More Info Critical Swim Speed Calculator is a simple tool that allows the swimmer or coach to calculate the athletes Critical Swim Speed (CSS) or Threshold Pace. *Note you will need either PB’s or to Time Trial for both 400 and 200m or yard swims. CSS Pace Calculator 3 Unit Meters Yards Enter your time: 1 Time for 400m minutes seconds 1 Time for 200m minutes seconds calculate CSS Pace: - Actual speed: -