Critical Swim Speed Calculator

About this calculator

Critical swim speed (CSS) is estimated as CSS = (400m − 200m) ÷ (t400 − t200): the extra distance covered in the extra time between a 400 m and 200 m time trial. It approximates the fastest pace a swimmer can sustain aerobically without accumulating lactate faster than they can clear it — the swimming equivalent of a running lactate threshold.

Swim coaches use CSS to set interval training paces, since threshold sets are typically swum at or just above CSS to build aerobic capacity without the fatigue of all-out sprinting, and competitive swimmers retest it every few weeks to track fitness gains and recalibrate their per-100m target pace.