Critical Swim Speed (CSS) test: how to do it and use the result
CSS is an estimate derived from the distance–time relationship, used to guide endurance interval pacing. The practical 400/200 test can overestimate sustainable critical speed; it is not a guarantee of race pace. The test needs only a pool, a clock and two time trials.
Open the Critical Swim Speed CalculatorThe 400 m and 200 m test
- Warm up for 400 to 600 m with easy swimming, drills and a few short build-ups.
- Swim 400 m at the fastest even pace you can hold. Record the time.
- Recover with easy swimming and rest for about 5 to 10 minutes. This is a practical starting point, not a guarantee of full recovery; repeat the test if fatigue affects the second effort.
- Swim 200 m at the fastest even pace you can hold. Record the time.
- Cool down with easy swimming.
Start each trial from a push-off and keep the pacing even. A fast first 50 m that you cannot hold will distort the result.
How CSS is calculated
The two distances differ by 200 m, so the extra 200 m took (400 m time minus 200 m time). Divide that by two to get the pace per 100 m.
CSS pace per 100 m = (400 m time in seconds minus 200 m time in seconds) ÷ 2.
Example: a 400 m in 6:20 (380 s) and a 200 m in 3:00 (180 s) give (380 minus 180) ÷ 2 = 100 s, a CSS of 1:40 per 100 m. For yards, multiply the metric pace by 0.9144, so 1:40 per 100 m is about 1:31 per 100 yd.
Using your CSS in training
- Threshold sets: for example 8 to 10 × 100 m at CSS pace with 10 to 15 seconds rest, or 5 × 200 m at CSS with 20 seconds rest.
- Easier aerobic swimming: a few seconds per 100 m slower than CSS.
- Faster work: 100 m repeats a few seconds per 100 m faster than CSS, with more rest.
Adjust the rest to keep the effort honest. If you cannot hold CSS pace through the set, the pace or the rest is wrong, not you.
Limits of the test
- It depends on the pool length and the quality of your push-offs, so test in the pool you train in.
- Pool CSS does not directly predict open-water pace: sighting, waves, currents, wetsuit buoyancy and drafting can change it in either direction.
- Check that both distances and times are correct, both efforts were maximal, and recovery was adequate; retest implausible results.
When to retest
As a practical training example, retest every 4 to 6 weeks while you are improving, using the same pool, the same warm-up and similar rest between trials.
FAQ
Is CSS the same as race pace?
No. CSS is a test-based estimate for training, not a race-pace prediction. Practise your intended race effort and account for the course, conditions and wetsuit.
Can I test with 200 m and 100 m instead?
400/200 is a common practical protocol, but longer-distance trials can improve validity. Do not assume results from different distance pairs are interchangeable.
Do I need a coach?
No. You need a clock, honest pacing and the same test conditions each time.