Free Swim Tool
CSS Calculator: Find Your Critical Swim Speed
Enter your two time trial results and get your threshold pace per 100 m straight away. It is the pace most aerobic and threshold sets are built around.
Enter both times to calculate your CSS.
How to do the CSS test
Warm up well, then swim a 400 m time trial as fast as you can hold evenly. Recover fully with easy swimming, then swim a 200 m time trial all out. Enter both times above.
Use the same stroke for both swims (usually freestyle) and push off the wall rather than diving, so the test is easy to repeat.
How CSS is calculated
CSS is the difference in distance divided by the difference in time between the two swims. The pace per 100 m follows from that speed.
CSS = (D₂ − D₁) ÷ (T₂ − T₁)
How to use your CSS pace
Threshold sets usually sit at or close to CSS pace with short rest, for example 10 × 100 at CSS pace with 15 seconds rest. Easier aerobic sets swim a few seconds per 100 m slower.
Write your CSS pace into each set as a send-off so swimmers know the target before they push off.
Frequently Asked Questions
What is critical swim speed (CSS)?
CSS is an estimate of the fastest pace a swimmer can hold for a long, continuous effort, close to lactate threshold. It is usually written as a pace per 100 m.
Which distances should I use for the CSS test?
The standard test uses a 400 m and a 200 m time trial. You can also use 50/400 or 100/400, or enter your own distances in the calculator.
How often should I retest CSS?
Every 6 to 8 weeks, or at the start of a new training phase, so threshold sets keep matching your current fitness.