Critical Swim Speed Calculator

Critical swim speed (CSS) is the fastest pace you can hold for a long, hard swim before fatigue builds quickly. Swimmers and triathletes use it to set the pace of their main threshold sets. This calculator finds it from two all-out swims.

What Critical Swim Speed Measures

CSS sits close to your threshold, the point above which blood lactate keeps rising. When swimmers held their CSS over 4 x 400 m, their blood lactate stayed steady at about 3.2 mmol/L, but it kept climbing at just 2% faster¹. Later work found CSS is a little faster than the true maximal lactate steady state (the fastest pace with stable lactate), at about 93% versus 88% of maximal aerobic speed². It does not measure sprint speed or VO2 max.

The Formula

CSS speed = (400 - 200) / (T400 - T200)

CSS pace per 100 = (T400 - T200) / 2

Other pairs: CSS pace per 100 = 100 x (T2 - T1) / (D2 - D1)

T400 and T200 are your times in seconds. D1 and D2 are the shorter and longer distances, and T1 and T2 their times. The method takes the slope of the straight line joining your two distance and time points, as Wakayoshi and colleagues did with 200 m and 400 m swims¹.

How This Calculator Computes Your Result

Choose meters or yards and enter your 400 and 200 times in minutes and seconds. You get your CSS pace per 100 in the unit you swam, the same speed per 100 in the other unit (1 yard is exactly 0.9144 m), your CSS speed in meters per second, and the pace you held in each trial. Paces are rounded to the nearest second.

The advanced option lets you use another pair, such as 50 and 400, 100 and 400 or 200 and 800, as long as the longer swim is at least twice the shorter one. Different pairs give different answers, so the result is labeled. There is no result if your longer swim pace is not slower than your shorter swim pace, which usually means a pacing, rest or typing error.

What Your Result Means

There is no official scale of good or bad CSS. Your number is most useful as a training pace and compared with your own past tests. The calculator shows three paces based on a study of competitive swimmers who swam to exhaustion at, below and above their critical speed⁴:

For example, a 400 m in 6:40 and a 200 m in 3:10 gives (400 - 190) / 2 = 105 seconds, a CSS pace of 1:45 per 100 m. The guide shows 1:51 for 5% slower and 1:40 for 5% faster. CSS is always a little slower than your 400 pace, here by 5 seconds per 100 m.

In the same study, lactate stayed steady over 10 x 400 m at CSS with 40 seconds rest, though the effort felt harder each round⁴. That is why CSS is usually swum as repeats with short rests, not as one long swim.

How to Do the CSS Test

1. Warm up well. One research protocol used a set 1 km warm-up of easy swimming, drills, kick and short fast efforts⁵.

2. Swim the 400 first, as fast as you can hold at an even pace⁸.

3. Take 5 to 10 minutes of active recovery, such as easy swimming⁶.

4. Swim the 200 all-out⁸.

5. Use the same stroke, pool length and type of start for both swims and every retest.

Pacing matters most. Swimmers who start too fast and blow up, or start too slowly, get a CSS that does not match what they can hold in training⁸. If the 400 fades, your CSS pace comes out too slow. Research uses trials of about 2 to 15 minutes each, so a 50 m or 100 m sprint as the short swim falls outside that range⁵.

How Often to Retest

Many coaches retest every 6 to 8 weeks, often in a lighter training week⁶. In competitive swimmers, CSS from a 200 and 400 varied by about 2 to 3.4% between repeat tests, and a change smaller than about 0.07 to 0.12 m/s could not be told apart from normal test variation in one person⁵. At a CSS of 1:45 per 100 m, 0.07 m/s is about 7 to 8 seconds per 100 m, so look for a trend over more than one test.

To check a new result, Swim Smooth suggests a set of 400s within 5 days of the test, starting at CSS pace plus 6 seconds per 100 m and getting 1 second faster each round. If that set is far too hard or too easy, adjust your CSS pace⁸.

Why the Distance Pair and the Pool Matter

The 200 and 400 pair is quick, but it gives a faster CSS than tests that add an 800 m or longer swim⁵. In one study, CSS from 200 and 400 m tests was close to the speed swimmers held in a 30 minute swim, but about 3% faster on average³.

Pool length changes your times too. Swimmers were on average 4.3% faster in 25 m pools, which have twice as many turns, than in 50 m pools⁷. Always retest in the same pool length. For the pace of any general swim, use the Swim Pace Calculator.