Critical Swim Speed Calculator
Your CSS threshold pace from 400 m and 200 m
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200 m time
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.
Frequently Asked Questions
- What is Critical Swim Speed (CSS)?
- Critical Swim Speed is an estimate of the fastest pace you can sustain over a long swim without rapid fatigue, similar to a swimming threshold. The calculator uses your 400 m and 200 m times to find your CSS pace per 100 m and speed.
- How do I calculate my CSS pace?
- Swim timed 400 m and 200 m trials at maximal effort, then enter both times. CSS is the difference in those times divided by the 200 m distance difference, giving your threshold pace per 100 m for setting training intervals.
- How should I use CSS in swim training?
- Use your CSS pace per 100 m to anchor threshold and endurance sets, holding it for repeats with short rest to build aerobic swimming fitness. As your CSS pace improves through training, retest periodically and adjust your interval targets.