Tools
CSS Calculator
Calculate your Critical Swim Speed and determine your swim training zones
CSS Calculator
What is Critical Swim Speed?
Critical Swim Speed (CSS) is the fastest pace you can theoretically sustain without exhaustion. It's comparable to lactate threshold and forms the foundation of structured swim training. CSS is calculated by dividing the distance difference (400m − 200m) by the time difference.
Why is CSS important?
- It gives you an objective measure of your aerobic swim capacity
- You can accurately determine training zones, similar to FTP in cycling
- It helps monitor progress without a full exercise test
- It prevents training too hard or too easy
Test Procedure
- 1Warm up well: 400m easy swimming, followed by 4×50m at increasing intensity
- 2Swim 400m all-out: give everything and record your exact finish time
- 3Rest 10–15 minutes: swim easy to fully recover
- 4Swim 200m all-out: maximum effort again, record the finish time
- 5Cool down: 200–300m easy swimming
Tip: Do the test in a 25m or 50m pool with accurate timing. Make sure you're rested — don't do the test after a hard workout.
Example Calculation
A swimmer completes 400m in 6:20 (380s) and 200m in 2:58 (178s). CSS = (400 − 200) / (380 − 178) = 200 / 202 = 0.990 m/s → that's 1:41 per 100m. This pace becomes zone 3: the CSS threshold.