How Does The CSS Swim Test Work?

What Is Critical Swim Speed (CSS)?

Critical Swim Speed (CSS) is your predicted pace per 100m/yards for a 1500m/yard swim.
It’s calculated using your 400 and 200 swim times from the CSS test.

Why These Distances?

The CSS test uses both your aerobic and anaerobic systems:

• Your 400 time reflects a strong, sustainable aerobic effort.

• Your 200 time is faster and includes more anaerobic contribution.

• The test focuses on the aerobic part, as this is what predicts sustainable race pace.

• The anaerobic portion is not used in the calculation (you can’t swim a full 200 anaerobically).

How CSS Is Calculated

The test assumes your 200 will be faster than your 400, and the gap between them helps identify your sustainable speed.

Formula:

1. Subtract your 200 time from your 400 time

2. Divide the result by 2

3. The answer is your CSS pace per 100m/yards

Example:
(400 time − 200 time) ÷ 2 = CSS pace per 100m/yards

CSS depends on both your times and the relationship between them.

You can use our Calculator to work out your CSS.
 

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