🏊 Swimming Pace & CSS Calculator

Turn a swim time into pace per 100 m, per length and full splits, then — if you have a 200 m and a 400 m test time — get your Critical Swim Speed and the training zones built around it.

Optional — Critical Swim Speed (CSS) from test times
How CSS and the zones are derived

Critical Swim Speed is the slope of a distance-versus-time line through your 200 m and 400 m tests: CSS = (400 − 200) ÷ (T400 − T200). It is a strong predictor of sustainable aerobic pace. The zones are percentages of that CSS pace (75% recovery, 85% easy, 90% steady, 95–100% threshold, 105% VO₂, 115% sprint).