CSS Calculator — Critical Swim Speed

Calculate your Critical Swim Speed from 400m and 200m time trials. Get personalized swim training zones — free, science-based.

Critical Swim Speed (CSS) is the swimming equivalent of lactate threshold or Functional Threshold Power (FTP). It represents the fastest pace you can sustain without accumulating excessive fatigue. CSS is calculated from two time trials — 400m and 200m — using the formula: CSS = (400 - 200) / (T400 - T200). Once you know your CSS, you can define 5 training zones to structure your swim workouts, from easy recovery to all-out sprints. This method was developed by Wakayoshi et al. (1992) and is widely used by coaches and triathletes worldwide.

Calculate Your CSS

Swim a 400m all-out time trial, rest for 10-15 minutes, then swim a 200m all-out. Enter both times below to calculate your Critical Swim Speed and training zones.

🏊Already know your CSS? Get your personalized swim training zones →
400m

Format: M:SS or MM:SS (e.g. 6:30 = 6 min 30 sec). Swim 400m/yd as fast as you can.

200m

Format: M:SS or MM:SS (e.g. 3:00 = 3 min). Swim 200m/yd as fast as you can.

Formula

CSS = (400 − 200) / (T400 − T200)

Source: Wakayoshi et al. (1992) — "Determination and validity of critical velocity as an index of swimming performance"

Frequently Asked Questions