What is Critical Swim Speed?

CSS is the fastest pace you can sustain over a long swim without fatiguing rapidly — your aerobic threshold in the water. It is the swimming equivalent of a runner's threshold pace and is the anchor for most endurance swim training.

How is CSS calculated from 400m and 200m?

CSS is the slope of the distance-versus-time line through your two trials: CSS = (400 − 200) ÷ (T400 − T200) metres per second. Because the difference is exactly 200m, it equals 200 divided by the time gap between your two swims.

How should I do the two time trials?

Swim a maximal 400m, rest fully, then swim a maximal 200m, both as evenly paced as you can. Do them on the same day after a good warm-up. Accurate, all-out efforts are essential — a soft trial skews your CSS.

How do I train with my CSS?

Hold CSS pace for threshold intervals such as repeated 100s or 200s with short rest. Easy aerobic swims sit a few seconds per 100m slower than CSS, while VO2 and speed work go a couple of seconds faster. The tool lists these paces.

Why must my 400m be slower than my 200m time?

Over a longer distance your pace naturally slows, so your total 400m time will exceed your 200m time. If the 400m were faster, the inputs are inconsistent and CSS cannot be computed, so the tool flags it.

What is the Swimming CSS (Critical Swim Speed) Calculator?

Free Critical Swim Speed calculator. Enter your 400m and 200m time-trial results to compute your CSS threshold pace per 100m, your CSS speed in m/s, and threshold-based training paces. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Swimming CSS (Critical Swim Speed) Calculator

Name: Swimming CSS (Critical Swim Speed) Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Critical Swim Speed (CSS) is the single most useful number for endurance swim training — the threshold pace you can hold for a long, steady effort. This tool computes it from the classic 400 m and 200 m time-trial test, then converts it into a pace per 100 m and a set of training paces so every swim has a target.

How it works

CSS is the slope of the line through your two time-trial points. Because the distances differ by exactly 200 m:

CSS (m/s) = (400 − 200) ÷ (T400 − T200) = 200 ÷ (T400 − T200)

The tool converts that speed into a pace per 100 m (100 ÷ CSS), then builds threshold-relative paces: easy aerobic a few seconds slower, endurance slightly slower, and VO2/speed work a touch faster. It validates that your 400 m time is slower than your 200 m time before computing.

How to run the test

The accuracy of your CSS depends entirely on doing two genuinely maximal efforts:

Warm up thoroughly (at least 400–800 m of easy swimming and a few pickups).
Swim 400 m all-out from a push start, as evenly paced as possible. Record the time in minutes and seconds.
Rest fully — at least 10 minutes of easy swimming or passive rest.
Swim 200 m all-out, again evenly paced. Record the time.
Enter both times above.

A soft trial on either distance shifts your CSS. If your 200 m feels easy, you held back too much on the 400 m — which happens often when swimmers fear blowing up at 300 m. Aim for even splits across the 400 m.

Worked example

A 400 m of 6:00 (360 s) and a 200 m of 2:50 (170 s):

CSS = 200 ÷ (360 − 170) = 200 ÷ 190 ≈ 1.053 m/s
Pace per 100 m = 100 ÷ 1.053 ≈ 94.97 s ≈ 1:35 per 100 m

From this CSS of 1:35 / 100 m, the training zone table might look like:

Zone	Pace per 100 m	Use
Easy aerobic	1:45–1:55	Recovery, warm-up
Endurance	1:38–1:43	Long sets, base building
CSS threshold	1:33–1:37	Threshold intervals
VO2 / speed	1:25–1:32	Short high-intensity sets

Using CSS in training

The most effective use of CSS is threshold intervals: repeated swims of 100 m or 200 m with 5–15 seconds rest, targeting CSS pace. Sessions like 10 × 100 on CSS + 10 s rest, or 5 × 200 on CSS + 15 s rest, build aerobic capacity efficiently. Unlike open-ended distance sets, CSS intervals give you a precise pace target that is directly linked to your physiology — not an arbitrary number.

Re-test every 4 to 6 weeks. As fitness improves your CSS improves; without re-testing you will train at a pace that is too easy and miss the adaptation stimulus.

Everything runs locally in your browser — nothing is uploaded.