Short course vs long course: why your two times differ

You swim a 100 free in 1:32. Two weeks later, in a different pool, the same effort reads 1:36 and you spend the drive home wondering what went wrong.

Nothing went wrong. You swam in a longer pool, and the clock is telling you something about walls rather than about you.

The two courses

Long course means a 50 m pool. It is the Olympic standard and the one the world records that get quoted are set in.

Short course means a 25 m pool — or, in the United States, a 25-yard pool, which is a third thing again and the source of most of the confusion in this article.

The water is identical. The difference is entirely in how often you meet a wall.

Why walls are free speed

A turn is not a pause in the swim. It is the fastest part of it.

You arrive carrying momentum, plant your feet against something solid and push. For that moment you are producing force against a fixed object rather than against water that moves when you press on it, which is why the push-off is the highest speed you will reach in the whole length. Then you travel in a streamline, with a dolphin kick, in a position that has far less drag than surface swimming.

Competition rules put a limit on how much of this you are allowed: after the start and after each turn, the head must break the surface by 15 m. That number exists precisely because underwater travel is faster than surface swimming, and without a cap the sport would become a breath-holding contest.

So the arithmetic of a 400 m swim looks like this. In a 50 m pool: eight lengths, seven turns. In a 25 m pool: sixteen lengths, fifteen turns. Eight extra opportunities to do the fast thing.

If your turns are sloppy, that advantage shrinks. The mechanics are in how to do a flip turn.

How big is the gap, really

This is where most articles wave a hand and say "a few seconds". There is a better answer.

A study published in the Journal of Human Kinetics examined 200 m and 400 m individual medley results across 2000–2011, comparing 25 m and 50 m course performances. The sample was substantial: 26,081 swims on the Swiss national high score list plus 382 world championship finalists.

Swiss and world-level swimmers of both sexes were, on average, 4.3 ± 3.2% faster on short course at both distances.

Two caveats worth keeping. That is individual medley, which contains four different turns and may exaggerate the effect relative to a straight freestyle swim. And that standard deviation of 3.2 is enormous next to a mean of 4.3 — meaning the gap for any individual swimmer might be 1% or might be 8%.

Which is the actually useful conclusion: the conversion is not a constant, it is a property of your turns. A swimmer with a strong underwater kick gains far more from extra walls than one who surfaces immediately and starts stroking.

The American complication: yards

In the United States most pools are 25 yards, and this trips up every swimmer who tries to compare themselves to international standards.

A yard is 0.9144 m, so a 25-yard pool is about 22.86 m and 100 yards is about 91.4 m. Your 100-yard time is therefore faster than your 100 m time for two independent reasons stacked on top of each other: you swam about 8.6% less distance, and you did it in a short course pool.

That is why an American swimmer reading a world record and comparing it to their own best is often comparing three different things at once. The time converter exists for this, and it is worth using before you draw conclusions about your own level from average 100 m times. And when the question is not two pools but two different events, the World Aquatics points calculator scores each swim against its own course’s records, so a 50 m result and a 25 m result are never mixed on one scale.

Where the 4.3% actually comes from

It is worth pulling the number apart, because doing so tells you where your own gap sits.

Each extra turn buys you three separate things. There is the push-off itself, a burst of speed produced against a solid wall rather than against water. There is the streamline and underwater kick, up to 15 m of travel in a body position with far less drag than surface swimming. And there is the free breath — the moment before the wall and the moment after are both times when you are not managing a breathing pattern.

Which means the advantage is not evenly distributed among swimmers. Somebody who pushes off shallow, surfaces after three metres and starts stroking immediately collects the first of those three and very little of the second. Somebody who holds a tight streamline to 10 m collects almost all of it, several times per swim.

That is the honest explanation for the ±3.2% spread. It is not measurement noise. It is a real difference between swimmers, and it is trainable.

The same logic explains a pattern coaches see constantly: swimmers who are relatively better at short course tend to be the ones with good underwater work, and swimmers who are relatively better at long course tend to be aerobically strong with weak walls. Whichever you are, the gap tells you what to train.

What this means for your training

The practical consequences are bigger than the tidy conversion factor suggests.

The same set is a different session. Ten 100s in a 25 m pool contain 30 turns. The same ten 100s in a 50 m pool contain 10. If turning is a strength, the short course version lets you hide behind it; if it is a weakness, the long course version makes you swim.

Long course exposes pacing. Fifty metres without a wall is a long way to hold a stroke together, and the second half of a long course 50 is where technique that only works when fresh falls apart.

Short course rewards a skill worth having anyway. Turns and underwater kick transfer to every distance in every pool. Time spent on them is never wasted, whatever course you race in.

If you are training for open water or triathlon, note that there is a third case: no walls at all. Every advantage discussed here disappears, which is why open water swimmers often find their pool times flattering. That is dealt with in swim training for triathlon.

Two sets that measure the difference on you

Rather than trusting an average from a study of other people, you can find your own number in two sessions.

In a 25 m pool: swim 4 × 100 m at a hard but repeatable effort, on plenty of rest so fatigue is not the variable. Record every time.

In a 50 m pool, within the same week: the identical set, identical effort, identical rest.

Compare the averages. The percentage difference is your conversion, not the study's, and it is the number worth writing in the front of your logbook.

If your gap is well above 4%, your walls are doing a lot of work and your surface swimming is the weaker half. If it is well below, the reverse: you are an efficient surface swimmer leaving free speed on every wall, and the turn article is the highest-return thing on this site for you.

A word of caution on the protocol. Do it at the same time of day, rested, and not in the week after a hard block. Otherwise you will measure your fatigue and attribute it to the pool.

How to compare your own times honestly

Three rules that will save you a lot of confusion.

Keep separate logs. One for each course you swim in. Comparing across them without adjustment is the single most common way swimmers convince themselves they are getting slower.

Note the course on every entry. Six months later you will not remember which pool a time came from, and an unlabelled personal best is a number you cannot use.

Treat conversions as approximate. They are built on averages, and the study above shows the individual spread is wide. A converted time is a rough guide to what you might swim, not a time you have swum.

And where a table already exists for your course, do not convert at all. USA Swimming publishes its motivational standards separately for 25 yards, 25 m and 50 m, so a yards time is read against the yards column and nothing is estimated on the way. Pick the course in the age group time standards and the cut you see is the published one.

If you want a sense of where any of these numbers place you, how long it takes to swim 1 km and what counts as a good mile time give the ranges, and the rest of the standards live in times.

This is general information rather than coaching for your specific race. If you spot an error or a rule that has changed, tell us and we will correct it and date the change.