Critical Swim Speed calculator

Two time trials in - your CSS pace per 100m, target splits and training zones out. The standard threshold test used by swim and tri coaches.

min / sec
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?

Close - CSS typically lands within a couple of seconds per 100m of a fresh 1500m race pace. It's an estimate, not a lab test, but it's consistent, free, and tracks 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?

No - test in skin so retests compare cleanly. Expect the wetsuit to make you roughly 3-6 seconds per 100m faster on race day (nice surprise, isn't it). If you need one, our wetsuit size finder is next door.