Critical Swim Speed (CSS) Calculator

1. Swim an all-out 400 metres and record the time.
2. Rest fully, then swim an all-out 200 metres and record that time.
3. Enter both times as minutes and seconds.
4. Read your CSS speed and threshold pace per 100.

Critical swim speed, or CSS, is the fastest pace you can hold for a long time without fatiguing, roughly your lactate threshold in the water. It is the single most useful number for setting swimming training paces, marking the line between steady endurance and hard efforts.

It is found from two all-out time trials, usually 400 metres and 200 metres. The difference in distance over the difference in time gives a speed that closely matches your sustainable threshold pace. Coaches favour it because it needs nothing more than a pool and a watch, yet it produces a pace precise enough to build an entire training plan around, from easy recovery swims to hard threshold sets.

1. Swim all-out time trials over 400 and 200 metres.
2. Subtract the 200 metre time from the 400 metre time.
3. Divide the 200 metre distance difference by that time difference for the speed.
4. Divide 100 by the speed for the threshold pace per 100.

The method works because the extra 200 metres of the longer swim is covered at close to your sustainable pace, once the fast start of the shorter swim is removed. In effect the maths cancels out the anaerobic burst you can only hold briefly, leaving the aerobic pace you could keep going at for far longer.

You swim 400 metres in 6 minutes, which is 360 seconds, and 200 metres in 2 minutes 50 seconds, or 170 seconds. The difference is 190 seconds for the extra 200 metres, so your CSS is 200 divided by 190, about 1.05 metres per second. That is a threshold pace of roughly 1 minute 35 seconds per 100 metres, the pace to build your training around. Retest the same two trials in a month or two and a faster CSS is proof your endurance has genuinely improved, in a way that a single fast swim on a good day never quite confirms.

Fitness

As your endurance improves, your CSS speed rises and your threshold pace per 100 falls.

Test Quality

Both trials must be genuinely all-out and well rested, or the CSS comes out wrong.

Technique

A more efficient stroke raises CSS without extra fitness, by covering more distance per effort.

Pool and Conditions

Turns and pool length affect the trial times, so test in consistent conditions.

Threshold pace per 100 for some CSS speeds.

Retest every few weeks; a falling threshold pace is a clear sign your endurance is improving.

Setting Training Paces

Base your interval and endurance sets on your CSS pace per 100.

Tracking Fitness

Retest to see your threshold pace improve over a season.

Planning a Race

Use CSS to judge a realistic pace for a distance event.

1. Not Swimming All Out

Both trials must be maximal efforts, or the CSS is too slow to be useful.

2. Too Little Rest Between Trials

Rest fully between the 400 and the 200, or the second time suffers.

3. Mixing Pools

Test both trials in the same pool, since turns and length affect the times.

4. Never Retesting

Fitness changes, so an old CSS soon stops matching your real threshold.

The CSS formula is exact for the two times you enter, and the method is well established. Its value depends on both trials being true maximal efforts.

CSS is your threshold swimming pace, the fastest you can sustain without fatiguing. It is found from a 400 metre and a 200 metre time trial.

Subtract your 200 metre time from your 400 metre time, then divide the 200 metre distance difference by it. A 6:00 400 and 2:50 200 gives about 1.05 m/s.

Swim an all-out 400 metres, rest fully, then swim an all-out 200 metres. Enter both times to get your CSS.

The extra 200 metres of the longer swim is covered close to threshold pace, so the difference gives a good estimate of sustainable speed.

It varies by level: around 2:00 per 100 is developing, 1:40 is fit, and 1:10 or faster is competitive.

Set your endurance sets near CSS pace, and your threshold intervals at or just faster than it. Most swim training is built around CSS.

Every four to six weeks. A falling threshold pace shows your endurance is improving.

Yes. A more efficient stroke covers more distance per effort, raising your CSS without any change in fitness.

No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Critical swim speed and threshold (Wikipedia).
• Swimming (sport) (Wikipedia).
• Time trial (Wikipedia).