Critical Swim Speed Calculator

Critical Swim Speed Calculator

Critical Swim Speed is the swimming equivalent of a runner's threshold pace — the speed you can sustain for a long time before fatigue forces you to slow down. Enter your best 400 m and 200 m times and this calculator returns your CSS pace per 100 m and your CSS speed using the standard two-distance formula. Use it to set threshold and interval targets, pace long swims, and track aerobic swim fitness over a season.

Critical Swim Speed (CSS) is the pace you can theoretically hold without exhaustion — roughly your threshold swimming pace. It is found from a 400 m and a 200 m time trial: CSS = (400 − 200) ÷ (T400 − T200). Swim both efforts maximally and rested for an accurate result. Most threshold sets are done at or just faster than CSS, with easy swimming slower than it.