One pace. Five zones. Smarter swimming.
Type your 400m and 200m time-trial times. We compute your Critical Swim Speed and derive 5 training pace zones used by pool and triathlon coaches worldwide.
Your CSS is 1:38 per 100m. Train in the zones below to build pool fitness and threshold endurance.
Find swimming eventsEach zone is a CSS offset (per 100m). Easy zones are slower than CSS; race zones are faster. Use for structured pool sets.
CSS is the swimming equivalent of FTP in cycling or threshold pace in running — the fastest pace you can sustain for ~30 minutes without crossing into anaerobic territory. The 400m + 200m test is from coach Paul Newsome (SwimSmooth); it’s the most-cited CSS protocol in triathlon and pool training.
Tempo per 100m per zone bij gangbare CSS-niveaus — van elite (1:30) tot amateur (2:30).
| CSS | E1 | E2 | CSS | A1 | A2 |
|---|---|---|---|---|---|
| 1:30 | 1:40–1:50 | 1:35–1:40 | 1:30 | 1:25–1:28 | 1:20–1:25 |
| 1:45 | 1:55–2:05 | 1:50–1:55 | 1:45 | 1:40–1:43 | 1:35–1:40 |
| 2:00 | 2:10–2:20 | 2:05–2:10 | 2:00 | 1:55–1:58 | 1:50–1:55 |
| 2:15 | 2:25–2:35 | 2:20–2:25 | 2:15 | 2:10–2:13 | 2:05–2:10 |
| 2:30 | 2:40–2:50 | 2:35–2:40 | 2:30 | 2:25–2:28 | 2:20–2:25 |
Best protocol: warm up 800-1000m. Swim 400m all-out (push from a static start) and record the time. Easy 200m recovery. Swim 200m all-out and record. Cool down 200-400m. Take CSS = (T400 − T200) / 2. The test stresses your aerobic system; allow 48 hours before another hard session.
The formulas and ranges above are grounded in the following peer-reviewed literature.
Original Critical Swim Speed (CSS) paper. Our two-distance estimate (T400 − T200) / 2 is the simplified field-test version.
Read paperTools die hier goed bij passen — open ze in een nieuw tabblad en plan verder.
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CSS is roughly the pace you’d hold for 30 minutes — your threshold ceiling. Race paces vary: a 400m race is faster than CSS, an Ironman swim is slower. Use CSS as the reference point for threshold sets, not as your race pace.
Yes — the math works the same. Just enter your 400 yd / 200 yd times instead. The output is per-100 (yards or meters) according to what you input.
Every 6-8 weeks during a swim block. CSS shifts as fitness improves, so out-of-date zones make threshold work too easy or too hard.
CSS is calculated from a single test. If a long threshold set (e.g. 5×400m at CSS) feels off by more than 5s/100m, your test was probably not max-effort or you’ve drifted. Re-test before adjusting zones manually.