What is CSS?

Critical Swim Speed (CSS) approximates your swimming lactate threshold - roughly the pace you could hold for a maximal 1500m or one-mile swim (about 20–30 minutes for most swimmers). It's calculated from the difference between two time trials (200m and 400m), which cancels out your start and turn overhead to isolate your true sustainable speed. CSS is the foundation for setting swim training zones, from easy recovery swims to threshold and VO2 max intervals.

How to Use This Calculator:

• Swim a 400m time trial at best sustainable pace.
• Rest fully, then swim a 200m time trial.
• Enter both times and select your pool size.
• Your CSS and training zones will be calculated.