What critical swim speed is

Critical swim speed, or CSS, is the pace you can hold for a long time without falling apart: in practice an estimate of your aerobic threshold in the water. It is the reference most used by swimming and triathlon coaches to set training sets, because it comes from a simple test and is expressed as a pace per 100 metres.

This calculator takes the times of a 400 and a 200 swum all out and returns your CSS pace, the speed in metres per second, the pace slowdown between the two distances and the 1500 m time at that pace, with a guide to training zones.

How to do the test

• Warm-up: 10-15 minutes of easy swimming with a few build-ups.
• 400 m: swim it at the hardest effort you can sustain evenly, timing it.
• Recovery: 5-10 minutes of very easy swimming, until your breathing returns to normal.
• 200 m: swim it all out, also freestyle and in the same pool.

Do the test in a pool, with turns, and repeat it every 6-8 weeks to track progress. The 400-then-200 order is the most common: with the 200 second, the fatigue of the first effort weighs less on the result.

How the calculation works

The method uses the nearly linear relationship between distance and time in efforts of a few minutes. The difference between the two distances, 200 metres, divided by the difference in times gives the speed you could hold for a long time.

• CSS: 200 / (400 m time − 200 m time), in metres per second.
• CSS pace: (400 m time − 200 m time) / 2, in seconds per 100 m.
• Pace slowdown: how much slower the average 400 pace is than the average 200 pace, as a percentage.
• 1500 m time: 15 times the CSS pace.

The verdict is based on the pace slowdown, which describes your profile: how well your speed holds when the distance doubles.

Example tests

CSS pace is always slower than your average 400 pace: it is the pace you could hold for about half an hour, not for a few minutes.

The four bands of the verdict

• Slowdown below 4%: you hold pace very well over distance; often speed is the margin to train, or the 200 was not swum all out.
• 4% to 8%: a balanced profile, the typical range for regular swimmers.
• 8% to 12%: a speed profile; pace fades over distance and sets at CSS pace are the most useful lever.
• 12% and above: a marked fade; endurance is the limit, or the 400 started too fast.

How to use CSS in training

The zones below are a guide, because each method defines them slightly differently. The example uses a CSS of 1:45 per 100 m.

A classic threshold set is, for example, 10 × 100 m at CSS pace with 10-15 seconds' rest: if you can complete it all at the target pace, your CSS is reliable.

CSS pace over race distances

At a constant CSS pace, here are the times over the most common open-water and triathlon distances:

Over short distances you swim faster than CSS, over long ones a little slower. In open water, wetsuits, currents, sighting and drafting also matter, so real times can differ a lot from pool times.

Limits of the calculator

CSS is an estimate based on just two efforts and depends heavily on how well you swam them: a 400 started too fast or a 200 not swum all out changes the result. It applies to freestyle and to the pool length of the test; in a 50 m pool times are usually a little slower than in a 25 m pool. The training zones are a guide and do not replace a coach's programme.

Frequently asked questions

How do you calculate CSS in swimming?

Swim 400 m all out and, after a recovery, 200 m. CSS is 200 divided by the difference in times in seconds: for example, with 6:00 and 2:50 the difference is 190 seconds and the CSS pace is 1:35 per 100 m.

What does critical swim speed represent?

It is an estimate of the pace you can sustain for a long time, close to your aerobic threshold. It is used as a reference to set training sets.

Why is my CSS pace slower than my 400 pace?

Because a 400 lasts a few minutes, while CSS represents the pace sustainable for about half an hour. It is normal for it to be a few seconds slower per 100 m.

What order should I follow in the test?

The most common is the 400 first and then the 200, with 5-10 minutes of easy recovery between them, after a good warm-up.

How often should I repeat the test?

Every 6-8 weeks, in the same conditions and the same pool, to track progress and update your training paces.

What does the pace slowdown mean?

It shows how much slower your average 400 pace is than your 200 pace. A small slowdown means good endurance; a large one means your speed fades a lot over distance or the 400 started too fast.

What are CSS-based training zones?

As a guide: aerobic 5 to 10 seconds slower than CSS per 100 m, threshold at CSS pace, fast 3 to 5 seconds quicker. Each method defines them slightly differently.

Can I do the test in a 50 m pool?

Yes, but always use the same pool to compare results: a long-course pool has fewer turns and times are usually a little slower.

Does CSS apply to other strokes?

The test is swum freestyle and the paces derived from it apply to freestyle. Other strokes would need a specific test.

Can I use CSS to predict my open-water time?

It gives an indication, but in open water wetsuits, currents, waves, sighting and drafting matter, so the real time can be very different.

Does the calculator work for beginners?

Yes, as long as you can swim 400 m continuously freestyle. For beginners the test is mainly useful to measure progress.

What if the calculator shows an error?

Check the times: your 400 pace cannot be faster than your 200 pace, and a much slower 400 usually means the test was not swum at a steady effort.