CSS swimming calculator
Determine your critical swim speed from a 400 m and 200 m test. You get threshold pace per 100 m and ready-made swim zones.
Everything is computed in your browser — the values you enter are never sent or stored.
Result
- CSS — threshold pace
- 1:53 / 100 m
- CSS — speed
- 0.889 m/s
| Zone | Pace / 100 m | Use |
|---|---|---|
| Z1 | > 2:21 | technika, regeneracja |
| Z2 | 2:05-2:21 | baza |
| Z3 | 1:55-2:05 | tempo |
| Z4 | 1:47-1:55 | okolice progu |
| Z5 | < 1:47 | mocne odcinki |
Swim measurement has real limitations: turns, stroke and watch accuracy all shift the result. Do the test after a warm-up, both distances on the same day.
How we calculate this
- CSS [s/100 m] = (T400 − T200) / 2
- CSS [m/s] = 200 / (T400 − T200)
- Zones: Z1 < 80%, Z2 80-90%, Z3 90-98%, Z4 98-105%, Z5 > 105% CSS
The formulas come from the app’s domain specification and are covered by unit tests. The same code powers zones inside the app.
Want these numbers to feed your plan automatically?
In the app, zones update after every test and the plan adapts to your data and how you feel.
This calculator is informational and does not replace a doctor, physiotherapist or coach. Results based on population formulas are estimates, not measurements. Read the full disclaimer →