Swimming pace calculator

How swimming pace and CSS are calculated

Swimming pace is time divided by distance, expressed as minutes and seconds per 100 metres or per 100 yards. The calculator has two modes. Pace mode takes any distance and time and converts it into pace per 100 m, pace per 100 yd, and speed. CSS mode takes two timed swims, a 400 m and a 200 m time trial, and works out the pace you could sustain for a long aerobic effort without the anaerobic push of a shorter swim skewing the number.

Pace mode

Pace per 100 m equals total time divided by distance in metres, multiplied by 100. Swim 1,000 m in 16 minutes 40 seconds, which is 1,000 seconds, and pace per 100 m is 1,000 divided by 1,000, times 100, which is 100 seconds, or 1:40 per 100 m. Speed follows directly from distance divided by time, so 1,000 m in 1,000 seconds is exactly 1.0 metres per second, or 3.6 kilometres an hour.

Converting to yards

Metres and yards are not the same distance, so pace per 100 yd is not the same number as pace per 100 m at the same speed. One yard is exactly 0.9144 metres, so 100 yd is 91.44 m, a shorter distance than 100 m. At the same swimming speed, covering 91.44 m takes less time than covering 100 m, so pace per 100 yd is always a smaller number than pace per 100 m for the same swimmer. Multiplying pace per 100 m by 0.9144 gives pace per 100 yd exactly, so a 1:40 per 100 m pace becomes about 1:31 per 100 yd.

CSS mode

Critical swim speed treats the 400 m and the 200 m time trial as two points on a distance and time line. The difference in distance, 200 m, divided by the difference in time between the two swims, gives a velocity in metres per second. Turned into a pace per 100 m, that becomes the 400 m time minus the 200 m time, divided by 2, in seconds. Swim the 400 m in 6:20 and the 200 m in 3:00, and the difference is 3 minutes 20 seconds, 200 seconds. Half of that is 100 seconds, a CSS pace of exactly 1:40 per 100 m, at a velocity of 200 m over 200 s, exactly 1.0 metres per second.

What critical swim speed measures, and where it comes from

The idea behind CSS comes from critical power, a concept from cycling and running research that Wakayoshi and colleagues adapted to swimming in 1992. They proposed that plotting swimming distance against the time it takes to cover it, across several efforts, produces a straight line, and the slope of that line, a velocity, marks the pace a swimmer can hold for a long time without exhausting their anaerobic reserves.

A follow up study by the same group found that swimming right at critical velocity kept blood lactate roughly steady across repeated stages, while swimming even slightly faster caused lactate to climb progressively. That supports critical velocity as a reasonable stand in for the maximal lactate steady state pace, the fastest pace a swimmer can hold aerobically before lactate starts to build up.

The two point shortcut

Wakayoshi's 1992 study measured critical velocity in a swimming flume, from swims at six set speeds. The 1993 follow up worked it out from just two maximal swims, 200 m and 400 m, which is the two point version used here, and the practical field test coaches and swimmers actually use, since it needs only a pool, a clock and simple arithmetic. Scott, Burden and Dekerle (2023) retested that 200 m and 400 m protocol in 32 national level swimmers, each in their own specialist stroke, with every trial on a separate day. Both times were highly repeatable (coefficient of variation under 2 percent) and the critical speed calculated from them varied by under 4 percent. The authors note that adding a longer third trial such as 800 m would make the estimate more valid, but many swimmers, butterfly specialists especially, were unwilling to swim one. Treat a two point CSS as a solid, retestable training number rather than a precise laboratory measurement.

How to run the 400 and 200 time trial

Swim both trials in the same session, in the same pool, on rested legs. Warm up properly first, since a cold 400 m or 200 m time trial understates true fitness and skews the gap between the two swims that CSS depends on.

Order and rest

Swim the 400 m first, the usual order, so the longer trial is swum fresh and the 200 m, which is less affected by leftover fatigue, comes second. Rest for around 5 to 10 minutes between the two, enough to recover breathing and heart rate without losing the feel of the session. Both swims should be a genuine maximal, sustainable effort for that distance, not a controlled pace held back for later. A 400 m paced too conservatively, or a 200 m swum as an all out sprint rather than a hard sustained effort, distorts the difference between the two times and with it the calculated CSS.

Timing and conditions

Time both swims to the second from a proper push start, ideally with a stopwatch or pool clock rather than a phone timer started late from the wall. Use the same stroke, usually front crawl, and the same pool for both swims. Retest every 4 to 6 weeks during a training block, in the same pool and stroke each time, so a change in CSS reflects a change in fitness rather than a change in test conditions.

Swim pace at a glance

The table shows exact finish times at four common swim paces across the pool and open water distances the calculator's splits list also covers. The 2:00 per 100 m row is highlighted as a common reference pace for recreational lap swimming.

Does pool length change your pace

Pool length does not change the arithmetic in this calculator, since pace comes from your total time and total distance regardless of how many walls you pushed off along the way. It does change how honestly that pace reflects your swimming fitness.

Short course versus long course

A 25 m or 25 yd pool gives a wall and a push off every 25 m, while a 50 m long course pool gives one every 50 m. Each push off and streamline glide covers a few metres essentially for free, without the drag cost of a stroke, so the same swimmer typically posts a faster pace in a short course pool than a long course one, purely from the extra walls. Comparing a short course time trial with a long course one is not a fair comparison of fitness, only of how many walls were available.

Metres versus yards pools

A 25 yd pool, common in the United States, is slightly shorter than a 25 m pool, 22.86 m against 25 m, which adds even more walls per length swum on top of the short course effect above. A time trial swum in a 25 yd pool is not directly comparable to one swum in a 25 m or 50 m pool even at the same reported number, since 400 yd and 400 m are different distances entirely, 365.76 m against 400 m. Always note which pool length and unit a time trial was swum in, and retest in the same pool where possible.

Open water versus pool pace

Open water swimming removes walls entirely, along with the small rest each push off provides, and adds navigation, sighting, waves and current, all of which typically make open water pace slower than pool pace for the same underlying fitness.

Sighting and drafting

Lifting the head to sight a landmark every few strokes breaks stroke rhythm and adds drag compared with a pool swim's straight line down a marked lane. Drafting off another swimmer, following close behind or beside them, can meaningfully reduce that cost in open water racing, an advantage a pool swim never offers since lane ropes keep swimmers apart.

Using pool CSS for an open water goal

A pool tested CSS is still a useful planning number for an open water swim, since it reflects genuine aerobic fitness, but expect the actual open water pace to run slower, more so in rougher water, colder temperatures, or a longer event where sighting and navigation cost adds up over more strokes.

Using CSS to guide training

CSS is most useful as a single, retestable number to track progress and to describe how a set should feel, rather than as a fixed pace to hold in every session. A set held right at CSS should feel like a genuinely hard, sustainable effort, one you could not have pushed much faster across the whole set.

Faster and slower than CSS

Sets swum meaningfully faster than CSS train the anaerobic side of swimming and cannot be held for long without a substantial rest between reps. Sets swum slower than CSS, at an easy or steady effort, build aerobic base without accumulating much fatigue. Exactly how many seconds per 100 to add or subtract for a given set is something a coach who knows the swimmer's other training and racing patterns is better placed to judge than a fixed offset applied to every swimmer.

Retesting

CSS moves as fitness changes, typically improving over a training block and easing off during a taper or after time out of the pool. Retesting every 4 to 6 weeks, at a similar point in the training cycle each time, keeps the number meaningful rather than comparing a rested taper time trial against one swum mid way through a heavy block. If pool time is part of a wider weekly routine, the swimming calories burned calculator folds session length and effort into a weekly calorie estimate alongside it.

Common mistakes

Sprinting the 200 m. An all out 200 m rather than a hard, sustainable one inflates the gap between the two times and pushes the calculated CSS pace faster than it really is.

Pacing the 400 m too conservatively. Holding something back on the longer swim narrows the gap between the two times and drags the calculated CSS pace slower than it really is.

Mixing pool lengths between tests. A short course result compared against an old long course result, or a yards result against a metres one, looks like a fitness change when it is really a change in how many walls were available.

Confusing pace with speed. A lower pace number is faster, a higher speed number is faster. Mixing the two up when comparing sessions or swimmers leads to real training errors.

Skipping the warm up before a time trial. A cold 400 m or 200 m understates fitness and can distort the gap between the two swims that the CSS formula depends on.

Treating CSS as fixed for a whole season. CSS moves with fitness, so a number from three months ago is probably a little slower than current form after a solid training block.

Where this estimate breaks down

CSS is specific to the stroke it was tested in. Wakayoshi's studies used front crawl, and Scott, Burden and Dekerle (2023) found the 200 m and 400 m test reliable when each swimmer used their own specialist stroke, so butterfly, backstroke or breaststroke can be tested the same way. Those strokes carry different drag and efficiency profiles, though, so a CSS worked out from freestyle swims does not transfer to another stroke; test in the stroke you plan to train.

Illness, poor sleep, a heavy training load carried into the test, or testing straight after a hard session all slow both time trials and can shift the calculated CSS without any real change in fitness. A wetsuit in open water changes buoyancy and drag enough that a pool CSS in a skins swim is not a precise match for a wetsuit swim pace. Treat any single test as one data point, and trust a trend across several tests over trusting one number in isolation.

How coaches use a swimming pace calculator

For a swim coach, pace and CSS are the numbers every set gets built around, and working them out by hand for every client, every few weeks as fitness changes, adds up over a season of clients.

A workable routine: retest CSS every 4 to 6 weeks in the same pool and stroke, write sets around the current number rather than one from months ago, and log both time trial times alongside the calculated CSS so a coach can see whether a change came from the 400 m, the 200 m, or both.

Scraler lets you build and assign swim sessions with target paces per client, and see what was actually logged next to what was planned in workout programming, rather than relying on a swimmer's memory of how a session felt. Retest results fit naturally into a client's check ins, alongside the rest of their training log.