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.

Swimming CSS (Critical Swim Speed) Calculator

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Swimming CSS calculator

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.

Example and tips

A 400 m of 6:00 (360 s) and a 200 m of 2:50 (170 s) give 200 ÷ (360 − 170) = 1.053 m/s, or about 1:35 per 100 m CSS. Swim both trials all-out and evenly paced on the same day for an accurate result, and re-test every 4 to 6 weeks to keep your training paces current.

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