Critical Swim Speed Test: 400 and 200

The standard critical swim speed test is two maximal time trials at different distances, almost always a 400 and a 200. Swim both as hard as you can hold from start to finish, then drop the times in below.

Two rest schemes, two different priorities

There is no single blessed protocol, and the disagreement is worth understanding before you pick one.

• Same session. U.S. Masters Swimming has you swim the 200 shortly after the 400, after a short active recovery such as easy kicking. One trip to the pool, one warm-up, done. The cost is that the 200 is swum on legs the 400 already taxed.
• Separate days. The stroke-specific research protocol put every trial at least 48 hours apart, and ran three repeats of each distance over three weeks. Fatigue carry-over disappears, and so does any chance of getting your number this afternoon.

Pick the practical one and be consistent, or pick the strict one and get a cleaner baseline. What you must not do is switch schemes between tests and read the difference as fitness.

The session, step by step

1. Warm up 10 to 15 minutes, including a few short fast efforts so the first hard length does not feel like a shock.
2. Swim the 400 at the fastest pace you can hold even — not a sprint start you pay for at 300. Record the time.
3. Recover. Easy swimming or light kicking if you are doing both today; at least 48 hours if you are following the research protocol.
4. Swim the 200 maximally, same stroke, same pool, same course.
5. Enter both times above.

Swim in one course throughout. A 400 in yards paired with a 200 in meters produces a number that means nothing, and the SCY to LCM page explains why the two cannot be reconciled with a factor.

Four ways to ruin the result

• Sandbagging the 200. By far the most common failure. A soft 200 shrinks the time difference between trials, and since that difference is the denominator, CSS comes out too fast. You then train above threshold all season and call yourself unfit.
• Going out too hard in the 400. The mirror image. A blown 400 inflates the time gap and drags CSS down, and every set afterwards is too easy.
• Not enough recovery. A 200 swum on a fatigued 400 is not a maximal 200.
• Changing stroke, course or equipment between trials. Fins, a pull buoy, a different pool — any of these breaks the comparison the subtraction depends on.

After the test

Sanity-check the output first: critical speed should land at roughly 91% of your 400 pace, and the two-trial method is known to read high rather than low, so treat the printed pace as the fast edge of your threshold. Then turn it into sets with the swimming training zone chart, and convert any distance you swim into a comparable pace with the swim pace calculator. The CSS calculator covers the formula itself in more detail.

This content is informational and does not replace coaching or medical advice.