What is Critical Swim Speed?

Critical Swim Speed (CSS) is an estimate of the fastest pace you could sustain aerobically - roughly your 1500m race pace. It is calculated from a 400m and a 200m time trial: CSS = (400 - 200) divided by (your 400m time minus your 200m time). Swim coaches use it to set threshold training paces.

How do I do the CSS time trials?

In the same session, after a thorough warm-up: swim an all-out 400m, take 5-10 minutes of very easy recovery, then swim an all-out 200m. Both from a push, not a dive. Pace the 400m evenly - a fast start ruins the result. Retest every 6-8 weeks.

What do I do with my CSS pace?

Train around it. Threshold sets sit at CSS (e.g. 10x100m at CSS with 10-15 seconds rest), endurance swims at CSS +4-6 seconds per 100m, and short speed work faster than CSS -3 seconds. Improving your CSS pace over retests is the clearest sign your fitness is moving.

Critical Swim Speed calculator

Swim a 400m and a 200m time trial and this works out your CSS pace per 100m, along with the training zones coaches set from it.

400m time trial

min / sec

200m time trial

min / sec

CSS = (400 − 200) ÷ (T400 − T200). Retest every 6-8 weeks in the same pool - a dropping CSS pace is fitness you can see.

Doing the test properly

Same session, proper warm-up. 400m all-out first, then 5-10 minutes very easy, then 200m all-out.
From a push, not a dive - and no wetsuit, so results stay comparable.
Pace the 400m evenly. The classic mistake is a sprint first 100m; it inflates your CSS and makes every threshold set feel impossible.
Same pool for retests. Short-course times don't compare with long-course, and open water is its own world.

What the zones mean

CSS approximates the pace you could hold for about 1500m fresh - your aerobic threshold in the water. Training slightly below, at, and slightly above it stresses different systems: the zones in the results table follow the ranges swim coaches commonly set around CSS.

FAQs

Is CSS the same as my 1500m race pace?

Very close. CSS typically lands within a couple of seconds per 100m of a fresh 1500m race pace. It's an estimate rather than a lab test, but it's consistent, free, and tracks your fitness well.

Why not just use my 400m pace?

A single trial mixes aerobic fitness with speed. Using the drop-off between 200m and 400m isolates the sustainable component - that's the maths the formula does.

Should triathletes test in a wetsuit?

Test in skin, not a wetsuit, so your retests always compare cleanly. On race day expect a wetsuit to make you roughly 3-6 seconds per 100m faster, which is a welcome bonus rather than something to bank on. If you need one, our wetsuit size finder is next door.