Critical Swim Speed (CSS) calculator: how to set your endurance swim pace for 33.3km

Critical Swim Speed (CSS) is the pace equivalent to your lactate threshold, the highest speed you can swim aerobically without accumulating fatigue. Unlike maximum pace or your best 100m time, CSS is the number that predicts performance over the distances that matter in multi day open water racing.

Calculated from two pool test sets: a 400m and a 200m time trial. Expressed as a pace per 100m that defines your aerobic ceiling.

Used to set training intensities across all sessions in your plan. Retested every 4 to 6 weeks to track fitness progress.

The calculation is straightforward. Swim a 400m time trial at maximum effort, rest fully, then swim a 200m time trial at maximum effort. Your CSS pace is derived from the difference in distance divided by the difference in time. Most swimmers find their CSS sits 10 to 15 seconds per 100m slower than their best 100m pace.

CSS = (400 − 200) ÷ (T400 − T200), expressed as seconds per 100m. Train at 100 percent CSS for threshold sets, 85 percent for aerobic base work.

Race pace for 33.3km is typically 90 to 95 percent of your CSS. Adjust for open water conditions: add 5 to 8 seconds per 100m for chop and navigation.