Swim CSS Calculator
Critical Swim Speed from 400m and 200m time trials: threshold pace, zones and target times.
Format m:ss, e.g. 6:40 and 3:10
What Critical Swim Speed is
CSS (Critical Swim Speed) approximates your anaerobic threshold pace in the water: the speed you could theoretically hold for 1500m. It's computed from the difference between two time trials: CSS = (400 β 200) / (T400 β T200). It's the most accessible way to set swim training intensity without a lactate meter.
How to run the test
After a standard warm-up, swim 400m all-out, recover fully (5β10 minutes of easy swimming), then swim 200m all-out. Both swims start with a push off the wall, on the same day, in the same pool. Pace evenly: going out too fast in the 400 is the main cause of a skewed CSS.
Frequently asked questions
How do I train with CSS?
The classic set is intervals at CSS pace with short rest: for example 10Γ100m with 10β15 seconds rest. Recovery swimming sits at CSS +10s/100m and slower; VO2max work runs 1β4 seconds faster than CSS.
How often should I retest?
Every 6β8 weeks. CSS responds to training faster than you'd expect: a stale pace turns threshold sets either too easy or impossible.
Why is my CSS slower than my 1500m race pace?
That's expected for trained swimmers: race day adds adrenaline, drafting and a fresh taper. CSS is a training anchor, not a result predictor β for projections, check the target times table.