Critical Swim Speed Calculator
Your critical swim speed — the pace you could hold for a long, hard swim — from 400 m and 200 m time trials, with the pace per 100 m and paces for endurance, threshold and race sets.
Swim a 400 and, after a rest, a 200, both flat out.
How the critical swim speed calculator works
Swim a 400 and, after a rest, a 200, both flat out. The 400 takes longer per 100 than the 200 because it draws more on aerobic endurance; the slope between them — extra distance over extra time — is a speed you could theoretically hold indefinitely: CSS = (400 − 200) ÷ (t₄₀₀ − t₂₀₀). Threshold sets are swum at CSS pace, endurance a few seconds per 100 slower, race pace faster.
Formula: CSS (m/s) = 200 ÷ (t₄₀₀ − t₂₀₀); pace per 100 m = 100 ÷ CSS
Worked examples
| Inputs | CSS pace per 100 m | Note |
|---|---|---|
| 400 in 6:40, 200 in 3:10 | 1:45 per 100 m | 1:45 per 100 m |
| 400 in 5:20, 200 in 2:32 | 1:24 per 100 m | 1:24 per 100 m |
| A strong swimmer: 4:30 and 2:08 | 1:11 per 100 m | 1:11 per 100 m |
FAQFrequently asked questions
How should I do the time trials?
Warm up well, swim the 400 flat out, rest 5–10 minutes, then the 200 flat out — or do the 200 first if you prefer. Push-off starts, no dive. Repeat every 6–8 weeks to update your paces.
Why is CSS faster than my 400 pace but slower than the 200?
Because it is the sustainable component: the 200 includes anaerobic energy you cannot keep producing, while the 400 pace is already fading. CSS sits between and is close to your 1,500 m pace.
What is a typical CSS set?
Something like 10 × 100 m at CSS pace with 10–15 seconds’ rest, or 6 × 200 m with 20 seconds. The short rest is the point — it trains the pace you can hold, not sprinting.
Is this the same as lactate threshold?
Close to it — CSS correlates well with the pace at which lactate starts to accumulate, without a blood test. Triathletes use it as swimming’s equivalent of FTP or threshold pace.
Where these figures come from
- Riegel PS, "Athletic records and human endurance", American Scientist 1981 — T2 = T1 × (D2 ÷ D1)^1.06
- Cooper KH, "A means of assessing maximal oxygen intake", JAMA 1968 — the 12-minute run test
- Ainsworth BE et al., Compendium of Physical Activities (2024 update) — MET values by activity
- OpenPowerlifting — DOTS coefficients (Tim Konertz, 2019) — the DOTS polynomial used for scoring
- Naismith's rule (1892) as used by UK and Australian walking bodies — 1 hour per 5 km plus 1 hour per 600 m of ascent
- Australian Institute of Sport — national sports-science body
Last checked: September 2026. Formulas are fixed by their published sources; the MET values come from the Compendium of Physical Activities.