What is Critical Swim Speed (CSS)?

Critical Swim Speed (CSS) is the pace you can sustain over a long distance without fatiguing — essentially, your swimming aerobic threshold. It's a practical and proven way to train smarter, track progress, and dial in your race pace.

Why CSS Matters

• Precision Training
  CSS gives you a personalized pace to guide your training — hard enough to drive progress, but sustainable enough to avoid burnout.

• Built Around You
  It's calculated from two simple time trials (400m and 200m), making it specific to your current fitness — no guesswork involved.

• Race-Day Relevance
  CSS closely reflects the pace you'll need for events like the 1500m swim in an Olympic triathlon or the 1.9km swim in a 70.3.

• Track Your Gains
  Repeating the test every 4–6 weeks helps you monitor improvements in fitness and endurance.

How to Use It

• Set Your Training Zones
  Use CSS to pace aerobic sets, threshold sets, and recovery swims.

• Build Better Endurance
  Training around CSS improves your ability to maintain speed over longer distances.

• Refine Pace Judgment
  CSS helps develop that internal clock — the skill to swim steady, consistent splits.