and multiple data points. Make your own local copy, and let me know if you find any bugs or have suggestions on how to improve it.

Most athletes are familiar with the traditional 400 + 200 test. This works well, but it has a limitation: it forces a straight line through only two points. Any pacing error or sprint bias can distort the result. Using three distances improves accuracy because it calculates a best-fit line across multiple efforts.

In simple terms:

For many endurance athletes, three data points produce a CSS that better reflects sustainable performance.

Rather than prescribing fixed distances, it is often more useful to choose test efforts based on duration. This allows faster and slower swimmers to select distances that target similar physiology. A practical guideline is to include efforts lasting approximately:

The goal is to include efforts across different durations that allow you to estimate threshold speed, rather than prescribing identical distances for everyone.

Examples of test protocols:

Whatever protocol you choose, consistency matters more than perfection. Always repeat the same test format when tracking progress.

D' describes how much speed you have above threshold, while CSS describes how fast you can sustain. It helps explain why athletes with similar CSS can perform differently over shorter distances.

Low D' is very common in endurance athletes and is not always a weakness. It often reflects strong fatigue resistance and efficiency.

Higher D' athletes typically show:

Lower D' athletes typically show:

For most training decisions, CSS remains the more important number. D' mainly provides insight into your athlete profile.

The calculator provides several outputs. Here is how to interpret them and how to use them in practice.

Two Data Points vs Three Data Points

If you enter two test distances, the calculator gives you a CSS based on those two performances. This is the traditional approach and works well when pacing is good. If you enter three distances, the calculator also provides a regression CSS. This uses all available data to calculate a best-fit line across your efforts.

In practice:

If you have three good maximal efforts, the regression value is usually the better number to use.

Which CSS Should I Use to Anchor My Zones?

Use the number that best reflects what you could repeat in training. For many endurance athletes with low D’ values, raw CSS slightly overestimates threshold pace. In these cases, it can be useful to anchor training zones on CSS + 2 to 3 seconds per 100 meters. This is not changing your fitness; it is aligning training intensity with physiology. The objective is repeatable quality, not validating the test result.

What Is the Manual Adjustment?

The calculator includes a manual adjustment factor so you can modify the pace slightly if needed. This exists because testing conditions vary, athlete profiles differ, and sustainable pace is not identical to test pace.

Think of this factor as a coaching correction, not a mathematical one.

Testing is not about producing the perfect number. It is about creating a consistent reference point that helps guide training. Used correctly, CSS becomes a tool for understanding your physiology, not judging it.

The number matters far less than how you use it.