Tools / Critical Swim Speed

Critical Swim Speed (CSS) calculator

Enter a maximal 400 and 200 time trial (same course, same session) to find your CSS threshold pace and a set of training-zone paces around it.

How do you calculate Critical Swim Speed?

CSS is your estimated threshold pace: the difference between your 400 and 200 time-trial times, divided by two. It approximates the fastest pace you could hold for a long, steady effort. Enter both times and this calculator returns your CSS per 100 plus training-zone paces around it.

Because CSS is anchored to real test swims, it's a practical, repeatable way to set threshold pace without lab testing. Recheck it every 6–8 weeks as fitness changes.

Tip: CSS gives you the pace; live heart rate confirms the effort is actually landing in the zone you intended.

FAQ

What's a good CSS test protocol?

Swim a maximal 400 and a maximal 200 in the same session, same course, with full rest between. Enter the two times here. Consistent conditions matter more than the exact distances.

How do I train with CSS?

Hold CSS pace for threshold sets, add a few seconds per 100 for aerobic and endurance work, and drop below it for short speed repeats. The zone paces in the result give you the targets.