Two times — 400 metres and 200 metres, both all-out — give you your critical swim speed, and from there the seven zones you'll swim in. Enter the times and you're done.
Each sport uses a different test and a different metric. Pick one and we'll walk you through it.
More than one test is valid and they all get you to the same place. Pick the one you did — the maths changes depending on which.
If you have more than one metric, trust them in this order: power > pace > heart rate. HR lags behind the effort and drifts with heat, sleep and caffeine — it's the backup, not the main signal.
Having the numbers is half of it. The other half is knowing which session grows each zone — how to build a Z2 that's more than easy miles, the marathon-pace session that justifies Zone X, and what a real threshold set looks like. We'll email you the full guide.
The full PDF guide: the sessions for every zone, for swimming, cycling and running. One email, nothing else, and you can unsubscribe whenever you want.
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The same ones I use with my athletes, built on this zone structure.
Running plans are organised by race distance rather than by zone — pick your distance in the running plans, and the zones you just calculated are already built into them.
Critical swim speed (CSS) is the quickest pace you can hold, in open water or in a pool, without accumulating fatigue that forces you to slow down. It behaves as a threshold: it's the number every zone is built from.
It's estimated from two maximal efforts over different distances. The time difference between 400 and 200 metres describes exactly what each additional 100 metres costs you at a maximal sustainable effort — and that is your CSS. The maths is simple: the difference between the two times, divided by two, is your pace per 100 metres.
The test is worth exactly what its execution is worth. Rested, after a short but real warm-up. Swim the 400 evenly and hard, not fast-then-dying: a badly paced 400 inflates the gap between the two distances and hands you a slower CSS than the one you actually have. Between the two efforts, full recovery — several minutes, until your breathing is back to normal.
If the test falls apart halfway or the pacing goes wrong, don't patch it with an estimate. Repeat it the following weekend.
Under Settings → Zones → Swim, enter your critical swim speed as threshold pace and choose the 80/20 Swimming method. You'll get exactly the values you see here — this calculator exists so you can see them first, and understand where they come from.