Error: Your 200 pace is slower than your 400 pace!

Critical Swim Speed

Your Critical Swim Speed (CSS) is the predicted pace you could currently swim a 1500 time trial. The 400 and 200 within your test are used to calculate your CSS.

Once you have completed your swim test, you will need to update your threshold settings in TrainingPeaks.

Enter or Update Your Thresholds and Training Zones in TrainingPeaks (Swim)

If you see the error message 'Your 200 pace is slower than your 400 pace!' when entering your swim times into the CSS Calculator, it means your pace over 200 was slower than over 400.

How the CSS Test Works

The Critical Swim Speed (CSS) test uses your 400m and 200m times to predict your current 1500m pace.

Both parts of the test use a mix of aerobic and anaerobic effort, but the calculation focuses on your aerobic ability — the pace you can sustain.
The test assumes your 200m should be faster than your 400m, as the 200m effort can't be fully anaerobic.

How CSS Is Calculated

CSS is based on the difference between your two swim times:

(400m time – 200m time) ÷ 2 = CSS pace per 100m

For this formula to work correctly, your 200m time must be quicker than your 400m time.

Why Your 200m Might Be Slower

Several factors can affect pacing in the shorter interval:

• Fatigue (common for newer swimmers or after a break)

• Not enough rest between the 400m and 200m

• Busy lanes or other disruptions

• Going out too hard on the 400m

When this happens, the calculator cannot give a reliable CSS value.

What To Do If Your Test Doesn’t Calculate

• If you already have a good estimate of your CSS, enter that for now.

• Pay attention to your swim sessions:

  • If paces feel too easy or too hard

  • Or your planned vs. completed TSS is far off
    → You may need to retest sooner than scheduled.

Need Help?

If unsure, please email our coach support team (from the Email the Coach button at the top).

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