Swim Pace Calculator: Pace per 100 and Critical Swim Speed (CSS)

Pace per 100 is the time you take for each 100 yards or meters, the number swimmers set their intervals by. This swimming pace calculator works it out from any swim, turns a pace into finish times from 100 yards to the 2.4-mile Ironman swim, and splits it into the time for each length of your pool. Yards and meters convert by the exact yard, 0.9144 meters, as defined by the US National Institute of Standards and Technology.

It is also a critical swim speed calculator. CSS grew out of research by Wakayoshi and colleagues in the early 1990s, who applied the critical power idea to swimming and showed that two all-out swims, a 400 and a 200, give a threshold pace for training. This CSS swim calculator turns your two times into that pace, into the training paces later research tied to it, and into the error you should expect from a two-swim test.

How to calculate swim pace per 100

Swim pace per 100 is your swim time divided by the distance in hundreds. A finish time runs the same sum backward.

Pace per 100 = swim time ÷ (distance ÷ 100)Finish time = pace per 100 × (distance ÷ 100)

1. Put the time in seconds. 20:00 is 1,200 seconds.
2. Count the hundreds. 1,000 yards is 10 hundreds, and 1,650 yards is 16.5.
3. Divide. 1,200 ÷ 10 = 120 seconds, a pace of 2:00 per 100 yards.
4. Split it by length. A 25-yard length is a quarter of 100 yards, so 2:00 per 100 is 30 seconds a length. In a 50-meter pool one length is half of 100 meters.

A yard is exactly 0.9144 meters, by the conversion table in NIST Special Publication 811, so 100 meters is 109.36 yards. To turn a pace per 100 yards into the same speed per 100 meters, multiply it by 1.0936. To go the other way, multiply by 0.9144. So 2:00 per 100 yards is 2:11 per 100 meters, and 2:00 per 100 meters is 1:50 per 100 yards.

That conversion changes the distance only. The length of the pool also changes how often you push off a wall. In a study of 11 competitive swimmers by Bosetto and colleagues (2026), the same effort over 400 meters was 5.2 percent faster in a 25-meter pool than in a 50-meter pool, and at the same speed the long pool cost about 4 percent more energy. The authors advise coaches to be cautious about prescribing the same paces in both. That is why this calculator asks for your pool course: a pace from a 25-yard pool describes swims in a 25-yard pool.

Swim pace chart: finish times at each pace per 100

Read across from your pace per 100 to the finish time for each distance. The chart works in yards or in meters, as long as the pace and the distance use the same unit: 2:00 per 100 yards gives 1,500 yards in 30:00, and 2:00 per 100 meters gives 1,500 meters in 30:00.

Each time assumes one even pace from start to finish. Holding a pace gets harder as the swim gets longer, so the right-hand columns are a best case unless you have already swum that distance at that pace.

How long does it take to swim a mile?

That depends on which mile you mean. A full mile is 1,760 yards or 1,609 meters, but pool swimmers often call shorter events the mile: the 1,650-yard freestyle in a yards pool and the 1,500 meters, the metric mile, in a meters pool. At 2:00 per 100 yards, 1,650 yards takes 33:00 and a full mile takes 35:12. At 2:00 per 100 meters, 1,500 meters takes 30:00 and a full mile takes 32:11.

Read the pace as per 100 yards for the two yard columns and per 100 meters for the two meter columns.

Swim pace for triathlon: sprint to Ironman

Triathlon swims are set distances in open water. World Triathlon sets the sprint swim at 750 meters and the standard (Olympic) swim at 1.5 kilometers, and the Ironman 70.3 and full Ironman swims are 1.2 and 2.4 miles. Ironman is a registered trademark, and FitCraft is not affiliated with the Ironman races. The table uses pace per 100 meters, with the same speed per 100 yards underneath for swimmers who train in a yards pool.

Open water has no walls to push off, and you also sight, round buoys and swim through chop, so do not expect a pool pace to carry over exactly. Turns alone made the same effort 5.2 percent faster in a 25-meter pool than in a 50-meter one in the Bosetto study, and open water has no turns at all. Read the times as a best case, then check them against a long open-water swim of your own.

What is critical swim speed (CSS)?

Critical swim speed is the swimming version of critical power. Plot the distances of several all-out swims against their times and the points fall close to a straight line. The slope of that line is the critical speed: in theory, the fastest pace you could hold for a very long time without exhaustion. In the first swimming study, Wakayoshi and colleagues (1992) tested 9 trained college swimmers in a swimming flume. Distance and time fitted a straight line almost perfectly (r² above 0.998), and the swimmers' critical speeds ran from 1.06 to 1.26 meters per second, about 1:34 to 1:19 per 100 meters. Critical speed tracked the speed at a blood lactate of 4 mmol/L (r = 0.949) and the swimmers' average speed over 400 meters (r = 0.864).

A second paper the same year drew the same line from swims in an ordinary pool as well as in a flume (Wakayoshi et al. 1992). Then the team showed that two swims are enough. In Wakayoshi and colleagues (1993), critical speed from just a 200 and a 400 correlated closely with the speed at 4 mmol/L of blood lactate (r = 0.914) and with 400 m race speed (r = 0.977) in 8 swimmers. When the swimmers held that speed for four 400s, their blood lactate stayed level at about 3.2 mmol/L, and at 2 percent faster it kept climbing. That study is the basis of the CSS test, and of the idea that CSS sits at the top of the steady range.

Researchers call it critical velocity, critical speed or critical swimming speed. Many coaches call it CSS, the name this page uses.

How to do a CSS test: the 400 and 200 time trial

The test is two all-out swims in the same pool and the same stroke. These steps follow the protocols in the studies cited on this page.

1. Warm up well. In the study by Nikitakis and colleagues (2019), swimmers did a 400 of front crawl, 200 of drills and 4 × 50 at their best 400 pace, then rested 8 minutes before the first trial.
2. Push off the wall for both swims. The Nikitakis study started each trial from a push in the water rather than a dive, so both swims begin the same way.
3. Swim the 400 all-out at an even pace. Going out too fast and fading makes the 400 slower than you can really swim it, and a slow 400 makes CSS too slow.
4. Recover fully. The Nikitakis study allowed 30 minutes between the two trials. Scott, Burden and Dekerle (2024) ran each trial on its own day, at least 48 hours apart.
5. Swim the 200 all-out. A 200 that is not truly all-out makes CSS too fast.
6. Time both swims to the tenth of a second if you can. Enter them in the calculator with the pool course you used.

Most of the research on this page tested front crawl. Scott and colleagues ran the same 200 and 400 test in all four strokes and found CSS repeatable in each, so test in the stroke you plan to train in.

Critical swim speed calculator: the formula

With two swims, the straight line runs through two points, and its slope is the distance between the swims divided by the time between them:

CSS = (400 − 200) ÷ (400 time − 200 time)CSS pace per 100 = (400 time − 200 time) ÷ 2

For the default swimmer, a 400 in 7:00 is 420 seconds and a 200 in 3:20 is 200 seconds. CSS = 200 ÷ (420 − 200) = 200 yards in 220 seconds, or 0.91 yards a second. Per 100 yards that is 220 ÷ 2 = 110 seconds, a CSS pace of 1:50. In meters per second, the unit the research uses, it is 0.83.

Because CSS pace is half the gap between the two times, every second you lose in either swim moves it by half a second per 100. Lose 2 seconds in the 400 and CSS pace slows by 1 second. Lose 2 seconds in the 200 and CSS pace gets 1 second faster, because the gap shrinks. That is plain arithmetic, and it is why both swims need to be all-out.

The same line gives a second number, D′ (D prime): its intercept, the distance you can swim above critical speed before you tire. This calculator leaves it out. When Scott and colleagues had swimmers repeat the 200 and 400 three times, D′ moved by 4 to 9 meters from test to test, a coefficient of variation of 13 to 45 percent, and they judged it not reliable.

Training paces from your CSS

CSS is most useful as a pace to train at. The bands in the calculator come from a 2013 review of critical speed research by Toubekis and Tokmakidis. It found that critical speed runs about 3 to 6 percent faster than maximal lactate steady state (MLSS), the fastest pace at which blood lactate stays level in a long, continuous swim, and 8 to 11 percent faster than the lactate threshold, an easier landmark below MLSS.

• CSS repeats, 98 to 102 percent of CSS speed. The review suggests repeats of 100 to 400 at these speeds for aerobic training. For the default swimmer that is 1:48 to 1:52 per 100 yards.
• Maximal lactate steady state, 3 to 6 percent slower than CSS. This is about the most you can hold in one continuous swim: 1:53 to 1:57 per 100 yards for the default swimmer.
• Lactate threshold, 8 to 11 percent slower than CSS. For the default swimmer that is 1:59 to 2:02 per 100 yards.
• Your 400 and 200 paces. Your own test paces, 1:45 and 1:40 per 100 for the default swimmer, mark the fast end of what you can hold.

Repeats with short rests are the format the research backs at CSS. In the Nikitakis study, 10 × 200 at critical speed with 35 to 45 seconds of rest produced physiological responses similar to swimming at MLSS. Continuous swimming at critical speed was harder: only 6 of 10 boys and 7 of 10 teenagers lasted 30 minutes. For the default swimmer, that study's set would be 10 × 200 yards at about 3:40 each, a demanding session. The review adds that critical speed sets paces best when it comes from swims lasting about 3 to 15 minutes, and that young swimmers' lactate can rise or fall at that pace, so coaches should adjust it for them.

Worked examples: CSS and training paces for six swimmers

Each row runs the calculator on a 400 and a 200 in the pool course shown. The national-level row uses the first-trial average times of 10 front crawl swimmers in the study by Scott and colleagues, and the other rows are our own examples. Paces are per 100 in the unit of the course.

Two patterns stand out. CSS always sits as far behind your 400 pace as your 400 pace sits behind your 200 pace, which follows from the formula: for the default swimmer each step is 5 seconds per 100 (1:40, 1:45, 1:50). And because the bands are percentages, they spread with the pace: the lactate threshold band starts 6 seconds per 100 behind CSS for the fast age-group swimmer and 12 seconds behind for the newer lap swimmer.

How accurate is this swim pace calculator?

The pace and finish-time math is exact arithmetic, so it is as accurate as your distance and your watch. The easy mistake is the unit: a 25-meter length is 9.4 percent longer than a 25-yard one, so a swim entered under the wrong course reads 9.4 percent off. The real accuracy question is about CSS, and it has two parts: how well a CSS from two swims matches the lactate landmarks it stands in for, and how much it moves when you repeat the test.

CSS compared with maximal lactate steady state

The studies disagree. In Wakayoshi's 1993 study of 8 swimmers, blood lactate held level at a CSS from a 200 and a 400 and climbed at 2 percent faster, so CSS matched MLSS. Dekerle and colleagues (2005) found otherwise in 8 well-trained swimmers, with CSS worked out from four swims to exhaustion rather than a 200 and a 400. CSS came out at 92.7 percent of maximal aerobic speed and MLSS at 88.3 percent, so CSS was about 5 percent faster (our arithmetic on their averages). On average CSS was 0.07 meters per second faster, and the 95 percent limits of agreement ran from about 0.06 slower to 0.20 faster, even though the two correlated closely (r = 0.87). The authors concluded that CSS does not represent MLSS and that precise work needs a direct lactate test.

Across studies, the Toubekis and Tokmakidis review puts critical speed 3 to 6 percent faster than MLSS and 8 to 11 percent faster than the lactate threshold. In young swimmers the gap can be smaller. Nikitakis and colleagues found CSS from a 200 and a 400 within 1.5 percent of MLSS speed in 10 boys (1.092 against 1.083 meters per second) and 10 teenagers (1.315 against 1.297), a difference that was not statistically significant. Even so, blood lactate at the end of a continuous swim at CSS was higher than at MLSS (4.5 against 3.1 mmol/L in the teenagers), and 7 of the 20 swimmers could not last 30 minutes. In a study of 36 young swimmers by Greco and colleagues (2007), CSS from a 200 and a 400 was faster than the speed they held for 30 minutes: 1.25 against 1.16 meters per second in boys and 1.15 against 1.11 in girls, with strong correlations (r = 0.89 and 0.94).

In practice, treat CSS as the top of your steady range rather than a pace for a long continuous swim. That is how the bands in the calculator use it.

How much a repeat test moves CSS

Scott, Burden and Dekerle had 32 national-level swimmers swim the 200 and the 400 three times each over three weeks, at least 48 hours apart, in a 25-meter pool. For the 10 front crawl swimmers, 200 times varied by 1.85 percent and 400 times by 1.25 percent from test to test, and CSS had a typical error of 0.04 meters per second, a coefficient of variation of 3.07 percent. Across all four strokes, CSS varied by less than 4 percent. At a CSS of 1:50 per 100, 3 percent is about 3 seconds per 100, so a retest that moves your CSS by less than that may not be a real change.

Your own timing adds to that error. Because CSS pace is half the gap between the two times, a second lost in either swim moves it by half a second per 100.

Why a 200 and a 400 give a fast CSS

Short swims pull the estimate up. Scott and colleagues note that the 200 and 400 combination gives higher critical speed estimates than models that include longer swims, and that the swims behind a valid estimate should last about 2 to 15 minutes. The Toubekis review recommends swims of about 3 to 15 minutes, and a fast swimmer's 200 lasts only about 2 minutes. That is one reason a CSS from this test can land above MLSS. Longer swims are harder to fit in, though. When Scott and colleagues asked about adding an 800 as a third swim, all the backstroke and front crawl swimmers said they could do it, but only 71 percent of breaststroke and 39 percent of butterfly swimmers did.

Strength training for swimmers

Swim times improved most when pool training was paired with strength work on land. A 2025 network meta-analysis by Wang and colleagues pooled 36 randomized trials with 844 competitive swimmers aged about 10 to 21. Pool training plus dry-land resistance training ranked first for 100 m times, about 2.35 seconds faster than the control groups (95 percent confidence interval 1.07 to 3.63 seconds), and second for 50 m times. Those were young competitive swimmers, so the size of the effect for adults who swim for fitness is not known. Swimming on its own builds only limited muscle, as our post on whether swimming builds muscle explains.

FitCraft does not track swims, but it can cover the work on land. Our page on the dumbbell workout app shows how FitCraft builds dumbbell and bodyweight strength sessions around your goal, with your AI coach demonstrating the form for every exercise.

When to ignore this calculator

The pace math works for any swim. The CSS test and its bands are built for healthy swimmers who can swim a hard 400. In the cases below, use the pace mode only, or follow a coach's or a clinician's plan.

• Children and young teenagers. Blood lactate can rise or fall at CSS in young swimmers, so the Toubekis review says coaches should correct the pace for them, and in the Nikitakis study 4 of 10 boys could not swim 30 minutes at CSS.
• Newer swimmers who cannot swim 400 without stopping. The test needs two continuous all-out swims. Use the pace mode on your normal swims until a 400 feels manageable, and see our swimming workout for beginners for sessions that build up to it.
• Heart or lung conditions, high blood pressure or chest pain. Two all-out swims are maximal efforts. Get medical clearance before you test.
• Pregnancy. Ask your obstetric care team before any all-out effort in the water.
• Open water. Pool paces and CSS do not carry over exactly to open water, and cold water, currents and distance from help add risk. Swim with others, where someone can see you.
• Strokes other than front crawl. The validation studies on this page mostly tested front crawl. Test in the stroke you train in.

Never swim alone, and stop if you feel dizzy, short of breath or unwell in the water.

Related reading

References

1. Wakayoshi K, Ikuta K, Yoshida T, Udo M, Moritani T, Mutoh Y, Miyashita M. "Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer." Eur J Appl Physiol Occup Physiol. 1992;64(2):153-157. doi:10.1007/BF00717953, PubMed 1555562
2. Wakayoshi K, Yoshida T, Udo M, Kasai T, Moritani T, Mutoh Y, Miyashita M. "A simple method for determining critical speed as swimming fatigue threshold in competitive swimming." Int J Sports Med. 1992;13(5):367-371. doi:10.1055/s-2007-1021282, PubMed 1521952
3. Wakayoshi K, Yoshida T, Udo M, Harada T, Moritani T, Mutoh Y, Miyashita M. "Does critical swimming velocity represent exercise intensity at maximal lactate steady state?" Eur J Appl Physiol Occup Physiol. 1993;66(1):90-95. doi:10.1007/BF00863406, PubMed 8425518
4. Toubekis AG, Tokmakidis SP. "Metabolic responses at various intensities relative to critical swimming velocity." J Strength Cond Res. 2013;27(6):1731-1741. doi:10.1519/JSC.0b013e31828dde1e, PubMed 23449237
5. Dekerle J, Pelayo P, Clipet B, Depretz S, Lefevre T, Sidney M. "Critical swimming speed does not represent the speed at maximal lactate steady state." Int J Sports Med. 2005;26(7):524-530. doi:10.1055/s-2004-821227, PubMed 16195984
6. Nikitakis IS, Paradisis GP, Bogdanis GC, Toubekis AG. "Physiological responses of continuous and intermittent swimming at critical speed and maximum lactate steady state in children and adolescent swimmers." Sports (Basel). 2019;7(1):25. doi:10.3390/sports7010025, PMC6359490
7. Greco CC, Pelarigo JG, Figueira TR, Denadai BS. "Effects of gender on stroke rates, critical speed and velocity of a 30-min swim in young swimmers." J Sports Sci Med. 2007;6(4):441-447. PMC3794483, PubMed 24149476
8. Scott BE, Burden R, Dekerle J. "Stroke-specific swimming critical speed testing: balancing feasibility and scientific rigour." J Hum Kinet. 2024;90:239-251. doi:10.5114/jhk/170882, PMC10875687
9. Bosetto P, Coloretti V, Lubrano M, Bonifazi M, Zamparo P, Fantozzi S, Cortesi M. "Swimming turns reduce energy demands of the aerobic performance in front crawl." Eur J Appl Physiol. 2026;126(5):2491-2500. doi:10.1007/s00421-025-06084-7, PMC13236779
10. Wang Z, Liu K, Zhao X, Gao J. "Comparative effectiveness of physical training modalities on swimming performance: a two-tier network meta-analysis." Front Physiol. 2025;16:1636595. doi:10.3389/fphys.2025.1636595, PMC12423047
11. Thompson A, Taylor BN. Guide for the Use of the International System of Units (SI). NIST Special Publication 811, 2008 edition, Appendix B.8, factors for units listed alphabetically (yard = 0.9144 m and mile = 1609.344 m, both exact). NIST SP 811, Appendix B.8

How this calculator differs from typical swim pace calculators

Most online swim pace calculators divide your time by your distance and print one number. Many treat yards and meters as a display setting, which hides the fact that the length of the pool changes the pace itself, and few say how a pace from a short pool carries to a long pool or to open water. Critical swim speed calculators usually add a table of training zones with no source, and they rarely say how accurate a CSS from two swims is.

This calculator treats the pool course as an input of its own and converts yards and meters by the exact yard, with per-length splits and finish times out to the 2.4-mile Ironman swim, read as a best case. Its CSS uses the two-swim formula from Wakayoshi's research, its training bands come from a published review rather than a coaching chart, and the page reports the published error: CSS ran 3 to 6 percent faster than MLSS on average, and a repeat test moved it by about 3 percent. It refuses a test in which the 400 was faster per 100 than the 200, and it leaves out D′, which did not hold up when swimmers repeated the test.