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Critical Swim Speed (CSS) Calculator
Find your swim threshold and training paces from a 400 and 200.
Training paces (per 100)
What critical swim speed tells you
Critical Swim Speed (CSS) is the pace you can theoretically hold for a long, steady swim before fatigue forces you to slow — your swimming threshold. It comes from two all-out time trials, a 400 and a 200, swum fresh on the same day. Because the gap between them is exactly 200, the maths is tidy: your CSS pace per 100 is simply (400 time − 200 time) ÷ 2.
Using your zones
CSS itself is your threshold pace — the target for long intervals like 8 × 100 or 5 × 200 off short rest. Add a few seconds per 100 for steady aerobic and easy swimming, or take a few off for short, sharp speed work. The paces below are offsets from your CSS, the way threshold-based swim sets are usually written.
Testing well
Warm up thoroughly, then swim each trial as an even, maximal effort with full recovery between — the 400 first, then the 200. Re-test every four to six weeks; as your CSS pace drops, your whole set of training paces moves with it.
Questions
What is Critical Swim Speed?
CSS is your swimming threshold — the pace you can hold for a long, steady swim before fatigue forces you to slow. It approximates the fastest pace you could sustain aerobically, and it anchors threshold-based training sets.
How is it calculated?
From two maximal time trials swum fresh: a 400 and a 200. Because the distance between them is exactly 200, your CSS pace per 100 is simply (400 time − 200 time) ÷ 2.
How do I use the training paces?
CSS is your target for long threshold intervals. Add a few seconds per 100 for aerobic and easy swimming, or take a few off for short speed reps. Re-test every four to six weeks as your fitness changes.