CSS: Finding Your Threshold Pace in the Pool
On the bike you have FTP. You know exactly how many watts you can hold, and which zone you are training in. In the pool, most triathletes swim by feel, too slow one day, too hard the next, with no number to check against. Swimming has its own equivalent of FTP. It is called Critical Swim Speed, and once you have it, it changes how you build every session in the water.
What CSS is, and why it is your threshold
CSS is the fastest pace you can hold continuously before lactate build-up forces you to slow down. Put simply, it is your threshold pace in the water. The same way FTP on the bike marks the line between riding for hours and blowing up in minutes, CSS marks that line in swimming.
That matters for a triathlete because most of the racing you do in the water happens right around this threshold. Knowing your CSS tells you how hard you can go off the start without cooking yourself in the first 200 metres and coming out of the water with your heart rate through the roof.
The test: 400 and 200 metres flat out
Working out CSS is straightforward and needs no equipment beyond a watch, or a stopwatch on the pool wall. In a single session, swim two maximum-effort time trials:
- 400 m for time, swum as fast as you can hold across the full distance.
- Full recovery between the two, 10 to 15 minutes of easy swimming or rest. This matters: too short a break means you start the 200 m tired, which inflates your CSS.
- 200 m for time, again flat out.
Record both times in seconds. That is everything the calculation needs.
The formula: working out your pace
CSS is the difference between the two times, divided by two:
CSS (seconds per 100 m) = (400 m time − 200 m time) ÷ 2
Take an example. You swim 400 m in 6:30 (390 seconds) and 200 m in 3:00 (180 seconds). The difference is 210 seconds. Divide by 2 and you get 105 seconds, a pace of 1:45 per 100 m. That is your CSS, and it is the number the rest of your training is built around.
The logic behind the formula is neat: the difference between the 400 m and the 200 m time is, in practice, how long it takes you to cover the “second” 200 metres, once the boost from the start has worn off. That is the best approximation of the pace you can hold over a longer distance.
Training zones built on CSS
Once you know your threshold pace, every session gets a specific target. Instead of “swim for a while”, there are zones, exactly like on the bike:
| Zone | % of CSS | Purpose |
|---|---|---|
| Recovery | 65 to 75% | Easy swimming you can hold a conversation through. Technique and drills. |
| Endurance 1 | 75 to 85% | Aerobic base. Long sets of 1000 m and beyond. |
| Endurance 2 | 85 to 90% | Comfortably hard. Repeats of 400 to 800 m. |
| Threshold | 90 to 100% | Lactate work. Repeats of 100 to 400 m with short rest. |
The most valuable work for a triathlete happens in the threshold zone, since that is what actually pushes the limit further. That does not mean grinding at threshold every day. Train at CSS pace once, at most twice a week. The rest is aerobic base and technique.
Fitting it into a plan, and when to retest
A classic threshold session is a set like 10 × 100 m at CSS pace with a short rest, for example 10 to 15 seconds. Over time you shorten the rest or lengthen the intervals, and that is the proof that fitness is improving. There is no guessing whether progress is happening. It shows up in the numbers.
CSS is not fixed. As fitness changes, the threshold moves, so retest every 4 to 6 weeks. If CSS drops from 1:45 to 1:42 per 100 m after a training block, that is proof the work is paying off, and every zone gets updated with it.
The end of swimming by feel
Anyone who rides with a power meter and runs to a pace on their watch, but still swims “whatever comes out”, is leaving free seconds on the pool wall. The 400/200 test takes one session, the formula fits on one line, and the payoff is the same as with FTP: knowing, for the first time, what is actually happening in the water.