Critical swim speed is the swim equivalent of a runner’s lactate threshold, the fastest pace you can hold sustainably. Two timed efforts (a short and a long) bracket the truth. Out comes your CSS, your SDI fade index, training-zone bands, and finish-time predictions for 750 m to 10 km.
Standard test: 400 m all-out, rest, then 200 m all-out (or any two solid time-trials at different distances). Distance 1 should be the shorter, faster swim. Times accept either mm:ss or just seconds.
The number is only as good as the test. Two protocols, both valid.
Warm-up 800–1000 m. 400 m all-out time trial. Easy 200 m recovery. 200 m all-out. Plug both in. Most accurate when efforts are spaced enough to recover but close enough that fitness hasn’t shifted.
Better for triathletes targeting 70.3+. 1500 m time trial, recovery, then 100 m flat-out. Gives a more endurance-anchored CSS but the SDI fade index is more dramatic (and informative).
CSS shifts with training. Re-test every block. Same pool, same time of day, same protocol, or the comparison is noise.
The Swim Distance Index is just your second pace divided by your first. It tells you how much you fade as distance climbs, and that fade pattern hints at what your training should focus on.
What each band is for, and what a typical session looks like.
25–150 m reps, long rest (1:30–10:00). Builds peak speed and lactate tolerance. Use sparingly, 1 session per 7–10 days.
100–300 m reps, 1:00–3:00 rest. The classic 8×100 on 1:30 lives here. Hard but repeatable. 1–2 sessions per week.
100–3500 m reps, 5–30 s rest, very short. The most cost-effective swim training there is. Hold this band 1–2×/week and your race pace climbs.
800–3500 m continuous or with token rest. Builds the oxidative base that lets you hold CSS longer. Frequent, long, easy, no heroics.
Any distance, any rest. Form work, drill ladders, recovery between hard sessions. Slow enough to do a full kick-set without the legs falling apart.
Stroke rate vs distance per stroke. Tells you whether to lift rate or improve DPS.
Open → Swim efficiency II, tipping pointsFind the speed where your DPS collapses. Pairs naturally with CSS for race pacing.
Open → All calculatorsBack to the full catalog, 30+ endurance calculators across performance, fueling, swim, bike, run.
Browse →The lab’s swim protocol uses a 5×200 step test with lactate sampling, gives you a true threshold pace plus stroke metrics. Compare it against this calculator and you’ll see exactly where the population averages stop describing you.
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